Progress report for LNE21-426R
Maine’s soft-bottom intertidal zone has witnessed a steady decline (75% since 1975) in landings of its iconic soft-shell clam fishery 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. A $100 million fishery, also known as the red-footed surfclam (“hokkigai”), exists in Atlantic Canada, and is highly-valued as sushi and sashimi in Asian markets and restaurants. 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. Fieldwork using cultured juveniles demonstrated Mactromeris can grow and survive in the lower intertidal, especially in eastern Maine where seawater temperatures are colder than elsewhere along the coast. Greatest impediments to commercialization are crustacean and bird predators that can shred protective netting and consume >90% of 6-12 mm individuals. Recently, we created a new growout unit that is effective in deterring predators. Field trials (April-Oct 2019) at two intertidal sites using small juveniles examined effects of stocking density in 2-ft2 growout units. Mean survival was >95% at both sites with grow rates >15 mm SL. We propose to repeat these pilot-scale trials with clammers from two eastern Maine communities during Year I. Working hand-in-hand with clammers provides the best opportunity to demonstrate growout techniques that will eventually lead to commercialization. Specifically, in Year I together with three clammers from each community we will evaluate effects of stocking density (25, 50, and 100 individuals ft-2), size of growout unit (2- and 4-ft2), and type of predator deterrence on growth and survival of cultured surfclam juveniles. In Years II and III, we will plan to vary growout unit size from 4- to 32-ft2 depending on results from Year I. Clammers will present their findings to their peers at an annual forum for fishers (Years II & III) depending on COVID restrictions.
- 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.
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. 1) on 16-17 June 2022, and in the town of Gouldsboro (Timber Cove; 44o27’48.99”N; 68o00’01.26”W; Fig. 2) 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. 3). 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. 4) 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. 5). Stainless steel screws were used to secure each box top to its bottom (Fig. 4). 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. 6). 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. 7). 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. 8], to the nearest 0.1 mm using Vernier calipers). The initial and final SL of each live surfclam (Fig. 9) 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
Figures 1 through 9 Second Annual Report
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 1; Fig. 9a). 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 1). While stocking density was not statistically significant (P = 0.052, Table 1), there was an indication that increasing density to levels greater than 25 individuals per square feet resulted in lower mean percent survival (Fig. 10). 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. 11). No source of variation associated with mean absolute growth was statistically significant (P > 0.25; Table 2).
Small surfclams (SL = 13.2 ± 0.74 mm)
Mean percent survival varied significantly (P < 0.001) with size of experimental unit (Table 3; Fig. 9b). 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. 12); however, stocking density played a significant role (P = 0.0254; Table 4) as surfclams exhibited a growth depression of ~13% at the highest density compared to the two lower densities (Fig. 13).
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. 14). 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 5), 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. 15).
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. 16). 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. 16a), and many had black, anoxic spots on the sediment surface where clams had died (Fig. 16b). 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. 16c), 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).
Tables 1-5 First Annual Report
Figs. 9-16 First Annual Report
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 6; Fig. 17). 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 6). 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 6).
Live surfclams were recovered in 39 of the 60 boxes. Mean absolute growth was 4.75 ± 1.00 mm (n = 39; Fig. 18). No source of variation associated with mean absolute growth was statistically significant (P > 0.25; Table 7); 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 8; Fig. 19). 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. 19). 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. 20). 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 9) 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. 21).
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. 22). 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 10).
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. 23).
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.
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.
Tables 6-10 First Annual Report
Figs 17-23 First Annual Report
All sixty boxes at both sites were present and undamaged (Figs. 10-11).
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. 12). 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. 13) 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 3). 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 4-6) 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 5, Fig. 14). 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 4-6; Figs.15-17). 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 4, Fig. 15). 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 5, Fig. 16). 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 6).
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. 18 vs. Fig. 15). Mean surfclam growth for each of the measurement variables (final shell length, relative growth, and absolute growth) varied significantly with initial clam size (Tables 7-9; Figs. 19-21). 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 8, Fig. 20), 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 9, Fig. 21). Size of growout box had a significant effect on both mean final shell length and mean absolute growth (Tables 7 and 9). 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 7-9).
Second Year Results Tables 3-9
Second Year Results Figures 10-21
Seawater temperatures (18 June 2022 to 21 January 2023)
Seawater temperatures (Figs. 22-24) 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. 24).
Second Year Results Figures 22-24
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.
Education & Outreach Activities and 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
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.
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. 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.