Changes in fish assemblages of a previously highly polluted river: The role of environmental recovery and alien fish invasion in the bormida river (Italy)

The Bormida River in southern Europe has been severely polluted by industrial chemical wastes for many decades, creating a “dead” river. However, since 1990, water quality has rapidly improved, with curtailment and abatement of the chemical manufacturing activities, but there is no information about the recovery of the fish assemblages in this river. Thus, we characterized the changes in the fish community of the Bormida River between 1991 and 2005 and found considerable change. Taxonomic richness, abundance, and biodiversity all increased noticeably, and there was a dramatic increase of allocthonous species. We hypothesize that the recolonization by fishes was due to repopulation from stocks in unpolluted tributary streams and/or from the upstream migration of species from the hydrographic network of the Tanaro River and the Po River downstream.