Like many other states in the Northeast region, the New Jersey shellfish farming sector is poised for growth. Shellfish aquaculture production has grown rapidly, expanding in volume by 5.1% and value by 11.1% nationally from 1998-2008 (FAO, 2016). Currently, only two species of shellfish are cultured commercially in New Jersey: the Eastern oyster (Crassostrea virginica) and the hard clam (Mercenaria mercenaria). The emerging oyster aquaculture industry in New Jersey consists of 17 oyster farms, 10 of which reported sales of $1.1 million in 2015 (Calvo 2016). Meanwhile, the more established hard clam industry consists of 39 farms and reported sales of over $2.3 million in 2013 (USDA, 2015). Although monocultures may increase efficiency, they pose a threat to food security because they are vulnerable to a variety of risks including heightened disease and predation pressure (Altieri et al., 2015). Consequently, many New Jersey farmers have expressed significant interest in expanding farm capacity and resilience by diversifying their crops (D. Munroe, personal communication).
Diversification can sustain the economic viability of farm operations. By cultivating diverse species, a farmer becomes insulated from any individual crop failure, whether it occurs from disease (Felton et al., 2016), predation (Russell et al., 1989), or fluctuating environmental conditions (Gaudin et al., 2015). Growing multiple species may also increase profitability, by allowing farmers to more easily navigate market forces as the price of each individual crop fluctuates (Chopin et al., 2012; Isaacs et al., 2016). Finally, growing multiple species in one area may enhance the environmental quality and the natural resources of the local ecosystem, upon which local aquaculture endeavors depend (Chopin et al., 2012). For example, enhancing filter feeders abundance can lead to the removal of excess suspended particles from the water column and can even provide habitat for other economically and ecologically important species (Newell, 2004; Luckenbach et al., 2016).
For the Northeast aquaculture sector, the Atlantic surfclam (Spisula solidissima) represents an ideal target for species diversification because it is native, grows rapidly, and fits into the region’s established farming framework. Surfclam aquaculture can add value to existing hard clams or oyster farm plans, especially since the rapid early growth rates of this species gives it the potential to reach marketable sizes within a year (Krzynowek et al., 1980). Moreover, this species experiences its fastest growth rates during the winter and early spring months (Goldberg and Walker, 1990), which would allow aquaculturists to stay engaged with farming operations and improve productivity during what would otherwise be a largely inactive winter season.
Although some of the techniques for cultivating surfclams were developed decades ago (Walker and Heffernan, 1990a, 1990b, and 1990c), recently efforts in Massachusetts (Murphy et al., 2017) and New Jersey (present authors) have focused on expanding upon established methods and innovating new approaches in order to address the challenges presented by the Northeast’s unique coastal environment. Through recent support from NJ Sea Grant and NOAA Sea Grant Aquaculture Extension, we have begun studies to optimize nursery and grow-out phase techniques. As this work continues, farm grow-out strategies and seasonal planting regimes are being evaluated, along with optimal nursery gear types. During experimental evaluation of the optimal nursery rearing temperature of early post-metamorphic juvenile surfclams (shell length = 0.600 – 3.00 mm), we demonstrated that surfclam seed under temperature stress survived less than half as well as those reared under cooler conditions (Acquafredda et al., in prep). Interestingly, surfclams produced from different parent stock responded significantly differently to temperature during these trials, suggesting that thermal tolerance is a heritable trait (Acquafredda et al., in prep). Selective breeding programs have been the foundation of viable Eastern oyster production in the Eastern US, (Haskin & Ford, 1979), and we believe this will likewise be a critical step in developing surfclams as a cultivation candidate in the region.
The purpose of this project is to evaluate whether selective breeding for heat-tolerant surfclams is a viable strategy for enhancing survival of cultivated surfclams exposed to high temperature conditions on shallow coastal shellfish farms. It is clear that this new crop (“shellstock”) species has strong potential to benefit farmers eager to build diversity and resiliency into their farm plans. Farm-raised surfclams also may provide a new domestic product for chefs eager to concoct new recipes and consumers eager to try a new, locally-sourced, high-protein food (Krzynowek et al., 1980). The development of a selectively bred heat-tolerant surfclam stock will lead to improved and consistent annual yields, providing stability to farmers and seafood consumers, alike.
Surfclams are known to be vulnerable to high temperature conditions (Goldberg and Walker, 1990; Weinberg 2005) – an issue that will be exacerbated by rising temperatures (Munroe et al., 2016), and one that will be problematic on shallow coastal farms. The goal of this project is to assess whether thermal tolerance is a potentially heritable trait in the Atlantic surfclam and to take the first steps towards developing a stock of selectively bred heat-tolerant surfclams. These goals will be achieved by comparing growth, survival, body condition, and overall performance of first-generation offspring of thermally-selected surfclams to that of offspring from non-selected surfclams.
