Final report for LNE21-426R
Project Information
Problem addressed: Maine’s soft-bottom intertidal zone has witnessed a steady decline in commercial landings of its iconic soft-shell clam fishery (85% since 1977) with concomitant job losses from 4,500 clammers to 1,500 today. Clams were ubiquitous from low water to the high tide line; however, as seawater temperatures have increased dramatically since the 1980s, the once-abundant clams now are restricted to thin strips along the upper shore – areas that are out of the reach of many waterborne predators, including the invasive green crab that has seen dramatic increases in population numbers over the same period. Surprisingly, the combination of climate change and lack of sustainable clam management practices has resulted in a tremendous opportunity to create new wealth in the lower intertidal zone. Arctic surfclams, Mactromeris polynyma, occur subtidally in the northwest Atlantic from Rhode Island’s offshore waters to Newfoundland, but they can survive and grow in eastern Maine near the low water mark. A $100 million fishery for this red-footed bivalve (also known as “hokkigai”) exists in Atlantic Canada where the animals are dredged from 100-200 ft depths, and is highly-valued as sushi and sashimi in Asian markets and restaurants.
Solution pursued: While no large, commercial beds occur in the Gulf of Maine, we obtained broodstock with a goal to create a new culture candidate and diversify the shellfish industry by growing individuals to sizes between 38-50 mm SL (shell length) that can be consumed raw on the half-shell, steamed, fried, or broiled, or used in chowders, stews, or even in salads. We worked for five years to close the hatchery and nursery phase of this species for the first time ever.
Research objective and research implemented: Prior to receiving funding through USDA, fieldwork using cultured juveniles demonstrated cultured juvenile Mactromeris polynyma can grow and survive in the lower intertidal, especially in eastern Maine where seawater temperatures are colder than elsewhere along the coast. Most significant impediments to commercialization are crustacean and bird predators that can shred protective netting and consume >90% of 6-12 mm individuals. Hence, our research objectives were to: 1) develop, validate, and deploy pilot-scale studies in two communities in eastern Maine (Beals and Gouldsboro) to produce marketable, cultured Arctic surfclams; and, 2) demonstrate growout technologies to clammers and other entrepreneurs who earn their income working in the soft-bottom intertidal. These efforts focused on comparative experiments that tested interactive effects of stocking density, surfclam size, size of the growout unit, and season on growth and survival of cultured individuals ranging in size from 8-20 mm in shell length.
Research findings & conclusions: Most efforts to produce commercial size animals were foiled by our inability to deter invasive green crabs as well as an endemic species of another brachyuran crab species. This meant we were unable to repeat results obtained from previous (Pre-2020) efforts where we saw >95% survival over a 7-month period from spring to fall. That work demonstrated clearly that the primary obstacle between success and failure was straightforward: when crabs are excluded from the growout units, surfclams survive and grow. The earlier efforts used a small mesh with a 3.2 mm aperture that excludes crabs as large as 4.5 mm. Since juveniles settle out at sizes ~1 mm, this did not deter the smallest crabs that became entrapped in the field units when they grew to a size that was greater than the mesh size. After six months in the field across multiple trials, experimental units at both sites contained > 5-8 green crabs on average. Reducing the mesh aperture size to 1.7 mm reduced green crab densities to an average of < 1/unit, but resulted in a significant growth penalty for surfclams that would stymie commercial production. Future efforts will focus on using floating gear to grow surfclams to sizes between 40-50 mm in length.
Outreach: Throughout the course of this study, we have reached out to the general public by creating an article for a local newsletter, meetings with shellfish committee, informal courses (Aquaculture in Shared Waters through the University of Maine and the Maine Aquaculture Innovation Center - Feb. 2021 & 2022), and conferences (Northeast Aquaculture Conference and Exposition - Rhode Island - January 2024; National Shellfisheries Association Annual Meeting - March 2024).
Project Objectives
- To develop and deploy pilot-scale field growout trials in two communities in eastern Maine to produce marketable, cultured Arctic surfclams, and;
- To demonstrate growout technologies to clammers and other entrepreneurs who earn their income working in the soft-bottom intertidal.
Arctic surfclams are a new culture candidate. We wish to explore with 3 clammers from two eastern Maine communities novel methods to grow cultured seed (10-15 mm SL) to commercial size (40-50 mm SL). Recent field trials have been encouraging (annual survival [> 90%] and growth rates [15-20 mm SL]). Field trials will test scale-dependent repeatability of these results.
Context for the project: Maine's commercial soft-shell clam fishery has been declining for the past 50 years due in large part to climate change that has seen seawater temperatures in the Gulf of Maine reach levels only recorded once before (early 1950s). In addition, the rate of increase of these warm temperatures has occurred faster than what is occurring elsewhere in most of the global ocean (Pershing et al. 2015). Soft-shell clams can thrive and grow relatively quickly in warm waters, as they range from Newfoundland to Virginia, but a warming ocean environment also brings with it other species that respond differently to the new abiotic conditions. Green crabs were first identified in North America in 1817 (Young and Elliot 2020), having arrived in the ballast water of wooden ships from the British Isles somewhere in Long Island Sound. Over time, green crabs have spread to cover the same range as soft-shell clams. While most native predators (birds, fish, other crustaceans) will consume green crabs, their fecundity precludes their population densities from being regulated biologically; instead, green crab populations are regulated mostly by wintertime weather events. When winters are relatively warm without any or with little ice formation in the intertidal zone, green crab survival is relatively high. Conversely, when winters are cold, and ice forms and lasts for weeks in the intertidal zone, green crab survival is relatively low. Since 1977, a decadal increase of 0.34oC in average wintertime sea surface temperatures has occurred along the Maine coast (Beal et al. 2020). This has resulted in an explosion of green crabs that have wreaked havoc on soft-shell clam populations so much that soft-shell clams in some areas of the Maine coast (Cumberland and York county) will likely be commercially extinct if there is no relief from the ocean warming. In these and other areas along the Maine coast, soft-shell clams are now restricted to the upper shore (i.e., they are absent/missing from most of the intertidal zone including the mid intertidal and lower tidal heights). Fewer and fewer clammers are now plying this trade as the now-variable catch has affected markets. Inconsistency in production levels result in market instability that affects the value of the product and prices paid to clammers. While a majority of the clamming population from only two decades ago has left the fishery, those who participate today are frustrated by the lack of clams to harvest, and the lack of alternatives that take advantage of a clammer's skills that could diversify their income stream.
We thought, and still do, that Arctic surfclams present that alternative for clammers who are willing to learn how to grow these animals to a commercial size of 1.5-2-inches in shell length. The Arctic surfclam, Mactromeris polynyma, is a circumboreal infanual bivalve that can grow in the lower intertidal, where soft-shell clams are lacking today. Previous work on the hatchery and nursery phase of culturing Arctic surfclams had been completed, and early research (2015-2019) to examine conditions required for growing this new culture candidate suggested that the major obstacle to a successful growout operation was designing field gear that would exclude green crabs. Crabs, like surfclams, have a 3-4-week planktonic stage after which they settle to the benthos at a size of ~1 mm in carapace width. Therefore, to exclude green crabs completely from experimental units containing Arctic surfclams any mesh used for this purpose must have an aperture with a size smaller than 1 mm. Unfortunately, while Arctic surfclams can survive in field units protected with mesh with an aperture size of 1 mm (or smaller), they grow very slowly (several mm/yr vs. > 10-15 mm/year in units with a larger aperture). That is, we understood that our efforts to exclude crabs from field units with mesh having an aperture size > 1 mm was risky, but necessary because with the small mesh, surfclams would survive, but not grow, and with larger mesh, surfclams would grow, but survival might be too low to sustain a commercial undertaking.
Having conducted a study to determine where along the Maine coast Arctic surfclams would survive, we discovered that while surfclams can grow anywhere along the coast in the lower intertidal zone, they can do so throughout the year in places where seawater temperatures remain below 20oC during the summer. This eliminates most of the Maine coast except the easternmost one-third from eastern Hancock county to the Canadian border. In late 2019 and early 2020, we began reaching out to towns and clammers to gauge interest in a hands-on field project involving cultured Arctic surfclams. We found clammers in two communities who had an interest in learning how to grow surfclams: Gouldsboro, in eastern Hancock county; and, Beals, in central Washington county. Gouldsboro is approximately 33 km west of Beals. Seawater temperatures at high tide in the lower intertidal in Gouldsboro range from -0.1oC to 18.8oC. In Beals, the range is from -1.7oC to 15.6oC.
Research pursued: In late March/early April 2021, we began fieldwork at soft-bottom intertidal flat in Gouldsboro (Grand Marsh Bay) and Beals (Mud Hole Cove) to assess the interactive effects of three factors (size of growout unit [2ft2 vs. 4ft2]; thickness of the top frame protecting the clams [1- vs. 2-inches]; and, stocking density [25, 50, or 100 clams/ft2]). Five replicates for each of the twelve treatments were used, and experimental units were arrayed in a 6 x 10 matrix in the lower intertidal (-0.5 ft) at both sites. Sampling at both locations occurred in early December 2021. In Gouldsboro, green crabs were discovered in 59 of the 60 experimental units (growout boxes), averaging 11.5 per box (N = 686) and ranging in size (carapace width, CW) from 3.5 mm to 57.3 mm. Surfclam survival varied from 13-20% in small growout boxes and from 40-45% in large growout boxes. No significant stocking density effect was observed on mean percent survival. Frame thickness did not explain a significant amount of variation in mean percent survival, but boxes with 2-inch frames had a lot of mud that had built up between the top of the frame's 1/2-inch vinyl-coated trap wire and the bottom of the frame's 4.2 mm flexible netting. This mud had apparently come from the marsh over the summer and fall associated with historic rainfall events. Growth ranged from an average of 3.2 mm for individuals > 20 mm in initial shell length to 7.9 mm for individuals < 15 mm in initial length. In Beals, surfclam survival varied similarly with size of the experimental unit. Mean survival was ~2x greater in the larger (57%) vs. smaller (27%) growout units. Mean shell length increased by 4.8 mm for larger clams and 7.4 mm for smaller clams.
