The use of meiofauna as an indicator of fish-farms impact: a case study from the middle eastern Adriatic Sea

This investigation aimed to analyze for the first time the impact of fish farming on the biochemical composition of sedimentary organic matter and meiofaunal assemblages on a sea bass/sea bream farm located in the central Croatian waters. We compared two aquaculture sites: one near the cages (ST 1) and one directly beneath them (ST 2), along with a control site (Control) located 700 m away. Our findings revealed significant organic matter enhancement in the sediments surrounding the cages, leading to increased total meiofaunal abundance but decreased diversity. Furthermore, meiofaunal community structure differed among aquaculture and control sediments, particularly in terms of rare taxa (less than 1 % abundance). Increased Nematoda abundances contrasted with the decline or absence of more sensitive taxa like Copepoda and Kinorhyncha near the fish farm. These results highlight the importance of considering meiofaunal rare taxa, such as Kinorhyncha, for better understanding benthic responses to increased anthropogenic impacts and support the suggestion of other authors to use the nematodes to kinorhyncs ratio as a useful indicator of organic enrichment associated with aquaculture activities. ● Organic matter enrichment was found in the fish farm sediments. ● Meiofauna showed increased abundance and decreased diversity under the cages. ● Community structure revealed differences between fish farm and control station. ● Dominance of nematodes increased from the outside to the inside of the cages. ● Gradual disappearance of kinorhynchs was evidenced under the fish cages.