Do salmon aquaculture sites alter wild fish communities in the Broughton Archipelago?




Stabel, Dane

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Open net-pen aquaculture allows for free exchange of materials between farm and wild environments. Increased habitat complexity in the form of farm infrastructure produces effects similar to fish aggregating devices and artificial reefs, altering the distribution and abundance of fish species within the greater area. The continuous input of nutrients via fish waste and uneaten food pellets can amplify such effects, leading to large and persistent aggregations of wild fish near aquaculture sites. These aggregations have been quantified in numerous geographical locations but data are lacking for salmon farms in coastal British Columbia. The footprint of the attractive effects is also poorly understood in all cases as research has focused on fish populations directly associated with the infrastructure. In this study wild fish populations were quantified at shallow rocky ecosystems adjacent to salmon farms in the Broughton Archipelago and compared to paired reference sites with similar habitat characteristics to test for aggregating effects. Two SCUBA divers performed visual surveys along six 25x4x4m transects at each site, three at each of two depth ranges: deep (12-16m) and shallow (6-10m). Species, abundance, and estimated total length, as well as temperature, salinity, rugosity, and visibility were examined. A combination of multivariate and univariate statistical analysis were performed to compare the physical characteristics, community composition, number of individuals, and biomass between farms and reference sites. The overall community composition was significantly different at farm sites despite no difference found in the physical habitat characteristics between treatments. This difference was predominantly driven by five fish species, yellowtail rockfish (Sebastes flavidus), copper rockfish (Sebastes caurinus) quillback rockfish (Sebastes maliger), shiner perch (Cymatogaster aggregata) and striped perch (Embiotoca lateralis). Presence/absence data showed no significant difference in species identity between farms and references, implying that the difference in community composition was primarily driven by changes in abundance rather than species identity. The total number of fish and biomass of all fish species was also significantly higher at farms. These results suggest that salmon farms in the Broughton Archipelago alter the community structure and increase the abundance of near-field wild fish populations. The aggregating effects are asymmetric within the community, with higher trophic level species showing the greatest increases in abundance. The potential implications of these results include a greater risk of disease and parasite transfer between farm and wild fish, as well as within each population. The ecological relationships among species may also be compromised with predator amplified communities potentially exhibiting top-down effects on the rest of the food web. Changes to wild fish communities seen up to 170m away from farm infrastructure suggest that the mechanism of the attractive effects may be driven more by nutrient subsidies than the presence of infrastructure. This work underscores the need to determine the full spatial extent and mechanisms of attractive effects as well as the ecological implications of persistent aggregations of wild fish near salmon farms.



Aquaculture, SCUBA, Marine Fish