Relationships between environmental variables and fish functional groups in impounded reaches of the Upper Mississippi and Yangtze Rivers

Abstract

Large rivers throughout the world have been transformed by anthropogenic stressors that are known to influence the structure and composition of fish assemblages. Management of large rivers requires balancing socio-economic and political considerations with biodiversity conservation efforts. By exchanging best management practices between rivers, management efforts can be improved. However, data limitations have largely prevented comparative analyses among fish assemblages in large rivers, potentially limiting the effectiveness of shared management strategies. To improve understanding of the similarities and differences between the Upper Mississippi and Yangtze Rivers, we (1) compared environmental variables and functional traits of fish assemblages between the two systems, (2) identified traits responsible for distinguishing functional groups from one another, and (3) investigated relationships between similar functional groups of fishes and environmental variables to establish expectations for how fish assemblages in large rivers might respond to anthropogenic stressors. Regional species pools in the Upper Mississippi and Upper Yangtze Rivers were characterized by a similar composition of functional traits; the majority of species were omnivorous, had affinities for gravel or sand substrates, and produced sinking eggs. Few species were pelagic, planktivorous, or herbivorous. Functional groups in both rivers were primarily distinguished according to species' trophic habits and substrate preferences, with secondary contributions from species’ water column positions and life history characteristics. Pelagic planktivores and small-bodied guarders with an affinity for structural habitat complexity tended to increase downriver in both systems, in direct association with total phosphorus concentrations, agricultural land use, and temperature. In contrast, proportions of lithophilic species with affinities for gravel or cobble substrates were highest in segments located furthest upriver. By highlighting the sensitivity of different groups of fishes to anthropogenic stressors, we provide insights into the functional ecology of fishes inhabiting the Mississippi and Yangtze Rivers.