Impact of cage aquaculture of cobia (Rachycentron canadum) on nematode assemblages in the tropics

Abstract The increased demand for food for human consumption has led to an exponential growth of aquaculture in the marine environment. This growth has raised concerns about environmental health, as fish farming can cause organic enrichment and eutrophication in sediments and, consequently, alter the benthic biota. This study evaluated changes in the structure of nematode assemblages under the influence of a cobia farm sited on the shallow continental shelf in an oligotrophic area off northeastern Brazil. Two and eight months after the beginning of the 9-month fish rearing cycle (S1 and S2, respectively), and two months after harvest (S3), sediment and bottom water samples were taken at distances of 0, 30, 80 and 100 m away from the cages, and at a reference site 200 m from the cages. Univariate indexes for the nematode genera and multivariate structure of assemblages in spatial and temporal scales were investigated through PERMANOVAs. The temporal Beta diversity index was calculated comparing distances through time. Significant changes were found in the assemblages along the rearing cycle, namely increased abundance, low diversity, and evenness. Changes in dominance of the genus Chromaspirina on the Epsilonematidae and Draconematidae families and, changes in the dominant trophic group were observed between S1 and S2, the period when the largest amount of feed had been offered to the fish. No clear trend in spatial scale was revealed except by losses of genera below the cage and gains of genera in the furthest distance station between S1 and S2. Higher nitrogen content and pH values in S2 indicates that nutritional enrichment were the main drivers of change although a DistLM analysis evidenced low percentage of explanation of assemblage patterns by the measured environmental parameters. The fish farm in the oligotrophic and dynamic tropical region studied led to changes in nematode assemblages.