In 2022, we moved the Gouldsboro study site from Grand Marsh Bay to Timber Cove in an attempt to reduce the amount of suspended sediments that could become entrapped in the top frame. In addition, we had proposed to increase the size of the growout boxes (i.e., experimental units) so that we could compare the efficacy of scaling up the field operation. Clammers from both communities rejected this idea saying that it was already difficult handling the 4ft2 boxes compared with the 2 ft2 boxes, and neither group was interested in pursuing that line of research. Therefore, we continued to focus on the two sizes of boxes used in Year I, along with manipulating clam size (Small: 9.5 mm SL; Large: 12.8) and stocking density (30-90 ind./ft2). Units were established with surfclams on 18-19 June 2022 at Timber Cove and the first sampling occurred on 21 January 2023 (216 days). A total of 418 green crabs (mean CW was 18.6 mm; range = 5.2 mm to 43.2 mm) and 31 rock crabs (mean CW was 52.2 mm; range = 29.4 mm to 57.4 mm) were discovered in the boxes in January 2023 (crab size was not related to any design variable). Live surfclams were discovered in only 27 of the 60 (45%) growout boxes. Typically, whenever crabs were found in boxes (green crabs occurred in 41 [6.9/unit]; rock crabs in 18 boxes [7 shared with green crabs; 0.5/unit]), the majority of clams were dead, having been crushed or chipped by foraging crabs. In terms of growth, small clams added ~11 mm of shell, on average, vs. ~8 mm for large clams. Density was not an important factor in regulating surfclam growth. A second sampling of the boxes occurred 0n 4 August 2023 (412 days). At that time, only five live surfclams were found - a mortality rate of > 99%. The five clams had added 8.4 mm of new shell over the 412 days.
The comparative field trial began 16-17 June 2022 at Mud Hole Cove in Beals, and first sampling occurred on 20 January (218 days). Live surfclams were found in all 60 growout boxes, and, unlike Timber Cove in Gouldsboro, only 7 green crabs and 3 rock crabs were discovered. Crabs occurred in 9 boxes (15%). Clams grew faster at this site than they did at Timber Cove. Mean final length of "large" clams was 30.4 mm compared to 28.7 mm for "small" clams. Stocking density did not play an important role in surfclam growth. A second sampling occurred on 3 August (413 days). Live surfclams were found in 44 of 60 (73%) of boxes. Survival depended on size of box. In small boxes, ~50% of surfclams survived compared with ~30% in large boxes. Green crabs help explain these results as approximately twice as many occurred in the larger (8.8/box) vs. smaller (4.8/box) boxes. Small clams grew 25% faster than large clams over the 413-day trial. Stocking density depressed shell growth of large clams by ~13%. No similar effects were observed for small clams.
Final field trials occurred at both Timber Cove (Gouldsboro) and Mud Hole Cove (Beals) from 20-21 April 2023 to 14-15 January 2024. Cultured surfclams ranging from 6-20 mm (mean SL = 13.9 mm) were added to 2 ft2 boxes with a protective top mesh measuring 0.9 x 1.7 mm (PetScreening). The bottom of each box was comprised of a piece of tightly-woven polyester landscape fabric. Boxes were stocked with clams at the following densities: 25, 50, 100, 150, 200, and 225 individuals (n = 5). Boxes were deployed on the surface of the mudflat at both locations near the extreme low water mark in a 6 x 10 matrix with 2 m spacing between rows and columns. At Timber Cove, mean survival was relatively high (~92%). While 51 green crabs were discovered in the 60 boxes (0.85/box), their size ranged in carapace width from 4.5 mm to 12.6 mm. Given that the average size of the surfclams at the beginning of the study was 13.9 mm, it is clear that few crabs settled into the boxes, but when they did, they were unable to grow fast enough to be large enough to consume the surfclams. It is not too far from imagining that had those boxes been sampled in June or July 2024 instead of January, the crabs would have been much bigger, and fewer surfclams would have survived. Results from the January 2024 sampling at Mud Hole Cove were disappointing as the combined effects of two back-to-back winter storms that occurred just three days prior to sampling left ~0.25-0.5 inches of soft mud covering all 60 boxes. The cumulative effects of the storms and their concomitant surge was to create pockets of anoxia within the boxes that had a catastrophic effect on surfclam survival. Overall survival was 12%. Most animals in boxes seeded with surfclams at densities exceeding 25 were dead. The majority of dead surfclams had intact (undamaged), blackened valves with some remaining (decomposing) tissue suggesting a recent mortality event, and that there was little to no biological explanation as valve margins were smooth rather than serrated – a common sight when green crabs or rock crabs are present and abundant. Green crabs were not abundant, as only eight animals were found that occurred in only five of the 60 boxes (8.3%) at a mean density of 0.13 individuals/box.
Next steps: Future attempts to grow this species will likely use floating gear that is buoyed up so that whatever is holding the surfclam juveniles has a layer of stable sediment that the individual clams can bury into. Sediment within this structure would provide stability for the animals that, if left to move around due to tide- and wind-driven waves, would not grow lengthwise, but would grow thick shells that prevent normal growth morphologies. In addition, the top should be accessible in a matter of seconds, as having that feature would allow anyone to inspect the contents of the gear and remove any crabs, worms, or other would-be predators. These floating structures (3-ft x 4-ft x 4-inches deep; 2-ft x 3-ft x 5-inches deep) would need to be located in a protected cove or embayment, access could occur at any time it is safe to visit the site, presumably by boat. Regular and routine inspection of field gear, which is not possible with intertidal gear given that lower intertidal sites are only accessible for 2-3 tides in each month.
The sediment would provide stability for the animals that, if left to move around due to tide- and wind-driven waves, would not grow lengthwise, but would grow thick shells that prevent normal growth morphologies. In addition, the top should be accessible in a matter of seconds that would allow anyone to inspect the contents of the gear and remove any crabs, worms, or other would-be predators, and secure when returned to the tray. Finally, since these structures would be located in a protected cove or embayment, access could occur at any time it is safe to visit the site, presumably by boat. Regular and routine inspection of the field gear, which is not possible in the lower intertidal since those sites are exposed only 2-3 tides during each month, should result in high survival rates. Because increasing densities often results in some growth penalty, field trials should be conducted to determine the most efficacious stocking density to maximize growth to commercial size. That is, the floating gear could be a permanent structure for a cultured Arctic surfclam that enters the structure at a SL of 3-5 mm and exits 2.5-3 years later when it reaches 40-45 mm.
References
Beal, B. F., C. R. Coffin, S. F. Randall, C. A. Goodenow, Jr., K. E. Pepperman, B. W. Ellis. 2020. Interactive effects of shell hash and predator exclusion on 0-year class recruits of two infaunal intertidal bivalve species in Maine, USA. Journal of Experimental Marine Biology and Ecology 530-531:151441.
Pershing, A. J., M. A. Alexander, C. M. Hernandez, L. A. Kerr, A. Le Bris, K. E. Mills, J. A. Nye, N. R. Record, H. A. Scannell, J. D. Scott, G. D. Sherwood, and A. C. Thomas. 2015. Slow adaptation in the face of rapid warming leads to collapse of the Gulf of Maine cod fishery. Science 350:809–812.
Young, A. M., and J. A. Elliott. 2020. Life history and population dynamics of green crabs (Carcinus maenas). Fishes 5:4.
Cooperators
- (Educator)
Research
Hypotheses from the approved proposal
The research questions are as follows:
At each of the two study sites (Town of Beals, ME; Town of Gouldsboro, ME), we will evaluate main and interactive effects listed below with respect to growth and survival of cultured Arctic surfclam seed.
1) What role does intraspecific Stocking Density (30, 60, 90 ind. ft-2) play in surfclam growth and survival?
2) What predator deterrent top Frame Thickness (1- vs. 2-inch) will yield the maximum survival and/or growth?
3) Does Size of Growout Unit (2-ft2 vs. 4-ft2) affect surfclam growth and/or survival?
Methods - First Year
Arctic surfclam, Mactromeris polynyma, juveniles were produced in the shellfish hatchery approximately one-year prior to receiving the SARE grant. During the period between December 2020 and February 2021, an accident at the Downeast Institute facility was responsible for the loss of hundreds of thousands of research individuals. This loss caused us to alter the scope of the proposal. That is, instead of a single size class of Arctic surfclams being available for the project as intended (i.e., animals ranging in size from ~10-15 mm in shell length, SL), two sizes of animals were used instead to keep stocking densities at the proposed levels. The mean SL ± 95% confidence interval of the Small clams was 13.16 ± 0.74 mm (n = 40; minimum = 9.41 mm, maximum = 19.86 mm). Large clams averaged 26.36 ± 1.55mm (n = 20; minimum = 19.88 mm, maximum = 32.43 mm).
A comparative experiment was initiated near the extreme low water mark at two intertidal locations (Figs. 1-3) in eastern Maine (the mouth of Grand Marsh Bay, Gouldsboro: 44o27’13.20”N; 68o00’48.75”W – 27-28 March 2021; Mud Hole Cove, Great Wass Island, Beals: 44o29’08.61”N; 67o35’18.31”W – 1 April 2021). Clams were added to wooden boxes of two sizes (1-ft x 2-ft, or 0.186 m2; 2-ft x 2-ft, or 0.372 m2; Fig. 4). Boxes were placed on top of the muddy sediments and anchored in place using wooden stakes (spruce strapping) that were forced into the sediments ~ 25-inches at the short ends of the small boxes and the middle of each side of the large boxes so that the top of each stake was flush with the top of each box. Two galvanized screws were driven through the top of each stake and into the end or side of each box. No boxes were lost during the course of the field trial. A 2-inch layer of play sand was added to each box prior to stocking with cultured Arctic surfclam juveniles (ratio: 70% Small, 30% Large; Fig. 5). The bottom of each box was lined with a heavy-duty mesh – PetScreen®, a vinyl-coated polyester with a 0.9 x 1.7 mm aperture – that kept the play sand within the box. Each box was covered with a top that was a wooden frame either 1-inch thick or 2-inches thick. Bottom and top of each frame was identical regardless of frame thickness. Two pieces of flexible, oriented polypropylene netting (4.2 mm aperture) were stapled to the bottom of each frame (Fig. 6a) while a piece of more rugged, extruded, polyethylene netting (6.4 mm aperture) was stapled to the top of each frame (Fig. 6b). To ensure additional protection from bird and large crustacean predators (e.g., lobsters, rock crabs and green crabs) a piece of 12-guage ½-inch x ½-inch vinyl-coated trap wire was stapled over the top of the extruded netting (Fig. 6c; Fig. 7). The predator deterrent frames were secured to the top of the box using six 2-inch stainless steel screws (for the 2ft2 boxes) and eight 3-inch screws (for the 4ft2 boxes).
