Sea cage aquaculture affects distribution of wild fish at large spatial scales

Aquaculture sea cages are fixed in space and inadvertently provide food to wild animals that is stable through time. We measured the effect of these novel and highly predictable resource patches on the distribution of wild fish across large spatial scales along the south coast of Newfoundland, Canada. Randomized stratified hydroacoustic surveys were used to compare the distribution and abundance of wild fish in bays that contained Atlantic salmon (Salmo salar) farms with control bays. Control bays were areas with no history of salmon farming but have been selected for future use by the industry. We found that measures of total area backscatter (nautical area scattering coefficient, NASC) were significantly greater in bays that contain salmon farming compared with control locations. The mean NASC in farmed bays was not significantly different from mean NASC measurements taken directly adjacent to sea cages. Variability around mean NASC estimates could not be explained by the quantity of feed available to consumers, when the number of sea cages in a farm site was used as a proxy for feed availability. Our results suggest that individual-level consumer responses at sea cages can be transmitted across larger spatial scales.