Expanding sustainable shellfish aquaculture: Optimizing growth and survival in a bay scallop nursery system

Expanding sustainable shellfish aquaculture: Optimizing growth and survival in a bay scallop nursery system

Summary

The bay scallop (Argopecten irradians) commercial fishery in New England which was once robust and profitable for fishermen, has been in rapid decline since the 1980’s due to overfishing, habitat loss, and coastal water quality degradation. Over the past decade there has been a renaissance in shellfish production throughout New England, though the oysters and clams are now being produced through farming instead of through wild harvest. Shellfish farmers are currently seeking new species to grow, in order to increase revenue and diversify risk, and given the high market demand for bay scallops combined with a lack of significant wild supply, they are an ideal aquaculture candidate. However, current bay scallop aquaculture techniques are not efficient, lead to high mortalities, high labor and gear costs, and therefore, further investigations into the nursery, as well as the growout and overwintering phases, are needed to help make bay scallop aquaculture viable. Ward Aquafarms starting farming bay scallops in 2014, and they quickly realized that simply copying existing oyster farming methods was not going to work to produce scallops. In 2015 Ward Aquafarms designed and constructed a floating downweller system for the bay scallop nursery phase, which greatly improved survival and growth, leading to much more efficient techniques. In 2016, Ward Aquafarms further investigated optimization of the nursery phase to increase cultivation viability, with the goal of faster growth rates, higher survival and higher stocking densities. During an eight week observation period (July-September 2016), Ward Aquafarms investigated the differences in bay scallop growth, survival, and food availability in relation to flow rates, and initial stocking densities. Stocking densities for each of twelve silos were modified at the beginning of each two week stocking period, dependant on scallop size, gear mesh size and desired initial stocking densities. Initial stocking densities ranged from 900 scallops/m² to 36,626 scallops/m^2, which resulted in survival ranging from 77.9% – 86.2% and growth rate range from 0.05 mm/day – 0.70 mm/day. Flow rates between the silos ranged from 1,478 cm³/sec – 5,666 cm³/sec, though there was no correlation between flow rate and growth or survival. As is common in the natural environment, there was high variability in observed chlorophyll a and phycocyanin values, which resulted in either no correlation, or a weak relationship between available microalgae and production statistics. Provided there was sufficient available food, initial stocking density (scallops/m²) was shown to be the primary factor which determined growth rate and survival. It was proven that with augmented water flow, bay scallops can be grown at very high densities, with no apparent decrease in growth or survival until the stocking density exceeds 18,000 scallops/m², at an average shell height exceeding 11.8 mm shell height. The results indicate that as the initial stocking densities are increased, overall bay scallop growth decreases; however, stocking density did not have significant effect on survival, as very little death was seen at any stocking density during the entire nursery phase. Neither flow rates nor food availability proved to significantly impact growth or survival of bay scallops at any tested stocking densities. The results of this work will allow a farmer to fine tune their bay scallop stocking densities to either maximize growth or survival based on the known microalgal food density in their waters. Results of the project are to be presented at the Northeast Aquaculture Conference and Exposition (NACE) in January 2017 to inform farmers and researchers how to optimize bay scallop growth for their specific requirements during the nursery phase.

Objectives/Performance Targets

From July to September of 2016, Ward Aquafarms investigated the best way optimize growth rates and survival of bay scallop seed from the hatchery (.75mm) to a size ready for growout (20-25mm) (Figs. 1-4).

1. Ward Aquafarms evaluated four different initial stocking densities over the entire nursery phase (July to September 2016), comparing flow rates, food depletion, survival, and growth rates in a bay scallop downweller system.

This objective was achieved by comparing four different initial stocking densities over three different mesh sizes over three, two-week sampling periods throughout the nursery season. Initial stocking densities were based on the findings of Leavitt and Karney (2010) for bay scallops in bags with no additional water flow. Given that the downwellers increase water flow, and maintain enhanced flow rates, and therefore higher food availability, we increased the stocking density in the four treatments to either 2X, 4X or 8X standard stocking density (1:2:4:8 ratios) as compared to the published stocking densities for a system without augmented flow. Scallops were grown in downweller systems for 10 weeks from July through September, with sampling starting after two weeks of acclimation to the system.

