Comparing the impact of bottom and suspended oyster culture on bay-scale food resources (Foxley/Trout River, PEI)
Bivalves, such as mussels and oysters, are filter feeders that extract naturally occurring food, such as plankton, from the water. While their culture does not require the addition of feed, , their growth depends on the availability of food in the environment. When farming these species, special care must be taken to ensure that the number of cultured animals - does not exceed area's carrying capacity (the number of living organisms that a region can support without environmental degradation). . Exceeding carrying capacity will ultimately result in decreased growth of the cultured animals and could potentially impact other components of the ecosystem.
Oyster (Crassostrea virginica) aquaculture is shifting from benthic (bottom culture) to suspension culture, where the animals are grown in or on structures suspended in the water column where higher growth rates are often observed. The Foxley/Trout River system in Prince Edward Island (PEI) is considered to be one of the more heavily utilized oyster producing areas on the island. Some oyster culturists in this area have experimented with this new approach and are requesting to convert their bottom leases to suspended leases. However, both industry and regulators recognize the need to evaluate the ecological impact of growing oysters in the water column before lease conversions are granted. Since suspended culture holds a greater density of shellfish than bottom culturing, food availability may be an issue if all leases were to become suspended culture.
This project was designed to address the issue of carrying capacity by examining the extent to which the diet of bottom and suspended oysters overlap; comparing the filtration rates of oysters from the two culture types (bottom and suspended); and incorporating this information into a simple bay-scale model to quantify the impact of different culture scenarios on available food resources.
Clearance rates (i.e., the rate at which shellfish remove food particles from the water) relative to dry tissue weight were significantly lower in the suspended oysters than in the bottom cultured category. Maximum clearance rates for fully stocked floating cages and bags were less than clearance rates for both bottom culture and natural oyster reefs. Data also suggested that bivalve clearance rates control phytoplankton availability over tides input. Furthermore, oysters also demonstrated efficient assimilation of small-sized phytoplankton (picophytoplankton) as a food source.
The modelling results showed the maximum changes in seston (i.e., suspended food) were 25% where current suspended leases are fully utilized without conversion and 28% where current suspended and converted leases are fully utilized compared to current conditions. Those results suggest that conversion alone could account for near 3% change and that the majority of the extraction pressure (25%) will come from current suspended leases being fully exploited in the near future. The outcome provides managers and regulators with a quantitative framework for assessing the impact of converting oyster leases from bottom to off-bottom status and having both bottom and off-bottom oyster leases within the same bay area.
Sonier, R. Tremblay, R., Olivier, F., Meziane, T. and Comeau, L. A. 2017. Cultured eastern oysters (Crassostrea virginica): retention and assimilation of picoeukaryotes using multi-biomarker approach. Aquatic Living Resources 30(31):1-13.
Sonier, R. 2017. Rôle nutritionnel du picophytoplancton pour les bivalves d'élevage. Ph.D. thesis. Université du Québec à Rimouski.
Comeau, L. A. 2013. Suspended versus bottom oyster culture in eastern Canada: Comparing stocking densities and clearance rates. Aquaculture 410–411: 57–65.
DFO. 2013. DFO publishes feature story "Assessing Impacts of Converting Oyster Leases from Bottom to Suspended Culture in Foxley/Trout River, PEI". https://web.archive.org/web/20131120154122/http:/dfo-mpo.gc.ca/science/publications/article/2013/10-16-13-fra.html
2012 - 2014
Luc Comeau, Research Scientist, Fisheries and Ocean Canada, Gulf Fisheries Centre, Gulf Region
Claudio DiBacco, Research Scientist, Fisheries and Ocean Canada, Bedford Institute of Oceanography, Maritimes Region
Réjean Tremblay, Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski
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