CARRYING CAPACITY ASSESSMENT OF A COASTAL ECOSYSTEM FOR MUSSEL AQUACULTURE

Blue mussel (Mytilus edulis) culture industry in Canada started in the early 1970s and has steadily grown since. Over the last 35 years, a significant proportion of Eastern Canadian mussel production has been from the Magdalen Islands, Gulf of St Lawrence, Canada and has increased by more than a factor of ten. Contrary to shrimp or fish that need to be feed, mussels can grow by extracting plankton directly in the water column (extractive aquaculture). The sustainability of the industry depends on the ability to forecast the impact of mussel farms on the surrounding ecosystem based on knowledge of local conditions and ecosystem vulnerability. The aim of this project was to determine the carrying capacity of the GEL to determine the total bivalve biomass supported by this ecosystem without significant disturbance of the ecosystem.In this presentation I will present the work published by myself and different members of the team from the Institute of Marine Science (ISMER) of the University of Quebec from Rimouski involved in the carrying capacity project of GEL. Monitoring was set up in summer 2003-2004 to assess the water column characteristics (physico-chemico-biological), and the impact of mussel filtration on plankton. The GEL is characterized in summer by: (1) low nutrient concentrations; (2) fairly high primary productivity; (3) dominance by small phytoplankton cells (<10 µm); and (4) high biomass of heterotrophic microplankton. This indicates that the dominant trophic pathway in GEL is the microbial food web. Path analysis (Fig. 1) suggests a negative interaction of mussels on ciliates and heterotrophic nanoflagellates in GEL. Based on clearance rates of mussels in the area, we found that the mussel farm removed 16% of the biomass of ciliates, 8% for phytoplankton and 4% for phytoplankton daily production rates

A calibrated fine resolution physical-biogeochemical model coupled with a dynamic energy budget (DEB) was used to investigate the local and system scale interactions between the mussel farm and the receiving coastal ecosystem. Using a set of published parameters for the DEB, the coupled model reproduces quite accurately both the local mussel growth and its spatial distribution over the farm area. Mussel related process rates are also well reproduced, allowing the study of mussel/environment interactions. The coupled model results show that the mussel stock could be greatly increased before reaching the maximum production capacity of Grande-Entrée lagoon. However, when the ecological aspect is accounted for, using model results along with objective criteria such as the depletion footprint curve, the overall carrying capacity of Grande-Entrée lagoon must be significantly reduced. The coupled fine scale numerical model developed for this study gives the opportunity to assess the ecological carrying capacity of a coastal region for shellfish culture accounting for both local and system scale processes