Every 14 days, all of the twelve stocked silos were assessed in an identical manner. Flow rates entering each silo were measured using flow meter. Water samples were taken both at the water intake of the downwellers, and at the water exit point of each silo to measure total chlorophyll a and phycocyanin content at each sampling period total Cochlodinium polykrikoides cells within each water sample, entering and leaving the silos was quantified as well.

Once flow rate and food availability measurements were taken, all of the nested trays holding the bay scallops were removed from the silos. Scallops from each silo were then graded to separate size classes. Once separate, shell heights were recorded for twenty individuals per size class, individual counts were taken of live scallops per 100 ml, total volumes were recorded for each size class for each individual silo.

2. Results will be made public so shellfish farmers in the northeast region can begin to grow bay scallops.

All analysis for this project is complete, and findings will be published in a leading aquaculture academic journal, and will be presented at the Northeast Aquaculture Conference and Exposition (NACE) in January 2017 to make findings available to farmers and scientists looking to pursue bay scallop aquaculture.

Accomplishments/Milestones

During the six-week sampling period, percent survival based on size ranges of initial stocking densities (< 2,000, 2,200-4,000, 4,500-5,000, 7,000-9,500, and >18,000 scallops/m²), using total number of scallops stocked per silo (900 scallops/m² to 36,626 scallops/m²) and total number alive at the end of the two-week sampling period (722 scallops/m² to 31,529 scallops/m²), ranged from 77.9 ± 11.0% – 86.2 ± 6.6% (all values mean ± SD unless otherwise noted). There were no significant differences in survival at the conclusion of each any of the two-week sampling periods, regardless of stocking density (ANOVA, α=0.05, n=5, P-value= 0.4278). While the differences in survival between treatments were not significant due to low variability, the differences between groups in terms of actual number of scallops was quite large. For example, during the second two-week sampling period, silo 1 stocked at the mid-low level (2X low stocking density; 9,156 scallops/m²) had a final survival value of 86.6%, which meant 1,227 scallops died during the two-week growth period. During the same time period silo 6 stocked at the high level (8X low stocking density; 36,626 scallops/m²) had an almost identical final survival value of 86.0%, though under the higher stocking density, this meant a loss of 5,097 scallops over the same sampling period. Therefore it is in the farmer’s best interests to decide on optimizing survival or growth during the nursery period, as with the higher stocking densities, even a slight decrease in survival as compared to a lower stocking density can have a much larger impact.

Growth rates for the first two week sampling period decreased as stocking density increased (Pearson r, α=0.05, two-tailed P-value= 0.0029, R²=0.6054), with growth ranging from 0.04 mm/day to 0.35 mm/day. For the second two week sampling week, the reduction in growth rate given an increase in stocking density was greater as compared to the first two week sampling period. Growth rates was observed as stocking density increased (Pearson r, α=0.05, two-tailed P-value= 0.0002, R²=0.7627), with growth rates ranging from 0.20 mm/day to 0.45 mm/day. Growth rates in the third two week sampling period showed an even greater reduction (Pearson r, α=0.05, two-tailed P-value= 0.0001, R²=0.7789), as stocking density increased compared to previous sampling weeks, with growth rates ranging from 0.16 mm/day to 0.39 mm/day. These results indicate, that as initial stocking density (scallops/m²) is increased, a reduction in bay scallop growth rate occurs. When each week is evaluated individually (average initial size of scallops was 9.8 ± 1.5 mm for the first two week sampling period, 11.8 ± 2.8 mm for the second and 18.6 ± 2.1 mm for the second sampling period), it can also be concluded, that both as scallops grow in size over the nursery stage, and as growth rate increases with increasing water temperature scallop size plays a larger role in determining growth rate when scallops are stocked at a larger size. Investigating each week individually, and knowing the average size of the scallops stocked during each sampling period, estimates can be made by a farmer as to which stocking density (scallops/m²) is optimum for each size to maximize number of individuals stocked and maximize growth to desired final size.