The experimental design resulted in four variables: 1) sites (a = 2; Beals, Gouldsboro); 2) box size (b = 2; 1-ft x 2-ft, 2-ft x 2-ft); 3) frame thickness (c = 2; 1-inch, 2-inches); and 4) stocking density (d = 3; 25, 50, 100 individuals/ft2, or ~ 270, 540, 1,080 individuals/m2). Five replicates of each treatment (b x c x d) were arrayed in a matrix at each site, and percent survival and growth rate of both sizes of clams were recorded. For each site, Analysis of Variance (ANOVA) was conducted using the following linear model for each metric (dependent variable):
Yijkl = μ + Ai + Bj + Ck + ABij + ACik + BCjk + ABCijk + el(ijk). Where:
Yijkl = dependent variable (percent survival; absolute growth for each size class of clams);
Ai = Box size (a = 2; factor is fixed);
Bj = Frame thickness (b = 2; factor is fixed);
Ck = Stocking density (c = 3; factor is fixed); and,
el = Experimental error (n = 5; replicates are random).
Percent survival data was arcsine-transformed to meet normality and variance homogeneity assumptions of ANOVA. Back-transformed means and their 95% confidence intervals are presented. Statistical significance was defined as a P-value ≤ 0.05.
Absolute growth was estimated by measuring with digital calipers to the nearest 0.01 mm the longest linear distance on each live animal (shell length – SL – longest linear [anterior-to-posterior] distance) and subtracting the initial length (as determined by a disturbance line that is laid down in the shell at the time when animals are transferred from the hatchery to the field (Fig. 8). This same “hatchery mark” occurs in cultured soft-shell clam seed (Beal et al., 1999) and hard clam seed (Beal et al., 2009).
Although initially each box contained only play sand and live surfclam juveniles, at the end of the experiment at both sites, other organisms occurred in the boxes. These included the invasive green crab, Carcinus maenas, rock crabs, Cancer irroratus, soft-shell clams, Mya arenaria, blue mussels, Mytilus edulis, and polychaete worms, Alitta virens. All crabs and bivalves in a particular experimental unit were enumerated, and the carapace width (CW) of crabs and the SL of bivalves measured to the nearest 0.01 mm using digital calipers. Mean number of green crabs and mean CW were analyzed using ANOVA according to the linear model referenced previously.
In addition, at each site, a HOBO data logger was deployed at the beginning of the experiment that recorded air (when exposed on spring tides) and seawater temperature every 30 minutes for the duration of the trial (Gouldsboro: 250 days; Beals: 246 days).
Second Year - Methods
Cultured juveniles of Arctic surfclams, Mactromeris polynyma, were added to field units at an intertidal study site in the town of Beals (Mud Hole Cove; 44o29’08.32”N; 67o35’15.93”W; Fig. 9) on 16-17 June 2022, and in the town of Gouldsboro (Timber Cove; 44o27’48.99”N; 68o00’01.26”W; Fig. 10) on 18-19 June 2022. These dates coincided with the monthly spring tides, and units were placed below mean low tide at both sites (i.e., during tides when the height of the low tide varied between -0.9 ft and -1.3 ft). Sediments at both sites consisted of poorly sorted, very fine sand and coarse silt (3-5 ϕ units; 37-125 μm).
Surfclams, produced at the Downeast Institute (see: https://downeastinstitute.org/), of two overlapping sizes (shell length, SL, maximum anterior-posterior distance) were used in the field trials (SMALL ± 95% CI = 9.6 ± 0.4 mm, minimum = 5.3 mm, maximum = 13.3 mm, n = 106; (LARGE = 12. ± 0.4 mm, minimum = 7.1, maximum = 18.0 mm, n = 86; Fig. 11). The cultured bivalve juveniles were added to wooden boxes of two different sizes (1-ft x 2-ft x 6-inches deep; 2-ft x 2-ft x 6-inches deep; Fig. 12) that were placed on top of the mudflat surface in a 6 x 10 array with 2 m spacing between rows and columns) at both study sites. The bottom of each box were lined with PetScreen® (vinyl-coated polyester screen wire; 0.9 mm x 1.7 mm; https://www.phifer.com/product/petscreen/) to keep out digging predators such as invasive green crabs, Carcinus maenas, endemic rock crabs, Cancer irroratus, as well as milky ribbon worms, Cerebratulus lacteus, and burrowing, predatory fish, Cryptacanthodes maculatus. Box tops were a 1-inch thick wooden frame. A piece of biaxially-oriented, polypropylene flexible netting (4.2 mm) was stapled to the bottom (i.e., the side closest to the surfclams). The top of each frame was covered with two pieces of protective netting. One was a layer of extruded, low-density polyethylene netting (6.4 mm) which was stapled to the frame. The other was a piece of half-inch vinyl-coated, 14-guage lobster trap wire meant to deter crabs, birds, and fish that was secured to the frame with stainless steel staples (Fig. 13). Stainless steel screws were used to secure each box top to its bottom (Fig. 12). Boxes were held in place on the mudflat surface by pushing 24-inch pieces of spruce strapping (wooden anchors) into the sediments and screwing the top of each piece of strapping into the side of the box. Two anchors were used for the smaller boxes (one at each of the short ends), and four were used for the larger boxes (Fig. 14). A HOBO temperature recorder was deployed on the mudflat surface at both sites that recorded temperature every 30 minutes.
In the field, a half-inch layer of play sand was added to each box (approximately 3L and 6L to each 1-ft x 2-ft box and 2-ft x 2-ft box, respectively) to provide a substrate for surfclams within which to burrow (Fig. 15). An equal number of small and large boxes (N = 30) were deployed at each site. The experimental design was a factorial one, with surfclam density nested within both size of box and size of surfclam (Table 1 & 2).
Boxes at both sites were sampled during the spring tides, approximately 7 months after the experiment was initiated (20 January 2023 – Mud Hole Cove, 218 days; 21 January 2023 – Timber Cove, 216 days). At each site, box tops were brushed to remove macroalgal growth, and then unscrewed from the bottoms. All crabs (C. maenas and C. irroratus) from each box were removed and placed into labeled bags along with two haphazardly chosen live surfclams. Crabs from each box were identified to species, counted, and measured (carapace width [Fig. 10], to the nearest 0.1 mm using Vernier calipers). The initial and final SL of each live surfclam (Fig. 17) was measured with the same calipers and to the same precision as discussed above. Box tops were then re-positioned and screwed in place. Final sampling to estimate survival and growth is planned for the one-year anniversary of the trial (i.e., June 2023).
Surfclam growth was estimated using three measures – relative growth (Equation 1), absolute growth (Equation 2), and final length.
Eq (1): Relative growth = [ln(Final length) – ln(Initial length)]/time in days (= 217);
Eq (2): Absolute growth = Final length – Initial length.
Tables 1 & 2 Second Annual Report
Third Year Methods (Experiment I)
Methods (20-21 April 2023 to 14-15 January 2024)
Field and Laboratory
To examine short-term effects of intraspecific stocking density on growth and survival of cultured Arctic surfclam, Mactromeris polynyma, juveniles, a comparative field trial was initiated on 20 and 21 April 2023 at Timber Cove, Gouldsboro, Maine (44o27’54.24”; 68o00’06.31”W) and Mud Hole Cove, Beals, Maine (44o29’09.67”N; 676o35’21.49”W), respectively. Relatively large cultured juveniles (Shell Length, SL, ± 95% CI = 13.9 ± 0.19 mm, n = 598; Fig. 18) were used because smaller individuals were not available from the Downeast Institute Shellfish Hatchery at that time.
Surfclams were added to nursery growout boxes constructed of spruce strapping, each measured 1-ft x 2-ft x 3-inches deep in outer dimensions. The surface area within each box was ~ 1.615 ft2. Each box had the same mesh top that was a 1-ft x 2-ft piece of PetScreen® (see: https://www.phifer.com/screening/features/pet-protection/). This material is constructed from a densely-woven vinyl-coated polyester, and has an aperture size of 0.9 x 1.7 mm. It is resistant to rips and tears caused by large crustaceans such as lobsters and large green crabs. The PetScreen® was stapled to the periphery of the wooden box top, and then wooden laths were placed on top of the screen around the periphery of the box and hammered in place with 1.5-inch galvanized nails (Fig. 19). The bottom of each box was comprised of a piece of tightly-woven polyester landscape fabric (see: https://farmplasticsupply.com/weed-control/5oz-landscape-fabric). No sediment was added to any of the boxes.
Surfclams were added to boxes the day before they were deployed in the field. Boxes were held overnight in a flow-through tank where they floated on the seawater surface. Boxes were stocked with clams at the following densities/box: 25, 50, 100, 150, 200, and 225 individuals. Ten replicates of each of the six stocking densities were used. Boxes were deployed on the surface of the mudflat near the extreme low intertidal mark in a 6 x 10 matrix with 2 m spaces between each row and column. A HOBO temperature recorder was left at each site on the same day boxes were deployed. Recordings occurred every 60 minutes for each of the 269 days boxes remained in the field at Timber Cove (14 January 2024) and Mud Hole Cove (15 January 2014). For each high tide during the trial, three temperature recordings (1 hour before; during; 1 hour after) were averaged to yield an “Average Temperature at High Tide.” High tide temperatures were plotted against date to yield a site-specific temperature profile through time.
Boxes, weighing 15-20 pounds each, were removed from the flat by placing three at a time into a plastic fish tote. The twenty totes were dragged approximately 1,200 feet to the shore (Fig. 20) and then transported to the sample processing laboratory at the Downeast Institute (Fig. 21). The contents of each box were removed and washed through a sieve (3.2 mm aperture) with ambient seawater. All live and dead surfclams, live recruits of soft-shell clams, Mya arenaria, and live green crabs, Carcinus maenas, were removed and placed into a plastic bag with a label indicating the name of the study site, the stocking density treatment and matrix position. Live surfclams, clams, and crabs were enumerated and measured within 24 hours after processing. A maximum of ten live surfclams was taken as a representative sample from each box containing a minimum of ten live surfclams. All clams were measured in boxes containing ten or fewer live surfclams. The initial and final shell length was measured for each surfclam (Fig. 22).
