Assessing Impacts of Converting Oyster Leases from Bottom to Suspended Culture in Foxley/Trout River, PEI
Oyster aquaculture in Canada is gradually evolving from the traditional use of the benthic environment to suspension (off-bottom) culture. Some oyster culturists in the Foxley/Trout River system in Prince Edward Island (PEI) have been experimenting with this new approach and many lease holders in this area are now seeking to convert their bottom leases to suspended leases (Figure 1).
From a farming perspective, there are several advantages to suspending oyster stocks in the upper water column. This strategy protects stocks from benthic predators and facilitates product grading and harvesting procedures. Also, the relatively warmer temperatures and better food sources near the surface water, enhances growth and shortens the production cycle. Oysters grown in suspension generally reach market-size within 3 to 4 years, which is much faster than the 5 to 8 years normally required using the bottom culture method. In addition, oysters grown in suspension have a rounded, more ornamental appearance, thus increasing consumer appeal and market value (Figure 2).
From an ecological point of view, potential advantages of suspended oyster culture include providing additional habitat for native fish and invertebrate species. Filter feeders, such as oysters, also play a key role in encouraging the growth of vegetation (e.g., eelgrass) and improving water quality in aquatic ecosystems. On the other hand, the removal of phytoplankton by farmed oysters should not surpass the capacity of the ecosystem to replenish the supply, since this could result in adverse conditions for wild shellfish beds.
The conversions from traditional bottom leases to suspended leases within the Foxley/Trout River system (an intensive culture area where oyster leases cover 22% of the bay) raised some concerns amongst regulators and stakeholders and pointed to the need for new information on the potential ecological impacts of increasing the number of oysters (Crassostrea virginica) being grown higher up in the water column. Fisheries and Oceans Canada (DFO) funded research under its Program for Aquaculture Regulatory Research (PARR) to develop a bay-scale model that could predict the impacts of converting oyster leases from bottom to suspended culture on food availability in this area.
The research team, led by Dr. Luc Comeau and Rémi Sonier (DFO), designed a three-part study to: 1) compare the stocking density of suspended versus bottom oyster culture in the Foxley/Trout River system; 2) estimate the capacity of these oysters to filter particles from the water column; and, 3) determine bay-scale impacts.
Part 1 - Stocking Density
Researchers began their work in June of 2012 to determine the stocking density of suspended cultured oysters compared with that of the bottom lease culture within the Foxley/Trout River system. This involved counting every bag and cage within the 32 suspended leases. SCUBA divers then surveyed the bottom, collecting oysters within 121 quadrats randomly distributed across bottom leases and wild stock from natural reefs. These oysters were counted and measured to determine the density and composition of bottom populations.
Researchers found that stocking density in the suspended culture (floating bags and cages) was generally lower than in bottom culture and natural oyster beds. The suspended leases are currently being underexploited, suggesting that the industry is still in the development phase. According to Dr. Comeau, "Only a fully exploited suspended lease, one containing floating gear moored according to guidelines throughout its entire area, would compare with a bottom lease in terms of stocking density."
Part 2 - Clearance Rates (CR)
Samples of suspended and bottom oysters were brought into the lab to allow researchers to measure their respective clearance rates Footnote 1 (filtration rates) when feeding on natural phytoplankton assemblages. They also recorded physical differences between these oysters such as their gill size, shell size, and weight (dry tissue weight or DTW).
Results showed that suspended oysters have higher dry tissue weight than bottom cultured oysters of comparable shell height. In spite of this, suspended oysters have significantly lower clearance rates than bottom cultured oysters, relative to their body size.
Using this data, along with the stocking densities, researchers were able to determine that the CR per unit area in the most heavily exploited leases was 66.5 ± 8.5 (floating bags), 86.5 ± 8.6 (floating cages), and 197.3 ± 144.4 (bottom culture) litres per hour per meter square (L h−1 m−2). Hence a conversion to suspended culture, at current stocking densities, could result in an actual reduction in CR compared to traditional bottom culture operations, with the potential to reduce grazing pressure.
Should floating bags and floating cages be maximally stocked, the theoretical maximum CR per unit area is less than both the CR for bottom culture and those calculated for natural oyster reefs.
Part 3 - Bay Scale Impact Assessment
Researchers incorporated the clearance rate information into a bay-scale model to establish the grazing potential of the oysters and quantify the impact of different culture scenarios on available food resources (phytoplankton).
Based on this model, the estimated time for oysters in the lease area (bottom and suspended stocks) to filter all the water in the estuary was 9.8 days. Taking into consideration the tidal action of the area, the research team was able to establish water renewal times in the bay. They estimate that the tides take 2.1 days during spring tides and 4.6 days during neap tides to replace the entire volume of the Foxley/Trout River system. This means that tides replenish the bay 3.4 times faster than oysters can filter out the phytoplankton and that the latter food source is mainly controlled by the tidal regime. If you were to convert all of the leases to suspended leases at current stoking densities, the tides would be 5 times faster at replenishing food than the oysters could filter out the phytoplankton. This bay scale assessment indicates that suspended oyster culture does not limit phytoplankton abundance in the Foxley/Trout River system.
However, a finer-scale model revealed that converting bottom leases to suspended leases may negatively impact phytoplankton resources in the upper bay region, i.e., Trout River (see figure 3). One reason is that most bottom leases in Trout River are no longer being exploited due to an abundance of bottom predators, namely oyster drills and sea stars. Hence converting to suspended culture would result in a net increase in oyster biomass in that portion of the bay. In addition, tides take longer to renew water in Trout River compared with other parts of the bay. For these reasons, a conversion to suspended culture would increase grazing pressure in the upper part of the bay, potentially resulting in localized depletion of phytoplankton stocks.
Results of the study show that, based on data collected in the field and lab, and with model scenarios, the conversion from fully-operational bottom leases to suspended leases actually represents a reduction in oyster grazing potential. "Converting all bottom leases into suspended leases should actually reduce the grazing pressure in the system", said Dr. Comeau.
However, this conclusion does not apply to the upper bay portion of the system, Trout River, where bottom leases are no longer being exploited, natural reefs of oysters exist, and tidal flushing is relatively weak. "The Trout River case is a good example of the complex interactions between natural features, environmental conditions, and human activities in coastal areas" explains research team member Thomas Guyondet (DFO), "It also shows the need for adopting spatially detailed approaches to sustainably manage these regions."
Next steps in this research include further studies in the Trout River area: "We are continuing our work in Trout River to examine and compare the specific diets of bottom and suspended oysters and how they overlap." said PhD candidate Rémi Sonier (DFO). "How quickly phytoplankton cells multiply in Trout River is another piece of information we hope to integrate into the models."
This research provides an initial assessment of the Foxley/Trout River system and the potential impact of converting oyster leases from bottom to suspended status as well as having both types of oyster leases within the same bay area. Preliminary results have been presented to the PEI Lease Management Board for their consideration. Final data and analyses of the research will provide a scientific base for their decisions about whether to grant the lease conversions and allow for fine-tuning of siting decisions leading to effective spatial planning of aquaculture operations within the Foxley/Trout River system.
Research under the Program for Aquaculture Regulatory Research provides essential information to managers and regulators for science-based decision making in support of a sustainable aquaculture industry in Canada.
Research Team: Thomas Landry,Rémi Sonier, Luc Comeau, Thomas Guyondet (DFO) and Réjean Tremblay (Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski)
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