Objective 1: Select heat-tolerant brood stock. Selecting the appropriate brood stock is paramount to any successful breeding program. We will select individuals from a large cohort of clams derived from a mix of wild stock parents. This cohort will be split into two groups: one that will be exposed to thermal stress, from which the heat-tolerant survivors will be selected, and a second that does not experience thermal stress and will act as control brood stock.
Objective 2: Produce heat-tolerant and control progeny. We will spawn and back-cross the heat-tolerant and control brood stocks to produce a thermally-selected cohort (TS-cohort) and a non-selected control cohort for comparison (NS-cohort). Through experimentation (Objectives 3 and 4), these progeny will allow us to determine if the heat-tolerance observed in the parent stock was transmitted to their offspring.
Objective 3: Evaluate survival, growth and performance of selected progeny on the farm. Clam farms experience fluctuating temperatures over tidal, diel, and seasonal temporal scales. By growing these surfclam cohorts on an actual farm for the entirety of the grow-out phase, we will be able to assess whether the thermally-selected progeny perform better than the non-selected progeny during both optimal (cooler water of late fall through early spring) and suboptimal conditions (summer), and determine differences in yield during the final harvest of market size animals.
Objective 4: Evaluate heat-tolerance of selected progeny in the laboratory. When the thermally-selected and non-selected cohorts are ready to be deployed to the farm, a portion of each cohort will be retained in the laboratory in order to assess the thermal tolerance of each group in a highly-controlled setting. By exposing the animals to more severe conditions than they would experience on the farm, we will be able to precisely determine the upper thermal tolerance of each cohort.
Methods to achieve Objective 1: Select heat-tolerant broodstock.
The broodstock surfclams (Spisula solidissima) used in this project were spawned at the New Jersey Aquaculture Innovation Center (AIC) in May of 2016 from a mix of wild and farm-raised parents; specifically, five wild females, one wild male, two first-generation farm-raised females, and four first-generation farm-raised males were crossed to generate these clams. The clams were out-planted onto Farm DP and Farm BA in October and November of 2016, respectively. Farm DP is located in the Lower Barnegat Bay and Farm BA is located in Absecon Bay.
A total of 505 clams were harvested from Farm DP on October 11th 2017, and 499 clams were harvested from Farm BA on November 20th 2017. During the time between the harvest and the start of the experiment, the clams were kept at the AIC in continuous flow-through upwellers supplied with raw water from the Cape May Canal. The clams from both farms were split into non-selected (NS) control groups and thermally-selected (TS) groups. The initial number of clams per group was as follows: NS-BA = 235; TS-BA = 239; NS-DP = 240; TS-DP = 240.
The experiment took place at the Haskin Shellfish Research Laboratory in Bivalve, NJ. The TS-BA and TS-DP clams were challenged with a warm water thermal shock. The water temperature that these clams experienced was increased from 16˚C to 30°C over a five day period. Then, the clams were exposed to temperatures that exceeded their known tolerance limit (between 28˚C and 30˚C) for five days (Goldberg & Walker, 1990). After the thermal shock, the TS-BA and TS-DP clams were cooled to 10˚C over a period of 16 days. The control groups, NS-BA and NS-DP, were maintained at 10°C.
Mortality was monitored daily, and dead animals were immediately removed from the treatments when detected. Each day, animals were fed algae paste (Shellfish Diet 1800, Reed Mariculture); the manufacturer’s recommendations for broodstock feeding were followed. Temperature was recorded every 10 minutes with Seabird Scientific SBE 56 temperature loggers. Daily temperature and salinity point measurements were also collected. Ammonia, nitrite, and nitrate concentration levels were monitored daily; concentrations of nitrogen waste products never reached dangerous levels. Water changes were conducted every 2-3 days.
Before the experiment began, the following morphometric data were collected on 25 individuals from each farm origin: shell dimensions (shell length, height, and width), whole wet weight, wet shell weight, and wet tissue weight. The wet shells and tissues of the sacrificed clams were placed in a 68˚C drying oven for at least 48 hours until all moisture was removed. Afterward, dry shell weight and dry tissue weight were measured and the following condition indices were calculated: dry body weight/shell length and dry body weight/shell volume.
Immediately following the temperature challenge and at the end of the experiment, the following data were also collected from individuals of each group: shell dimensions (shell length, height, and width) and whole wet weight. Since only a limited number of clams remained in each group after the experiment, no clams were sacrificed. Consequently, wet and dry shell weight data as well as wet and dry tissue weight data were not collected.