Analyses
Analysis of variance (ANOVA) was used to test the effect of stocking density on mean percent survival. Survival data was not normally distributed, and, while an arcsine-transform improved normality in some treatment groups, it did not result in normality across all groups. Both analyses resulted in the same conclusion that supported results from a Kruskal-Wallis test, the nonparametric analog of the one-way ANOVA. A series of a priori orthogonal contrasts were developed to examine potential relationships between stocking density and mean percent survival. These included: 25 vs. 50-225; 2) 50 vs. 100-225; 3) 100 vs. 150-225; 4) 150 vs. 200-225; and, 5) 200 vs. 225. In addition, linear regression was performed to examine the relationship between survival/box vs. stocking density. This was followed by a lack-of-fit test to determine if the relationship was curvilinear. In addition, number of live surfclams/box was regressed on number of live soft-shell clams/box to determine if a linear relationship existed.
Three separate estimates of growth were collected: 1) Absolute growth: Final SL-Initial SL; 2) Relative growth: (Final SL-Initial SL)/Initial SL; and 3) Final SL. Each estimate, however, was linearly related to initial SL (Fig. 23); therefore, it was not possible to use any of the three variables alone to examine effects of stocking density on growth. Instead, analysis of covariance (ANCOVA) was conducted on the relative growth vs. initial shell length data (Fig. 24). Prior to ANCOVA, it was determined that individuals less than or equal to 10 mm in initial shell length should be removed from the analyses as their inclusion resulted in curvilinear data that complicates data analysis (N = 34 for the Timber Cove data set; all live surfclams at Mud Hole Cove were > 10 mm SL). Therefore, all analyses involving ANCOVA were conducted using a subset (~ 94%) of the data for the Gouldsboro location, whereas all data was used for the growth analyses from the Beals location.
Unless otherwise stated, all means are given with their 95% confidence interval (CI).
Third Year Methods (Experiment II)
Methods (16-19 June 2022 to 20-21 January 2023 to 3-4 August 2023)
This study comprised the bulk of the Second Annual Progress Report (16-19 June 2022 to 20-21 January 2023). The field trial was set out as a comparative experiment at Timber Cove (Gouldsboro, Maine) and Mud Hole Cove (Beals, Maine) to examine effects of box size (2 ft2 vs. 4 ft2), surfclam size (Small: 9.6 ± 0.4 mm vs. Large: 12.4 ± 0.4 mm) and intraspecific surfclam density (7.5-90/ft2) on surfclam growth and survival. Progress Report #2 examined the growth of a subset of animals taken from the boxes in January 2023, approximately 6 months after the trial was initiated. Here, the remaining information (overall survival and subsequent growth) taken from the growout boxes at both locations is given for sampling dates in early August 2023, nearly 14 months after the trial began. Surfclams inside both sizes of boxes were protected from predators with a 1-inch thick wooden frame top. The top of each frame was covered with a piece of black, UV-stabilized extruded polyethylene mesh (6.4 mm aperture; https://www.industrialnetting.com/xv1170-black-polyethylene-mesh.html), similar to VEXAR, and on top of the plastic mesh was a similar size piece of vinyl-coated lobster trap wire (12.7 mm aperture). This combination of two types of mesh was designed to reduce entry into the boxes by large, mobile crustaceans such as rock and Jonah crabs (Cancer borealis), as well as American lobsters (Homarus americanus). A piece of UV-stabilized, flexible (oriented), polypropylene mesh (4.2 mm aperture; https://www.industrialnetting.com/ov7100-168-polypropylene-netting.html) was stapled to the bottom of each frame. This arrangement would enable small crabs < 6 mm to enter boxes. Each box was 6-inches deep, the bottom was covered with PetScreen®, and approximately two and four liters of play sand was distributed evenly over the bottom of the small (2 ft2) and large (4 ft2) boxes, respectively.
Daily seawater temperatures were recorded at both locations during the field trial (Fig. 25) showing the peak in Gouldsboro (the westernmost site) during summer was approximately 3.5oC warmer than temperatures in Beals. Wintertime minimum lows were approximately 1.6oC colder in Beals vs. Gouldsboro, but for most of the winter, seawater temperatures at the Gouldsboro location were colder than those at Beals.
Results
First Year
Gouldsboro (27-28 March to 1-2 December 2021)
Large surfclams (SL = 26.4 ± 1.5 mm)
Mean percent survival varied significantly (P < 0.0001) with size of the experimental unit (Table 3; Fig. 26a). Mean survival was nearly 3x greater in the large (4ft2) vs. small (2 ft2) boxes (38.4 ± 5.1% vs. 13.4 ± 4.7%, n = 30). The relative percent of total variation explained by this one source was nearly 50% (Table 3). While stocking density was not statistically significant (P = 0.052, Table 3), there was an indication that increasing density to levels greater than 25 individuals per square feet resulted in lower mean percent survival (Fig. 27). That is, the ~30% difference in mean survival observed between the lowest and two highest stocking densities was not significantly different from zero.
Live surfclams were recovered in 52 of the 60 boxes. Mean absolute growth was 3.16 ± 0.39 mm (n = 52; Fig. 28). No source of variation associated with mean absolute growth was statistically significant (P > 0.25; Table 4).
Small surfclams (SL = 13.2 ± 0.74 mm)
Mean percent survival varied significantly (P < 0.001) with size of experimental unit (Table 5; Fig. 26b). Mean survival followed a pattern for small surfclams that was similar to that observed for large surfclams. That is, ~2.4x more clams were recovered in large vs. small boxes (47.1 ± 7.8% vs. 19.7 ± 7.1%, n = 30). Size of experimental unit explained ~33% of the total variability in small surfclam survival.
Live surfclams were recovered in 53 of the 60 boxes. Mean absolute growth was 7.85 ± 0.44 mm (n = 53; Fig. 29); however, stocking density played a significant role (P = 0.0254; Table 6) as surfclams exhibited a growth depression of ~13% at the highest density compared to the two lower densities (Fig. 30).
Invasive green crabs
Green crabs were recovered in all but one experimental unit (N = 686; minimum CW = 3.53, maximum CW = 57.27 mm), and averaged 11.45 ± 2.50 individuals per unit (n = 60). Most crabs (~75%) recovered had CWs ≤ 12 mm, and 90% were ≤ 20 mm (Fig. 31). Mean number of green crabs varied directly with size of experimental unit, but not proportionately. That is, mean number of green crabs in the small boxes was 5.3 ± 1.5 (n = 30), but in large boxes, mean number was more than 3x greater (17.6 ± 3.7, n = 30; P < 0.0001, Table 7), even though the surface area of the large boxes was only twice that of the smaller boxes. Crabs neither responded significantly to initial stocking density (P = 0.6924) nor frame thickness (P = 0.3568; Table 5).
It seems counterintuitive that survival of both sizes of Arctic surfclams increased with size of box along with green crabs; however, in some instances, most or crabs were smaller than the initial SL of surfclam. Therefore, a test was conducted to determine if maximum size of green crab in an experimental unit was associated with percent survival of both sizes of Arctic surfclams. No significant association was observed for the small size group, but a positive association was observed for the large size group (Fig. 32).
Other observations
The spring and summer weather in coastal Maine was wet, windy, and relatively cool, but especially wet. The study site was located in the lower intertidal below mean low water so that experimental units were only exposed to the air during spring tides, and only for an hour at most on most spring tides. The amount of rainfall and stormy weather we experienced from May through August exceeded most years and was greater than the long-term average. For example, during July 2021, precipitation in the region where the Gouldsboro study site was located ranged from 150 to 250% above normal. The precipitation and stormy weather likely resulted in a lot of sediment from the land and the adjacent Great Marsh Bay that ended up in the water column, and this was evident from the amount of sediment that built up inside many of the experimental units (surfclam growout boxes). The site was visited on 10 September 2021 and the tops of ~20 boxes were temporarily removed to inspect the contents of each (Fig. 33). When initiated in late March 2021, each box had approximately 2-inches of play sand that acted as the substrate into which the juvenile surfclams could burrow. Upon the site visit, most of the boxes inspected were full of mud (Fig. 33a), and many had black, anoxic spots on the sediment surface where clams had died (Fig. 33b). Further inspection of the protective tops indicated that so much mud from the water column had entered the boxes that the gap between the top and bottom of the wooden frame comprising the predator-deterrent top had also filled with muddy sediments (Fig. 33c), and that prevented a normal exchange of seawater from the overlying water column. That is, the excessive build-up of muddy sediments in the boxes and protective tops likely resulted in an environment that starved the growing surfclams of adequate oxygen that many simply died due to anoxia. This was the first time this phenomenon was observed in the time the PI has been experimenting with cultured Arctic surfclam juveniles (i.e., 2011).
Beals (1 April to 3 December 2021)
Large surfclams (SL = 26.4 ± 1.5 mm)
As with the study site in Gouldsboro, mean percent survival varied significantly (P < 0.0027) with size of the experimental unit (Table 8; Fig. 34). Mean survival was ~ 2x greater in the large (4ft2) vs. small (2 ft2) boxes (56.9 ± 14.8% vs. 28.4 ± 13.8%, n = 30). The relative percent of total variation explained by this one source was 12.6%, which was more than double any other source of variation (Table 8). The only other factor that warranted attention was frame thickness where survival in boxes with the thinner tops (1-inch) was nearly 40% greater than in boxes with the thicker tops (2-inches) (49.8 ± 15.8% vs. 35.5 ± 14.3, n = 30; P = 0.072, Table 8).
Live surfclams were recovered in 39 of the 60 boxes. Mean absolute growth was 4.75 ± 1.00 mm (n = 39; Fig. 35). No source of variation associated with mean absolute growth was statistically significant (P > 0.25; Table 9); however, mean absolute growth was ~68% greater in the larger vs. smaller experimental units (5.62 ± 1.5 mm, n = 24 vs. 3.36 ± 0.97 mm, n = 15), and ~50% greater in boxes with 2-inch protective frames than those with 1-inch frames (5.58 ± 1.5 mm, n = 22 vs. 3.68 ± 1.25 mm, n = 17).
