Environmental drivers of fish bycatch composition in small-scale shrimp trawling along the southern Brazilian coast

Small-scale shrimp trawling is crucial for the economy and culture of coastal communities worldwide, providing several ecosystem services. However, bottom trawling is well-known for its negative impacts on habitat structure and marine communities. Bycatch, or the accidental capture of non-target species during fishing, can offer valuable insights into the composition and variation of fish assemblages. This study was conducted between 2009 and 2010 and focuses on the Barra do Sul, Penha, and Porto Belo areas, which are traditionally used for artisanal shrimp fishing. We evaluated the variation in the composition of fish species caught as accompanying fauna, as well as the influence of environmental variables on the associated fish assemblages. The results indicate that differences in species composition were more pronounced spatially than temporally, suggesting that the structural characteristics and environmental filters of different study areas play a crucial role in shaping marine fish bycatch assemblages. Porto Belo showed higher abundance, biomass, and diversity, likely due to its greater substrate heterogeneity and habitat complexity, which promote higher environmental quality and species richness. The gam models used for abundance and diversity confirm the complexity of ecological interactions and highlight that depth, salinity, temperature, chlorophyll a and nutrient concentration (NO₂ and PO₄³-) were key environmental variables in determining the abundance and diversity of fish assemblages, indicating that the response of these communities to environmental variations is influenced by a specific combination of these factors. Additionally, species from the Sciaenidae and Paralichthyidae families were identified as significant contributors to the variation in fish communities. The presence of threatened species, such as Zapteryx brevirostris and Atlantoraja cyclophora, in the bycatch is particularly concerning for conservation efforts. Finally, our findings emphasize the importance of implementing management strategies tailored to local environmental conditions and protecting endangered species to promote sustainable management of marine resources and mitigate the impacts of trawling on a global scale.