Diversified & Profitable: Overcoming Challenges of Winter Mortality in Bay Scallop Culturing to Meet Increasing Demand Left by Wild Fishery Decline

Stage 1 Evaluation of Nursery Culture --To evaluate best practices for nursery culture of bay scallops, knowledge accumulated from previous studies will be leveraged while testing some novel techniques. Three nursery culture techniques will be studied, to evaluate growth rates, survivability, operation costs and labor per unit. 

Raceway—Raceways are a commonly used land-based nursery system. Ambient water will be pumped using a variable speed 3/4hp iceeater pump to flow through the raceways which hold seed and use 0.5mm mesh catch sieves on the output to eliminate loss. Flow rates will be monitored and altered to make sure animals are properly fed without causing too much turbulence. Daily visual inspection will be conducted on the raceway which will identify if animals are stocked at proper densities. Typically scallops will attach to the tanks and detach only when food becomes limited. When this happens, the operator will either increase flow, or reduce stocking densities. Raceways will be drained and cleaned weekly and are typically energy intensive due to pumping water at grade. Energy costs will be monitored via meter.

Downweller—Floating downwellers have recently been identified as a successful means to rear bay scallop through the nursery stage. Downweller systems as described in FNE16-861 will be constructed with multiple silos and inserts to house scallops. The system will be fed by a 3/4 hp iceeater pump and monitored to maintain ideal flows. Energy costs will be monitored via meter. Downwellers will be drained and cleaned weekly to maintain proper flow rates by eliminating biofouling from the system. Similar to raceway culture, daily inspection/adjustments will be conducted.

Netron—NetronTM, a plastic mesh material, has been used as a substrate for shellfish setting in different capacities. By culturing the shellfish in ambient water, the necessity for pumping water (as in the other proposed methods) to maintain food and oxygen levels is alleviated, thus reducing energy consumption. Each replicate of this treatment will include 3 feet of Netron mesh folded into thirds, and added to a settling tank where scallops will adhere to the mesh. 3 replicates will be sacrificed to sample baseline conditions (size and number of scallops). Replicates will be placed in ambient water to grow. Animals will be removed only during sampling periods, and each Netron system will only be sampled once. Removing the animals from the media and cleaning them for the final grow out stage could be a potentially laborious endeavor. 

Seed–Seed will be obtained from the Cornell extension program at ~ 1mm following clean pathology report and clearance to import into Rhode Island growing area 4a. Seed will then be split amongst the nursery culture techniques described above and stocked ~5,000 to 10,000 scallops/m2. Several replicates of each nursery technique will ensure sufficient sampling (number of replicates depends on seed availability). During the nursery phase of this experiment, scallops will be removed and graded using the vibratory seed separator biweekly for a total of 4 sampling periods (8 weeks). Following grading, enumeration and recording, random subgroups will be randomly sampled and measured for shell height and total volume. A random sample of 100 animals for each group will be measured to the nearest 0.1 mm using vernier calipers. As scallops outgrow the nursery system (~14mm shell height) they will be removed from the nursery systems and put into final growout stages. Growth rates and mortality rates will be calculated and recorded for different subgroups at each sampling period. Labor required for all aspects of nursery culture will be monitored and recorded for each of the respective nursery techniques to calculate effort/unit to provide guidance on scalability.

Stage 2 Assess Overwintering Strategies—Extreme mortality of submarket product during the winter months accounts for a huge bottleneck in bay scallop culture. To evaluate methods of overwintering scallops, 2 gear types will be tested in 2 environmentally different regions. Additionally, stocking densities will be varied at 3 different levels to test the impact of stocking density on overwinter mortality. Overwintering experiments will start when water temperatures drop to around 50 degrees Fahrenheit, when scallop growth is expected to slow dramatically (Rhodes and Widman 1984).

Site selection— Environmental conditions including, wind, waves, fetch, depth, temperature, salinity, etc all play a vital role in site selection and gear selection for shellfish aquaculture. Particularly for scallops, gear selection is critical and largely influenced by surrounding conditions. To evaluate gear effect on survivability, two differing sites will be used during the overwinter mortality study. The first site is in the Sakonnet River, Portsmouth, RI which has extremely fast flow rates and is subject to great turbulence and wind/wave energy. This site is representative of most open-water leases like those located throughout Narragansett Bay, RI. The second site is in Nanaquaket pond, Tiverton RI which has moderate flow and is largely protected, and thus much less impacted by wind and wave energy. This site is representative of salt-pond or small embayment type leases like those located in Southern RI.

Bottom—Bottom gear is commonly used in rearing different species of shellfish. By securing gear on the bottom of the waterbody, adverse impacts of weather are typically avoided and the shellfish remain unaffected by any turbulence on the surface. Bottom grown shellfish are typically slower growing as opposed to floating. 8-bay bottom cages fit with HDPE bags (14mm aperture) will be used to test designated stocking densities. Cages will be added to a bottom trawl long line to secure them in place and will be marked/accessed by a float line.

Suspended—Suspended gear has been proven to be a highly effective grow out technique for shellfish in a variety of environments, particularly lantern nets. Suspended gear has the benefits of being higher in the water column encouraging faster growth, without extreme exposure to the turbulence that floating gear is subject to. 7-tier lantern poly lantern nets with 14mm apertures will be stocked respectively to test overwintering. Lantern nets will be secured to a longline system with buoys at each of the connections to keep the lanterns afloat. In addition a small section of chain will be added to the bottom of the lantern net array, to keep the net’s vertical and reduce gyration of animals caused by wind/wave turbulence. 

Stocking density–Bay scallops from stage one will be continue growing throughout the summer and then in stage 2 to test gear selection and stocking density effect on overwintering survivability. Medium sized animals (~ 50mm or less) that did not make it to marketable size will be used in this study. Animals will be measured and deployed into respective gear types at low, medium, and high stocking densities in early December. The effect of stocking density on winter mortality will be tested at 3 different treatments, Low (15/ft2), medium (30/ft2) and high(60/ft2) (Leavitt 2010).Several replicates of each treatment will be tested at each site to ensure sampling is sufficient (replicate numbers depend on availability). When waters warm in the spring and growth should be expected (around 50 degrees Fahrenheit), scallops will be pulled out and sampled to measure mortality and meat quality (visual inspection).Statistical analysis–As data becomes ready and is prepared for analysis, one/two-way ANOVA will show whether treatments were statistically significant or not.