Small surfclams (SL = 13.2 ± 0.74 mm)
Mean percent survival varied significantly (P < 0.041) with the main and interactive effects of size of experimental unit and stocking density (Table 10; Fig. 36). Mean percent survival in the larger boxes was ~85% greater than in smaller boxes (26.9 ± 9.5% vs. 14.6 ± 7.4%, n = 30); however, effects of stocking density varied differently across experimental unit size. For example, mean percent survival in the smaller boxes stocked with surfclams at the two lowest two densities was 21.2 ± 9.9% (n = 20), but declined to 1.4 ± 2.1% (n = 10) in boxes stocked at 100 surfclams/ft2, a drop of nearly 95% (Fig. 36). In the larger boxes, mean percent survival was nearly 100% greater in the lowest (40.1 ± 20.4%, n = 10) vs. the two highest stocking densities (20.4 ± 10.0%, n = 20).
Live surfclams were recovered in 38 of the 60 boxes. Mean absolute growth was 7.41 ± 1.18 mm (n = 38; Fig. 37). Surfclams experienced an approximate 75% decrease in mean absolute growth with increasing stocking density (8.9 ± 1.7 mm, n = 16 @ 25 individuals/ft2 vs. 6.3 ± 1.6 mm, n = 22, P = 0.0094, Table 9); however, a significant 3-way interaction (P = 0.0002, Table 11) suggested that the factors combined in a more complex way that affected growth (Fig.21). For example, in both small and large experimental units, no significant difference was observed in mean absolute growth when contrasting low (25/ft2) vs. high (50 and 100/ft2) density (SM: P = 0.7669; LG: P = 0.0807). The contrast between the two highest densities (50 vs. 100 individuals/ft2) for both sizes of experimental units yielded statistically significant results. For small units with 1-inch protective tops, mean absolute growth increased from 2.7 ± 5.3 mm (n = 3) to 8.6 mm (n = 1), but for units with the 2-inch tops growth decreased from 7.5 ± 1.0 mm (n = 5) to 5.6 mm (n = 1). For larger units, a dissimilar pattern in absolute growth was observed for the same contrasts across the two different tops. For surfclams in boxes with 1-inch tops stocked at 50 individuals/ft2, mean absolute growth was the highest at 12.3 ± 12.4 mm (n = 2) vs. 4.5 ± 3.5 mm (n = 3) for surfclams stocked at 100 individuals/ft2. The opposite trend was observed in boxes protected with the 2-inch thick tops, where mean absolute growth increased from 2.5 ± 3.7 mm (n = 3) to 8.5 ± 5.4 mm (n = 4) (Fig. 38).
Invasive green crabs
Green crabs were recovered in 19 of 60 (~33%) experimental units (N = 47; minimum CW = 2.76, maximum CW = 47.25 mm), and averaged 0.78 ± 0.56 individuals per unit (n = 60). Fifty percent of crabs recovered had CWs ≤ 7 mm, and 75% were ≤ 30 mm (Fig. 39). Mean number of green crabs was significantly greater in the large vs. small boxes (1.4 ± 1.1 vs. 0.2 ± 0.2, n = 30; P = 0.0416, Table 10), even though the surface area of the large boxes was only twice that of the smaller boxes. Crabs neither responded significantly to initial surfclam stocking density (P = 0.8047) nor frame thickness (P = 0.0699; Table 12).
As with data from the Gouldsboro site, a test was conducted to determine if maximum size of green crab in an experimental unit was associated with percent survival of both sizes of Arctic surfclams. No significant association was observed for the small size group, but a positive association was observed for the large size group (Fig. 40).
Other observations
Many (> 75%) of the predator-deterrent tops had become fouled with various species of macroalgae as noted at the time of sampling in December 2021. The assemblage of species included Desmerestia aculeata, Saccharina longicruris, and Pylaiella littoralis. The growth of macroalgae on the tops of some boxes likely prevented an adequate exchange of seawater into and out of the boxes, and several boxes (independent of stocking density) contained 100% dead clams that had grown a few millimeters before they had all perished. In those cases, few of the dead clams were crushed, typical of damage due to green crabs, as most had undamaged valves and were black, which occurs when animals have been exposed to anoxic conditions. Future trials will require regular checks on growout boxes during spring tides to inspect and, when necessary, brush clean the tops and inspect the contents of boxes for green crabs.
References
Beal, B. F., R. C. Bayer, M. G. Kraus & S. R. Chapman. 1999. A unique shell marker of juvenile, hatchery-reared individuals of the softshell clam, Mya arenaria L. Fish. Bull. (Wash. D. C.) 97:380–386.
Beal, B.F., G.C. Protopopescu, K. Yeatts & J. Porada. 2009. Experimental trials on the nursery culture, overwintering, and field grow-out of hatchery-reared northern quahogs (hard clams), Mercenaria mercenaria (L.) in eastern Maine. J. Shellfish Res. 28:763-776.
Second Year
All sixty boxes at both sites were present and undamaged (Figs. 41-42).
Timber Cove – Gouldsboro (18-19 June 2022 to 21 January 2023)
Live surfclams were discovered in 27 of the 60 (45%) boxes (n = 13 in Large boxes [2-ft x 2-ft]; n = 14 in Small boxes [1-ft x 2-ft]). Crabs (rock – Cancer irroratus; green – Carcinus maenas) were found in 41 (68.3%) boxes, and a majority (in many cases 100%) of surfclams in those boxes were dead, having been crushed or chipped by foraging crabs (Fig. 43). Green crabs ranged in carapace width (CW) from 5.2-43.2 mm, with an overall mean CW of 18.6 ± 0.6 mm (n = 418). Size-frequency distribution of C. maenas varied significantly (G = 14.9, df = 5, P = 0.011) with the size of the box (Fig. 44) as crabs in the smaller (2-ft2) boxes generally were larger than those in the larger (4-ft2) ones. For example, ~30% of crabs in smaller boxes were less than 15 mm CW vs. ~40% of crabs that colonized the larger boxes. Similarly, ~45% of crabs in smaller boxes were larger than 20 mm CW vs. 37% for those in larger boxes. Mean green crab CW did not differ significantly by any main or interactive factor (Table 13). In addition, 31 rock crabs (CW range = 29.4 mm to 67.4 mm; mean CW = 52.2 ± 3.9 mm) were found in boxes (nLarge = 18, nSmall = 13, P = 0.369 [binomial test]). Rock crabs occurred either singly or in pairs in 18 boxes, and together with green crabs in 7 boxes. This result was statistically significant (binomial test, P = 0.028). Rock crabs were, on average, 37% larger in boxes where they occurred singly or in pairs ( = 58.4 ± 3.3 mm, n = 19) than when they occurred together with green crabs ( = 42.6 ± 4.9 mm, n = 12). These two means were significantly different in a two-sample t-test (Tobs = 5.99, P < 0.0001, df = 29).
Analysis of variance (Tables 14-16) demonstrated the lack of a significant effect of size of the growout box and stocking density on surfclam growth for each of the three measured variables. A priori contrasts looking at specific effects of density on growth were detected in only one of the analyses – final length of large clams in small boxes (P = 0.0249, Table 15, Fig. 45). Surfclams stocked at 60/box (30/ft2) were significantly smaller (by ~25%) than those at the two other densities (100 & 160/box representing 50 & 80 surfclams/ft2).
Initial surfclam size was a statistically significant source of variation in each of the three analyses (Tables 14-16; Figs.46-48). Mean final length of initially “large” clams (22.4 ± 0.7 mm, n = 28) was only 1.6 ± 1.0 mm greater than that of “small” clams (20.8 ± 0.8 mm, n = 27), but this was statistically significant (P = 0.0481, Table 14, Fig. 46). Relative growth rates were significantly greater in small vs. large clams with smaller clams growing ~1.6 faster than larger clams (P < 0.0001, Table 15, Fig. 47). In addition, mean absolute growth of small clams (10.6 ± 0.9 mm, n = 27) was ~35% greater than that of large clams (7.9 ± 0.8, n = 28; P = 0.0017, Table 16).
Mud Hole Cove (16-17 June 2022 to 20 January 2023)
Live surfclams were found in all 60 (100%) growout boxes. Unlike the growout boxes in Gouldsboro, only 7 green crabs, Carcinus maenas, were discovered. Five boxes contained a single green crab ( = 9.4 ± 3.9 mm, minimum = 6.7 mm, maximum = 14.9 mm), and a sixth box contained two green crabs (CW = 7.7 mm and 9.3 mm). A total of three rock crabs, Cancer irroratus, were discovered – each separately (CW = 11.0 mm, 45.7 mm, and 46.5 mm).
Regardless of initial size, surfclams generally grew to a larger final shell length at Mud Hole Cove compared with those at Timber Cove (see Fig. 49 vs. Fig. 46). Mean surfclam growth for each of the measurement variables (final shell length, relative growth, and absolute growth) varied significantly with initial clam size (Tables 17-19; Figs. 50-52). Mean final shell length for large surfclams was ~6% greater than for small surfclams ( = 30.4 ± 1.1 mm, n = 60; = 28.7 ± 1.2 mm, n = 60). Small clams grew ~1.4x faster than large clams (Table 18, Fig. 51), and added significantly more new shell than large clams (mean absolute growth for small surfclams = 18.5 ± 1.0 mm, n = 60 vs. 14.3 ± 0.8 mm, n = 60 for large surfclams; Table 19, Fig. 52). Size of growout box had a significant effect on both mean final shell length and mean absolute growth (Tables 17 and 19). Mean final shell length of surfclam juveniles was ~7% greater in large vs. small boxes (30.6 ± 1.1 mm, n = 60 vs. 28.5 ± 1.2 mm, n = 60, respectively), while mean absolute growth (amount of new shell deposited during the 218-day trial) was ~11% greater in large vs. small boxes (18.2 ± 0.9 mm, n = 60 vs. 16.4 ± 1.1 mm, n = 60, respectively). Stocking density did not play a significant role in surfclam growth (Tables 17-19).
Seawater temperatures (18 June 2022 to 21 January 2023)
Seawater temperatures (Figs. 53-55) demonstrate differences between sites with the western location (Timber Cove, Gouldsboro, Maine) having higher readings than the eastern location (Mud Hole Cove, Great Wass Island, Beals, Maine) beginning as soon as boxes and clams were deployed in June (e.g., on 21 June 2022, temperature at high tide at Timber Cove was 12.0oC vs. 10.4oC at Mud Hole Cove). The maximum temperature recorded at Timber Cove (18.8oC) occurred on 8 August 2022. At Mud Hole Cove, maximum temperature recorded was 15.7oC on both 6 and 23 August. From approximately 25 September 2022 to the end of the trial, high tide seawater temperatures were similar at both locations (Fig. 25).
