Ecological uniqueness and species contribution to beta diversity differ between fishes and crustaceans’ bycatch in subtropical shallow marine ecosystems

We assessed the ecological uniqueness of fish and crustaceans in traditional fishing grounds from a tropical shallow marine ecosystem, where bycatch is historically high. Trimestral trawling was carried out between November 2009 and August 2010 in nine sites along 80 km of coastline in South Brazil. We investigated the local (LCBD) and species contribution to beta diversity (SCBD) using beta regression models, disentangling the influence of environmental (water, sediment characteristics) and biotic (S, abundance, diversity, dominance) parameters over LCBD; and species occurrence, total and mean local abundance association with SCBD. The shallow marine areas presented high beta diversity of fish and crustacean. We identified two ecological unique sites for fishes that occurred in colder sites, while the only unique site identified for crustaceans occurred, where crustacean species richness was lower. The ecological unique sites for both taxa were those with lower species richness and abundance, with distinct assemblages’ composition, although the location of the unique sites differed between taxa. Species contribution to beta diversity was mostly driven by species with intermediary–high distribution on the region with high variability in occurrence and abundance. High biodiversity is the general rule when assessing bycatch species, and the singularity of the species composition in the unique sites stems from the occurrence of rare species, which increases the size of the species pool.