Potential impacts of finfish aquaculture on eelgrass (Zostera marina) beds and possible monitoring metrics for management: a case study in Atlantic Canada

Eelgrass (Zostera marina) has been designated an Ecologically Significant Species in Atlantic Canada. The development and rapid expansion of netpen finfish aquaculture into sensitive coastal habitats has raised concerns about the impacts of finfish aquaculture on eelgrass habitats. To date, no studies have been done in Atlantic Canada to examine these impacts or to identify potential monitoring variables that would aid in the development of specific conservation and management objectives. As a first step in addressing this gap, we examined differences in environmental variables, eelgrass bed structure and macroinfauna communities at increasing distances from a finfish farm in Port Mouton Bay, a reference site in adjacent Port Joli Bay, and published survey results from other sites without finfish farms along the Atlantic Coast of Nova Scotia. Drawing on research done elsewhere and our results, we then identified possible metrics for assessing and monitoring local impacts of finfish aquaculture on eelgrass habitats. Our results suggest some nutrient and organic enrichment, higher epiphyte loads, lower eelgrass cover and biomass, and lower macroinfauna biomass closer to the farm. Moreover, community structure significantly differed between sites with some species increasing and others decreasing closer to the farm. Changes in the macroinfauna community could be linked to observed differences in environmental and eelgrass bed variables. These results provide new insights into the potential impacts of finfish aquaculture on eelgrass habitats in Atlantic Canada. We recommend a suite of measures for assessment and monitoring that take into account response time to disturbance and account for different levels of eelgrass organizational response (from physiological to community).

Sea-cage aquaculture impacts market and berried lobster (Homarus americanus) catches

Sea-cage finfish aquaculture frequently spatially overlaps and competes with traditional fisheries and ecologically important habitats in the coastal zone. Yet only few empirical studies exist on the effects of sea-cage aquaculture on commercially important fish and shellfish species, due to the lack of data. We present results from a unique collaboration between scientists and lobster fishers in Port Mouton Bay, Atlantic Canada, providing 11 yr of market (market-sized) lobster catches and berried (ovigerous) lobster counts in 5 spatially resolved areas adjacent to a sea-cage finfish farm. The time series covered 2 stocked (feed) and 2 non-stocked (fallow) periods, allowing us to test for the effects of feed versus fallow periods. Our results indicate that average market lobster catch per unit effort (CPUE) was significantly reduced by 42% and berried lobster counts by 56% in feed compared to fallow periods. Moreover, both market and berried lobster CPUE tended to be lower in fishing region 2, which included the fish farm, and higher in region 5, furthest away from the farm. Bottom temperature measurements in one region suggest that differences in CPUE between feed and fallow periods were not driven by temperature, and that berried lobsters may be more sensitive to both aquaculture and temperature than market lobster. We discuss possible mechanisms of how finfish farms as well as other abiotic and biotic factors such as habitat quality and temperature could affect lobster catch. Our results provide critical information for the management of multiple human uses in the coastal zone and the conservation of shellfish habitats that sustain traditional fisheries.