Third Year
Third Year Results & Summary - (Experiment I)
Gouldsboro (Timber Cove – 20 April 2023-14 January 2024)
Mean survival was relatively high (91.9 ± 1.9%, n = 60) compared with previous field trials at this and another location in the town (see First and Second Year Results). No significant effect of stocking density was detected, and none of the five orthogonal contrasts were statistically significant (Table 20; Fig. 56). In addition, there was no linear or curvilinear relationship between percent surfclam survival per box vs. stocking density (Table 21; Fig. 57). Several reasons may explain the seemingly astronomical survival rates. These include the combination of initially large seed (mean SL = 13.92; minimum SL = 5.99; maximum SL = 20.5), the small aperture of the top mesh needed to exclude large predators, and the lack of large green crabs in the boxes at the end of the trial. For example, 51 green crabs were observed across the 60 boxes (mean number per box: 0.85 ± 0.46 individuals). No significant effect of stocking density on mean number of crabs per box occurred (F = 1.57, df = 5, 54, P = 0.1833), and no orthogonal contrast was statistically significant (P > 0.075; Table 22). Crabs ranged in carapace width from 4.52 mm to 12.58 mm, with a mean of 7.77 ± 0.92 mm, n = 51; Fig. 58), and there was no significant difference in size-frequency of green crabs across stocking density treatments (4 x 5 Fisher’s exact test; P = 0.6744). A weak (r2 = 0.0796), but significant negative relationship (P = 0.0289) was discovered between number of live surfclams per box and number of live recruits (3.2-16.4 mm SL) of the soft-shell clam, Mya arenaria (Fig. 59)
Growth rates were slower than in previous field trials using similar size animals over similar stocking densities. The primary reason was due to the top mesh – the PetScreen® – that tends to restrict flow compared with other types of predator-exclusion netting with larger apertures, and yields lower growth rates in other species of cultured bivalve juveniles such as soft-shell clam (B. Beal pers. obs.; Beal et al., 2018). Collectively, surfclams increased in mean SL by ~35% from a initial mean SL of 13.92 ± 0.19 mm to a final mean SL of 19.05 ± 0.18 mm (n = 598) (Fig. 20b). ANCOVA (relative growth vs. initial SL) was used to generate least-square means for each density treatment (means from treatment were adjusted for a common initial mean SL of 13.92) that showed a significant difference for all comparisons (Fig. 60) except for the two highest densities where the difference in relative growth was not significantly different from zero.
The HOBO temperature recorder deployed near the growout boxes at Timber Cove on 20 April 2023, was lost. Storms occurring on 10 and 12 January may have caused the loss, as the site experienced sustained winds in excess of 60 mph for several hours during both storms (see: https://gouldsboroshore.me/2024/01/12/the-january-10-storm/).
Beals (Mud Hole Cove – 21 April 2023-15 January 2024)
The combined effects of two winter storms that battered the downeast coast 48 hours apart on 10 and 12 January 2024 left ~0.25-0.5 inches of inch of soft sediment covering the tops of all 60 boxes. The cumulative effect of the storms and the surge was to create pockets of anoxia within the boxes that had a catastrophic effect on surfclam survival. Highest mortalities occurred in boxes with the highest stocking densities. Animals stocked at 25/box had the lowest mortality. While the Timber Cove location in Gouldsboro received high winds, the orientation of Mud Hole Cove is such (ESE-NNW) that it experienced the most direct effect of the wind during both winter storms. Unlike the boxes at Mud Hole Cove, sediment was not deposited on the Timber Cove boxes. Unfortunately, the storms resulted in >85% mortality of the cultured surfclams at Mud Hole Cove (overall survival was 12.00 ± 7.99%, n = 60). The HOBO temperature logger was recovered from Mud Hole Cove, and the high tide readings (Fig. 61) during the storm that brought the most significant damage to infrastructure along the downeast coast suggest a prolonged storm surge. For example, for the three days before the January 10th storm, seawater temperatures over five high tides averaged 2.67 ± 0.54oC compared with an average temperature 5.16 ± 0.77oC for the five high tides immediately after the storm.
Most animals in boxes seeded with surfclams at densities exceeding 25 were dead. The majority of dead surfclams had intact (undamaged), blackened valves with some remaining (decomposing) tissue suggesting a recent mortality event, and that there was little to no biological explanation as valve margins were smooth rather than serrated – a common sight when green crabs or rock crabs, Cancer irroratus, are present and abundant. Green crab were not abundant, as only eight animals were found that occurred in only five of the 60 boxes (8.3%) at a mean density of 0.13 ± 0.12 individuals/box (n = 60). Average carapace width of the eight green crabs was 8.61 ± 1.76 mm (min: 6.23 mm; max: 11.57 mm). In addition, no significant effect of stocking density on mean crab size was detected (P = 0.8009).
Analysis of variance demonstrated a significant effect due to stocking density (P = 0.0019; Table 23; Fig. 62). The sums of squares for the orthogonal contrast comparing mean survival in the lowest density group vs. the mean of all other treatments (25 vs. rest) explained nearly 85% of the variability in the stocking density source of variation (N25/box = 46.00 ± 34.74% alive, n = 10 vs. rest = 5.21 ± 5.94%, n = 50; P < 0.0001). No other contrast was statistically significant. A significant negative linear relationship best described the association between the number of living surfclams in January 2024 and stocking density (r2 = 0.2054, F = 14.99, P = 0.0003, df = 1, 58; = 35.625 - 0.1889X), and this model could not be improved upon as no significant deviations from a linear model were detected (F = 0.2584, P = 0.8872, df = 4, 54).
Because survival was density dependent, surfclam growth was essentially limited to the three lowest densities (N25/box = 50 individuals; N50/box = 20 individuals; N100/box = 10 individuals; N225/box = 1). Three growth variables were examined: relative growth; absolute growth; and, final SL (Fig. 63). All three were at least linearly related to initial SL, and since the relationship between relative growth and initial length had the highest coefficient of determination (r2) (Fig. 64a), analysis of covariance and the method of least-square means was used to assess effects of stocking density on growth rate. ANCOVA demonstrated the lack of a significant stocking density effect on the relationship between relative growth and initial size (Fig. 64b, 65).
Summary (Year III - Exp. I)
Relatively large cultured seed of the Arctic surfclam, Mactromeris polynyma, were deployed in nursery field cages (cages not intended to bring clams to market size, but to hold temporarily during a time [season and clam size] when individuals would grow quickly to a large size prior to growing it commercially in deeper cages) at two eastern Maine locations. Size of cultured seed varied in SL from 6-20 mm, but grew relatively slow during a time of year when growth rates are typically the highest (April to November). In addition to stocking density that resulted in a growth penalty for clams at Timber Cove, the mesh on the top of the cage was small enough so that it likely restricted flow that resulted in overall slow growth. The mesh aperture on the nursery field cages (0.9 mm x 1.7 mm) was much smaller compared with growout boxes from previous years (4.2 mm x 4.2 mm). The decision to use the smaller mesh did improve survival rates by limiting access to boxes by crabs and other large predators >2.1 mm in carapace width.
Survival rates at Timber Cove increased dramatically in 2023-2024 (> 90%) compared with previous field trials (< 10% see below). Nursery field boxes restricted green crab sizes and densities (< 1 crab/box on average and the CW [carapace width] of all individuals was < 15 mm CW), the net effect was to slow down growth so that the average linear increase (pooled across stocking density) was only 5.2 mm at Timber Cove and 8.6 mm at Mud Hole Cove. In other words, the mesh on the nursery field cages resulted (for surfclams at one location) relatively high survival, but growth rates could not be considered commercial. Use of PetScreen as a predator deterrent was effective, but growth rates were too slow to warrant widespread use of the material in any commercial setting.
Survival rates were lower at Mud Hole Cove in Beals than at Timber Cove in Gouldsboro, even though the opposite was true for the first two field trials. It would appear that the two back-to-back winter storms within 48 hours of each other were powerful enough and from a direction (ESE) that resulted in unusually large sediment load that covered boxes at the easternmost location (Beals) but not at the westernmost location (Gouldsboro). There, the boxes were relatively sediment-free, and little to no sediment occurred on the top of the boxes.
We have attempted to use the low intertidal to grow this species, which is largely a subtidal organism, because it is found intertidally throughout its circumboreal global distribution. Several logistical problems arise that limits access to the growout boxes, which is the fact that the boxes are only accessible during 2-3 spring tides that occur monthly. That is, the boxes are inaccessible for over 90% of each month, and this limits when boxes could be checked for green crabs and other predators. In addition to limited access due to the position along the tidal gradient where surfclams grow best, these low, nearly subtidal regions of the low intertidal have, characteristically, extremely soft mud making work in the area of the boxes difficult at best. Sometimes, small springs occur under the soft mud and when one steps in an area that has these springs, there is a danger of becoming stuck, as the environment is more like quicksand.
Future attempts to grow this species will likely use floating gear that is buoyed up so that whatever is holding the surfclam juveniles has a layer of stable sediment that can be buried into. The top of the floating gear will require an impenetrable mesh (extruded polypropylene or vinyl-coated wire) with an aperture as large as 12 mm since it will be above the surface of the water and need only exclude birds or other organisms that could sit on top of the floating gear. The sediment would provide stability for the animals that, if left to move around due to tide- and wind-driven waves, would not grow lengthwise, but would grow thick shells that prevent normal growth morphologies. In addition, the top should be accessible in a matter of seconds that would allow anyone to inspect the contents of the gear and remove any crabs, worms, or other would-be predators, and secure when returned to the tray. Finally, since these structures would be located in a protected cove or embayment, access could occur at any time it is safe to visit the site, presumably by boat. Regular and routine inspection of the field gear should result in high survival rates. Because increasing densities often results in some growth penalty, field trials should be conducted to determine the most efficacious stocking density to maximize growth to commercial size. That is, the floating gear could be a permanent structure for a cultured Arctic surfclam that enters the structure at a SL of 3-5 mm and exits 2.5-3 years later when it reaches 40-45 mm.