Initial stocking density (scallops/m²) directly affected the volume and total number of produced small, medium and large scallops after each two week sampling period. As initial stocking density is increased, the percent of which large scallops comprise the overall final volume of each silo is decreased (Pearson r, α=0.05, two-tailed P-value= < 0.0001, R²=0.6871). Although as initial stocking density increases the percent of which the final total volume of large scallops decreases, the average final density (scallops/m²) of large scallops increases. This is a result of a higher number of scallops being stocked at higher densities. However, since the final percent total volume of large scallops decreases with increasing stocking density, the percent and total average final density of small and medium scallops increases as stocking density increases as well. Additionally, as the number of large scallops after two-weeks is greater at a higher stocking density, the mortality of scallops at all size classes increases as well. If one desires mainly large scallops, but also wishes to optimize the number of large scallops produced, a trade-off between initial stocking density and final density of large scallops must be determined based on individual farmer needs.

Measured chlorophyll a and phycocyanin RFU (relative fluorescence unit) levels, as an assessment of available food (phytoplankton) for the bay scallops, indicated that flow rates (cm³/sec) did not have a significant influence on chlorophyll a availability when measured for the second two-week sampling period (Pearson r, α=0.05, two-tailed P-value= 0.3787, R²=0.02508), whereas there was a weak, but significant correlation between flow rate and phycocyanin levels (Pearson r, α=0.05, two-tailed P-value= 0.0049, R²=0.2281). Chlorophyll a and phycocyanin RFU levels, when compared to final scallop stocking density (scallops/m²), also show little effect of stocking density on the availability of food. While there not a significant correlation between chlorophyll a and flow rates, there was a weak but significant correlation between chlorophyll a levels and final scallop stocking density (Pearson r, α=0.05, two-tailed P-value= 0.0107, R²=0.1921). Additionally, while there was a weak but significant correlation between phycocyanin level and flow rate, there was not a significant correlation between phycocyanin and final scallop stocking density (Pearson r, α=0.05, two-tailed P-value= 0.1677, R²=0.06048). All of the analysis between food availability (chlorophyll a or phycocyanin) and either stocking density or flow rates, resulted in either no significant correlation, or a weak relationship between variables. Thus, it appears as though, in an environment with high rates of flow, constantly resupply food to the scallops in the downweller system, that food is not limiting the growth of scallops, but rather space limitation caused by overstocking.

Impacts and Contributions/Outcomes

We have completed all of the sampling for this project, as well as all of the analysis and report writing. We will be presenting the results to farmers, regulators and scientists alike at the Northeast Aquaculture Conference and Expo (NACE) in Providence, RI, January 11-13. We will also be publishing the results following the conference in the spring, when we will submit the final report to SARE as well.

In terms of impacts, the 2016 NE SARE project made a significant impact on our bay scallop production in 2016 which will continue in 2017 and in the future. While it would be great if other farmers would start building downwellers like were constructed through this project, first the farmers need to be able to try growing bay scallops before they invest the money in a nursery system. However, until now, there was nowhere to buy bay scallops, unless they were less than 1 mm, which very few farmers can grow. Now that we have been able to document both that bay scallops can be grown in this manner, and how the shellfish survive and grow under various conditions, we will be able to supply other farmers in the region with bay scallop seed in 2017. We already have a farmer in RI who is interested, as well as several farmers on Cape Cod, and that is simply by word of mouth. Once we present at NACE, I anticipate a lot of interest increasing throughout the spring and summer. Building a new industry is a slow process, but looking back on a sustainable bay scallop industry in the northeast in 5-10 years, we’ll be able to point to this particular project as one of the main reasons the industry exists. This was an incredibly successful project, we will continue with the methods we learned through this project, and others throughout the northeast will benefit as well.

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