Analyzing functional diversity to determine the effects of fish cages in insular coastal wild fish assemblages

Abstract

Fish cages attract a high variety of wild fish, and therefore are similar to fish aggregation devices. Aquaculture cages enhance local fisheries, with a more diverse and abundant fish assemblages compared to control stations, not influenced by fish cages. Previous studies have been mainly focused on taxonomic and ecological indices, such as species richness, individual abundances or Shannon-Weaver index. In the present study, we explore if functional diversity may be feasible to understand the consequences of changes in the composition of fish aggregates around cages. We studied fish aggregates from three cages on the coast of Tenerife (Canary Islands, Atlantic Ocean) over several years. All fishes were identified and counted using rapid visual counts (RVCs) in cage and control stations. An increase of species richness and individual abundance occurred in fish aggregates around the studied cages persisting over time. Functional diversity showed increase on cage stations, with higher values of functional evenness and functional divergence relative to control stations. A slight dissimilarity in the taxonomic composition was only observed in the eastern farm. In terms of functional diversity, the studied aquaculture cages exhibited a slight increase in wild fish aggregates over time. These results are similar to previous studies based on taxonomic and ecologic diversity in which a long lasting effect was observed on wild fish aggregates. Thus, aquaculture cages might be considered as a figure of conservation or fishery interest.

Statement of relevance Our results showed a functionally diverse fish community around offshore cages. Thus, aquaculture leases could work as a “reservoir”, promoting the development of diverse fish assemblages, as marine protected areas (MPAs) are designed for.

The development of aquaculture offshore cages might include their positive effects on wild fish aggregates, with a “buffer” fish-ban zone to protect fish stocks from the fishermen.