Third Year Results (Experiment II)
Results
Timber Cove – Gouldsboro (18-19 June 2022 to 21 January 2023 to 4 August 2023)
When boxes were opened in January 2023, after their first six months in the field, live surfclams were recovered from only 45% of the boxes (27 of 60), and crustaceans (indigenous rock crabs, Cancer irroratus; and, invasive green crabs, Carcinus maenas) were found in 68% of the boxes (41 of 60). That was a density of 6.9 ± 2.4/box for green crabs and 0.5 ± 0.2/box for rock crabs (n = 60). When crabs of either species exceeded ~ 15 mm CW, the majority of surfclams in those boxes were dead, having been chipped or crushed by the foraging crabs. Green crabs ranged in size from 5 mm to 43 mm CW, and rock crabs from 32 mm to 68 mm CW (Fig. 66).
At the end of the trial, in August 2023, only five live surfclams were found (3 in one box; 2 in another) – a mortality rate of 99.89 ± 0.18% (n = 60). An additional 104 green crabs were collected from the sixty boxes that ranged in CW from 10 mm to 50 mm CW (Fig. 67; density = 1.7 ± 0.6 crabs/box). Green crabs occurred in 42 of 60 boxes (70%). The five clams had a mean initial SL of 16.9 ± 2.5 mm, a mean final SL of 25.4 ± 2.8 mm, and a mean absolute growth of 8.4 ± 1.6 mm.
Mud Hole Cove – Beals (16-17 June to 20 January 2023 to 3 August 2023)
Boxes opened in January 2023 revealed live surfclams in all 60 growout boxes, and only seven crabs that were distributed between six boxes (0.12 ± 0.09 crabs/box, n = 60). Four green crabs varied in CW from 6.7 mm to 14.9 mm ( = 9.4 ± 3.9 mm), and the three rock crabs from 11.0 mm to 46.5 mm.
In August 2023, live surfclams occurred in 44 of 60 (73.3%) boxes. Survival rates varied significantly with size of box (Table 24). While surfclams of both sizes experienced no negative effect on survival due to stocking density, surfclam survival in small boxes (2 ft2) was ~77% greater than in large boxes (4 ft2) (52.1 ± 10.9% vs. 29.3 ± 11.7%). Green crabs were abundant, and crab density varied significantly with size of box (Table 25). Approximately twice as many green crabs occurred in the larger (8.8 ± 2.4/box, n = 30) vs. smaller (4.8 ± 1.4/box, n = 30) boxes (P = 0.005); however, crab size-frequency distribution (G = 6.25, df = 4, P = 0.183; Fig. 69) and mean CW (F = 1.54, df = 1, 45, P = 0.22) was similar between box sizes. In addition, a single instance occurred where crab density increased with surfclam density. This happened among the large boxes holding large clams (Fig. 70) where crab density increased by 1.9× when the mean of the two lowest densities (7.5/box & 30/box: 7.0 ± 3.2, n = 15) was compared with the mean of the highest density (50/box: 13.0 ± 10.09, n = 5) (P = 0.0201; Table 25; Fig. 70). In those particular boxes with large clams, no significant correlation was observed between green crab number/box and stocking density (r = 0.343, P = 0.211, n = 15).
In January 2023, six months after the experiment was initiated, mean final shell length for large surfclams was ~6% greater than for small surfclams ( = 30.4 ± 1.1 mm, n = 60; = 28.7 ± 1.2 mm, n = 60). Small clams grew ~1.4x faster than large clams, and added significantly more new shell than large clams (mean absolute growth for small surfclams = 18.5 ± 1.0 mm, n = 60 vs. 14.3 ± 0.8 mm, n = 60 for large surfclams). Size of growout box had a significant effect on both mean final shell length and mean absolute growth. Mean final shell length of surfclam juveniles was ~7% greater in large vs. small boxes (30.6 ± 1.1 mm, n = 60 vs. 28.5 ± 1.2 mm, n = 60, respectively), while mean absolute growth was ~11% greater in large vs. small boxes (18.2 ± 0.9 mm, n = 60 vs. 16.4 ± 1.1 mm, n = 60, respectively). Stocking density did not play a significant role in surfclam growth.
When boxes were revisited in August 2023, all three growth variables (final SL, relative growth, and absolute growth) were found to be significantly correlated with initial SL. While the coefficient of determination associated with the dependent variables final SL (r2 = 0.105) and absolute growth (r2 = 0.062) were statistically significant (both yielded a P-value < 0.0001), initial SL did not explain as much of the variation in the dependent variable for these two growth variables as it did for relative growth (r2 = 0.530; Fig. 71). A lack-of-fit test indicated a significant departure from linearity for the relative growth vs. initial SL data (F = 5.24, df = 4, 406, P = 0.0004), but not from a quadratic model (r2 = 0.548, F = 1.80, df = 3, 406, P = 0.1457; = 4.341 - 0.321X + 0.006). Therefore, an attempt was made to linearize the relative growth data by transforming both dependent and independent variable using a loge-transform on both dependent and independent variable. This resulted in a statistically significant model that did not depart significantly from a linear model in a lack-of-fit test (F = 2.427, df = 3, 404, P = 0.0651); hence, analysis of covariance was used to determine if box size, clam size, or stocking density could explain any of the variation in relative growth.
None of the four interaction sums of squares involving initial SL (with box size, clam size, box size x clam size, and density[box size, clam size]) were statistically significant (P >0.26). Not unexpectedly, ANCOVA (Table 26) demonstrated a significant difference in transformed relative growth between the two sizes of clams, with the smaller clams having a higher back-transformed least-square mean (+ standard error) than larger clams (1.594 ± 0.031 vs. 1.254 ± 0.032) – an approximate 25% difference. In addition, one of four stocking density tests was statistically significant. Using back-transformed least-square means for relative growth, large surfclams in small boxes grew significantly slower (by ~13%) at the highest density (160/box) compared to mean relative growth in the two lower densities (60/box & 100/box) (P = 0.0033; Table 26; Fig. 72). In addition, surfclams at the intermediate density (100/box) grew significantly slower (by ~9%) than those at the lowest density (60/box).
Summary (Year III - Exp. II)
Results from this study (June 2022 – August 2023) were disappointing given the goal was to produce commercial size clams (> 40 mm SL) beginning with animals averaging 9 mm and 12 mm SL. Initial sampling after the first six months (January 2023) showed a remarkable density of invasive green crabs and endemic rock crabs in growout boxes at the westernmost location (Timber Cove, Gouldsboro, Maine). Green crabs occurred in 58 of 60 boxes (averaging 6.9 ± 2.4/box) and rock crabs in 24 of 60 boxes (averaging 0.5 ± 0.2/box). Crabs entered boxes through the top mesh (4.2 mm) – both as larvae and early juveniles. Since intertidal growth of surfclams is seasonal (May – October), and this time coincides with green crab settlement and subsequent recruitment, it would seem that there are three options for would-be farmers; none, however, would result in commercial size clams in less than 24 months. The first would be to plant clams outside the window of seasonal growth/green crab settlement (November – March). The second would be to use a smaller mesh aperture such as PetScreen® that excludes crabs larger than 2.1 mm in carapace width. The third would be to seed clams larger than 12 mm SL.
References
Beal, B.F., Bayer, R., Kraus, M.G., Chapman, S.R. 1999. A unique shell marker in juvenile, hatchery-reared individuals of the softshell clam, Mya arenaria L. Fishery Bulletin 97(2), 380-386.
Beal, B.F., Coffin, C.R., Randall, S.F., Goodenow, C. A., Jr., Pepperman, K.E., Ellis, B.W., Jourdet, C. B., Protopopescu, G.C. 2018. Spatial variability in recruitment of an infaunal bivalve: experimental effects of predator exclusion on the softshell clam (Mya arenaria L.) along three tidal estuaries in southern Maine, USA. Journal of Shellfish Research 37(1), 1-27.
First-year research conclusions
Environmental conditions during 2021 for both rainfall and seawater temperature were at/near extremes (high), and the survival and growth data from both locations suggests that repeating trials is warranted to help make decisions regarding the efficacy of the growout model. Top thickness will be eliminated as a factor in 2022, that will be replaced with three new stocking densities so that a more clear assessment of density effects on both survival and growth can be obtained using regression analysis. If sufficient animals are available, there will be six levels of stocking density: 12, 25, 40, 60, 75, and 100 animals per square foot. Experimental units will remain as they were in 2021 (i.e., 2-ft2 and 4-ft2).
Second-year research conclusions
The research is showing that environmental conditions favoring higher temperatures along with an abundance of invasive green crabs at the western study site in Gouldsboro are not conducive for growing Arctic surfclams. Average density of green crabs per growout box at Timber Cove (Gouldsboro) was 6.9 ± 2.4 individuals (n = 60), or 2.45 ± 0.8 individuals/ft2. In addition, rock crabs occurred at a density of 0.5 ± 0.2 individuals per box (0.2 ± 0.07 individuals/ft2). These densities were significantly higher than those at the eastern study site in the town of Beals where 0.1 ± 0.09 individuals per box (0.05 ± 0.04 individuals/ft2) were observed. That is, in both years, conditions were more favorable for farming this species in the colder waters of Beals vs. Gouldsboro. It is unclear at present whether the reduced growth rates and final mean sizes of surfclam juveniles observed in Gouldsboro compared to Beals was due to differences in temperature maximums between the two locations, numbers of green crabs and rock crabs, or a combination of these two factors. The growout units (1-ft x 2-ft x 6-inches deep; 2-ft x 2-ft x 6-inches deep) appear too large for a single person to manipulate in the field. The clammers for whom the demonstration project is supposed to benefit have indicated their difficulty with the size of the growout boxes (i.e., container size). We will use smaller boxes (1-ft x 2-ft x 3-inches deep) with smaller aperture mesh in 2023 to determine if we can reduce the number of crabs accessing boxes specifically at the western site. It is likely that the smaller mesh (1.7 mm x 1.9 mm) will reduce flow rates and slow down growth rates, but the trade-off may be higher survival.
Third-year research conclusions
Planting clams outside of their seasonal growth period only delays the time when surfclams will encounter green crabs. Crabs settle between early July and late August. If clams are planted within the no-growth window, they will begin growing in May and by early July/late August will have attained sizes that are > 5-8 mm in shell length greater than they would be at deployment. Green crabs will become trapped in the boxes and grow by feeding on small invertebrates such as polychaete worms, amphipods, and juvenile gastropods that colonize boxes when larvae of these organisms settled into the boxes and then remain. Depending on when boxes are inspected, green crabs could be as large as 15-20 mm CW, and we have found that green crabs in that size range are capable of consuming surfclams as large as 25-30 mm. Using a smaller mesh aperture will potentially result in fewer crabs (as was observed with the nursery growout boxes with PetScreen® mesh netting), but the tradeoff is growth as the smaller apertures restrict water flow and increase competition for food among the surviving clams. Using larger seed may be an option, but it would be an expensive one since rearing surfclams in the hatchery to sizes > 12 mm, would cost farmers >$40/1,000 seed compared with $7/1,000 for 3-5 mm SL animals.
