Changes in demersal wild fish aggregations beneath a sea-cage fish farm after the cessation of farming

Abstract

Demersal, non‐cryptic, wild fishes were counted in replicate 100 m2 transects beneath a floating sea‐cage fish farm and two nearby sandy locations at Gran Canaria (Canary Islands, eastern Atlantic) four times before and after the cessation of farming. Cessation of farming involved the removal of farmed fishes and ceasing of the daily feeding, although farm structures (cages and moorings) remained. A ‘beyond‐BACI’ sampling design provided the framework to detect the effect of the cessation of farming, which produced qualitative and quantitative changes in the composition and structure of the fishes assemblages beneath the sea‐cage fish farm compared with two nearby controls. The aggregative effect on wild fishes due to the existence of the farm decreased from c. 50 times compared to nearby controls when the farm was in full operation to less than two times when only the farm structures remained. Abundances of particulate organic matter feeders (large‐sized mugilids), large benthic chondrichthyid rays and Pagellus spp. declined markedly at the fish farm after the cessation of farming, suggesting that the removal of daily feeding was responsible for their disappearance. In contrast, abundances of herbivores, benthic macro‐ and meso‐carnivores were similar beneath the fish farm both before and after the cessation of farming. Benthic macro‐carnivores, however, were more abundant beneath the sea‐cages compared to control locations, supporting the hypothesis that the increase in the physical structure beneath farms plays a role in aggregating these species. Sparids occurred beneath the sea‐cages only after the cessation of farming, while the two natural control locations did not show differences from before to after the cessation of farming. Overall, the results show that the wild fish assemblage beneath the farm partially changed after the cessation of farming to a more natural state, approaching the assemblages observed at the control sites.