Following the experiment, approximately half of the clams from each group were out-planted, so that they would naturally develop gonad (ripen) over the winter. As a precaution, the remaining clams are being held at the Bivalve facility. These clams will be maintained in conditions that should facilitate gonad development. All TS-BA clams have been retained in the Bivalve facility due to the small fraction of surviving clams in this group.
Methods to achieve Objective 2: Produce heat-tolerant and control progeny.
The methods to achieve Objective 2 are being planned. Objective 2 research will begin in the spring of 2018.
Methods to achieve Objective 3: Evaluate survival, growth and performance of selected progeny on the farm.
The methods to achieve Objective 3 are being planned. Objective 3 research will begin in the autumn of 2018.
Methods to achieve Objective 4: Evaluate heat-tolerance of selected progeny in the laboratory.
The methods to achieve Objective 4 are being planned. Objective 4 research will begin in the summer of 2018.
Objective 1 Results:
At the onset of the experiment, the average shell length of DP and BA clams was 32.37+/-2.03 mm and 32.21+/-4.13 mm (mean+/-standard deviation, SD), respectively. The average whole wet weight of DP and BA clams was 9.661+/-1.421 g and 9.245+/-2.706 g (mean+/-SD), respectively. The average wet tissue weight of DP and BA clams was 3.094+/-0.535 g and 2.820+/-0.795 g (mean+/-SD), respectively. None of these morphometric variables differed significantly between the clams of the two farm origins (Student t-test, p>0.16).
Conversely, the initial health or condition of the clams, as measured by dry tissue weight/shell length, was significantly greater for clams from Farm DP as compared to those from Farm BA (Student t-test, p<0.001).The average condition of DP and BA clams was 1.25e-2+/-3.43e-3 g/mm and 7.43e-3+/-1.97e-3 g/mm (mean+/-SD), respectively.
During the experiment, no morality was observed for either control group, NS-BA or NS-DP. Immediately after the warm water thermal shock, the survival of the TS-BA group was reduced to 23.8% while survival of the TS-DP group was reduced to 60%.
Some mortality continued to occur after the thermal shock, even as the water temperature became more hospitable. By the end of the experiment, the survival of the TS-BA group was reduced to 11.7% (28/239) while survival of the TS-DP group was reduced to 45.4% (109/240).
After one day in the thermal shock, TS-BA and TS-DP clams displayed behaviors that suggest the animals were succumbing to heat stress. The clams were frequently observed gaping. Clams were also seen extending their feet into the water column for prolonged periods of time, but they did not attempt to walk or explore their immediate surroundings by feeling the bottom of the tank. Similarly, the clams in the thermal shock responded more slowly to touch stimuli than clams in the control conditions; when probed, gaping clams in the thermal shock did not immediately close their valves or retract their extended feet or siphons.
After five days in the thermal shock, the shell margins of TS-BA and TS-DP clams had become very soft, almost pliable. The shell margins easily cracked or chipped under moderate finger pressure.
During the thermal shock, the soft tissues of TS-BA and TS-DP clams took on a moribund appearance, becoming thin, watery, and translucent.
Sixteen days into the cool down period, the soft tissues of most TS-BA and TS-DP clams had regained their color and turgor, their shell margins hardened, and their response times to touch stimuli were closer to that of the non-selected control clams.
Shell length, height, width, and whole wet weight did not differ significantly among the four groups immediately following the thermal shock or at the end of the experiment (ANOVA, p>0.05).
Objective 2 Results: NA
Objective 3 Results: NA
Objective 4 Results: NA
Objective 1 Conclusions: NA
Objective 2 Conclusions: NA
Objective 3 Conclusions: NA
Objective 4 Conclusions: NA
Education & Outreach Activities and Participation Summary
On December 7th 2017, I presented an overview of my project through the Rutgers Cooperative Extension – Marine Extension Program Seminars Series. The title of the presentation was, “Diversifying the New Jersey aquaculture sector by developing culture techniques for the Atlantic surfclam (Spisula solidissima)”. The content of this seminar focused on previous and continuing efforts to develop surfclam husbandry, with special attention given to my NESARE Project Objectives. A question and answer session immediately followed the 30-minute seminar.
Approximately 15 people were in attendance. The audience included community members specifically interested in fishing and farming surfclams and other shellfish, academics, NOAA Fisheries personnel, and representatives from the environmental non-profit organizations, ReClam the Bay and Save Barnegat Bay. Nearly all attendees agreed to receive periodic updates about the status of the project through a Surfclam Aquaculture Email Listserv; the first update will be sent to the listserv in early 2018.