Two other potential options may pay dividends. The first would be for farmers to check (inspect) boxes monthly, especially in August/September/October. Boxes would have to be manufactured in such a way that they are easily opened and closed. We used stainless steel screws (8 in tops protecting 4ft2 boxes, 6 in tops protecting 2 ft2 boxes), and that proved time-consuming since inspection means that tops are unscrewed and then re-screwed once the inspection is finished. While this method would seem obvious, it is likely one that would not be chosen as it would require a farmer give up an entire tide to inspect boxes rather than earning income from harvesting wild bivalves (soft-shell clams; razor clams; blue mussels) or marine bait worms (sand worms; blood worms). Since the bulk of a “farmer’s” income would come from these sources, it is unlikely that an individual would forego $500-$700 from working a tide to inspect boxes to remove small green crabs. Another potential option that we intend to pursue is removing the boxes from the intertidal to create a floating, shallow (3-inches) sediment-filled tray. Trays would float at the sea surface using Styrofoam attached to the periphery of each tray. As long as the sediment remains in the tray, and is stable through time, surfclams will grow. Tops can be manufactured so that they are easily removed and replaced. Boxes would float in a protected cove or embayment, and could be inspected at any time the boxes are floating. If the site is located subtidally, then access to the shallow trays would not be related to tidal cycle, but weather, which would guarantee periods of time when “farmers” would not have to trade their time inspecting trays for crabs for an income-generating activity.
This effort has also resulted in a realization that summer temperatures > 16oC are likely not conducive for fast growth for surfclams. In every instance, surfclams grew faster in Beals, where maximum summer seawater temperatures were ~3oC cooler than in Gouldsboro. Future efforts with Arctic surfclams likely would be more effective in locations east of the town of Beals.
The mesh used on the top of the nursery boxes was much smaller than the mesh used in the previous two years when we used larger, growout boxes. The mesh protecting surfclams in nursery boxes in 2023 had aperture dimensions of 0.9 mm x 1.7 mm, which was much smaller than that used in previous years with the growout boxes (4.2 mm x 4.2 mm). The decision to use the smaller mesh did improve survival rates by limiting access to boxes by crabs and other large predators >2.1 mm in carapace width. For example, survival rates at Timber Cove increased dramatically in 2023-2024 (> 90%) compared with previous field trials (< 10%). Nursery field boxes restricted green crab sizes and densities (< 1 crab/box on average and the CW [carapace width] of all individuals was < 15 mm CW). Unfortunately, results from the Beals site were catastrophic in terms of survival that was influenced by back-to-back winter storms that stirred up sediments and deposited up to a half-inch of silt on top of boxes that resulted in mass mortality of the surfclams inside. While we cannot know if the storms did not occur would survival at the Beals location been higher; but, from previous work at that site, the answer is that survival would likely have been much higher. We do know that the use of the smaller mesh likely restricted flow that resulted in overall slower growth of surfclams compared with previous years. The net effect was to slow down growth so that the average linear increase (pooled across stocking density) was only 5.2 mm at Timber Cove and 8.6 mm at Mud Hole Cove. That is, the mesh on the nursery field cages resulted (for surfclams at one location) in relatively high survival, but growth rates could not be considered commercial. Use of PetScreen as a predator deterrent was effective, but growth rates were too slow to warrant widespread use of the material in any commercial setting.
We have encountered several logistical problems that limits access to the growout boxes. First, boxes are only accessible during 2-3 spring tides that occur monthly. That is, the boxes are inaccessible for over 90% of each month, and this limits when boxes could be checked for green crabs and other predators. Boxes could be placed higher in the intertidal, but the net effect would be to slow growth even further because animals cannot grow unless they are covered with seawater (that contains the phytoplankton they feed on). Moving boxes to the mid- or upper-intertidal zone would shorten the time surfclams have to feed during each tidal cycle. In addition to limited access due to the position along the tidal gradient where surfclams grow best, these low, nearly subtidal, regions of the intertidal have, characteristically, extremely soft mud making work in the area of the boxes difficult at best. Sometimes, small springs occur under the soft mud and when one steps in an area that has these springs, there is a danger of becoming stuck, as the environment is more like quicksand.
Key Conclusions and Recommendations
Future attempts to grow this species will likely use floating gear that is buoyed up so that whatever is holding the surfclam juveniles has a layer of stable sediment that can be buried into. The top of the floating gear will require an impenetrable mesh (extruded polypropylene or vinyl-coated wire) with an aperture as large as 12 mm since it will be above the surface of the water and need only exclude birds or other organisms that could sit on top of the floating gear. The sediment would provide stability for the animals that, if left to move around due to tide- and wind-driven waves, would not grow lengthwise, but would grow thick shells that prevent normal growth morphologies. In addition, the top should be accessible in a matter of seconds that would allow anyone to inspect the contents of the gear and remove any crabs, worms, or other would-be predators, and secure when returned to the tray. Finally, since these structures would be located in a protected cove or embayment, access could occur at any time it is safe to visit the site, presumably by boat. Regular and routine inspection of the field gear should result in high survival rates. Because increasing densities often results in some growth penalty, field trials should be conducted to determine the most efficacious stocking density to maximize growth to commercial size. That is, the floating gear could be a permanent structure for a cultured Arctic surfclam that enters the structure at a SL of 3-5 mm and exits 2.5-3 years later when it reaches 40-45 mm.
Education & Outreach Activities and Participation Summary
Educational activities:
Participation Summary:
One presentation on the project was given in 2021 for an online course (Aquaculture in Shared Waters) that is organized by the Maine Aquaculture Innovation Center (24 attendees), and another, for the same course, was given in February 2022. Once the project is completed, the plan is to give a presentation at the National Shellfisheries Association's Annual Meeting.
Mactromeris Aquaculture in Shared Waters Class Part A
Mactromeris Aquaculture in Shared Waters Class Part B
A presentation was given on 11 January 2024 at the Northeast Aquaculture Conference and Exposition (NACE).
The abstract is here: 10_15_2023 ARCTIC SURFCLAM
The presentation is here: Slides 1-23 NACE 2024 - Rhode Island - Arctic surfclam (B. Beal)
Slides 24-46 NACE 2024 - Rhode Island - Arctic surfclam (B. Beal)
A presentation was given on 21 March 2024 at the 116th Annual Meeting of the National Shellfisheries Association in Charlotte, North Carolina.
The abstract is here: 03_21_2024 ARCTIC SURFCLAM - B. Beal NSA
The presentation is here: Slides 1-24 NSA 2024 - North Carolina - Arctic surfclams (B. Beal)
Slides 25-54 NSA 2024 - North Carolina - Arctic surfclams (B. Beal)
Learning Outcomes
This project is unlike anything that the five clammers we have been working with have experienced. All harvest wild soft-shell clams, and none have conducted research or intertidal aquaculture prior to this endeavor. We appreciate their assistance and willingness to learn about a new species. Our objectives are to introduce the concepts of farming in the intertidal zone with a new species of bivalve that will help diversify the types of shellfish that are sold and marketed in Maine and New England. None of the farmers participating in this project said they would be interested in farming Arctic surfclams in the lower intertidal given our struggles with weather-related events and green crabs. They did say that they thought moving the growout units from the mud to a cove where units float in surface waters may be a sensible alternative to what we tried over three years.
Project Outcomes
A grant proposal was written and sent to the Atlantic States Marine Fisheries Commission in 2021. This was not funded, but would have provided additional resources for further fieldwork in both Gouldsboro and Beals along with one other community.
The "farmers" that we work with on this project are clammers who are hunters/gatherers. That is, they dig wild clams for a living. We feel it a success that the individuals are willing to participate in a project that has never been done before, and that involves culturing this species in the intertidal zone. Intertidal aquaculture is rarely practiced in Maine, and never with Arctic surfclams or even soft-shell clams, the species they harvest. Each participant/clammer is an independent worker who works 3-5 hrs/tide to earn his living. This project is so far removed from their daily routine, but we are encouraged to have them participate, ask questions, and want to learn about the clam farming process.
Assessing the contents of the growout boxes on a regular basis is important, but difficult given that these are placed in the lower intertidal/shallow subtidal zone where they are exposed for only 1 or 2 tides in a given month. When foul weather (low pressure weather events) coincides with these tides, the boxes may not be exposed for 1-3 months at a time. This was the case during 2021 and 2023. The boxes are placed in areas to maximize surfclam growth, and we may want to determine what the growth penalty is if the boxes were to be placed above the extreme low water mark.
It was clear after the first two years that trying to culture Arctic surfclams in the lower intertidal using the structures that we were using was not a long-term solution to growing this species in eastern Maine. While these animals will grow in the intertidal zone, they do not grow at commercial rates much above the lowest intertidal mark. That is, they can be planted at the upper and mid intertidal where crab densities are low; however, because they are covered for only a few hours by the tides at these higher intertidal sites, they will not grow very fast and would not reach a size of 40-50 mm in under 5-6 years. Instead, planting them in areas lower on the shore that are only exposed for a few hours each month and where they would be covered with seawater and able to feed nearly 100% of the time seemed like a good plan. If not for green crabs, it would have been a good plan; however, the difficulty with accessing the study sites, due to their low intertidal locations, prevented regular inspection of the field gear. Some months, when storms coincided with the spring tides, the sites were not exposed and the boxes could not be inspected. The lack of regular inspection of the contents of each box and the time it takes to remove small crabs given the length of time boxes are exposed on a given spring tide make this a difficult farming strategy. Clammers (would-be farmers) were not interested in scaling up the size of the growout boxes as we had initially proposed. They thought that the work involved to deploy and collect data from the 1-ft x 2-ft boxes and from the 2-ft x 2-ft boxes was all they could do, and encouraged us not to use larger growout units. In addition, clammers were instrumental in helping us re-think the growout strategy. We are now focused on a completely different methodology to grow animals to commercial size.