Climate-induced changes in river hydrological and thermal conditions in a large basin: implications for diadromous fish migration

Abstract

Climate-induced changes in river conditions, such as water temperature and current velocity, can disrupt species migration patterns by altering the conditions encountered during migration. We assessed these impacts across the Loire River basin in Europe by compiling long-term fish passage data for three diadromous fish species at fishways, alongside reconstructed time series of water temperature and current velocity. We considered both water temperature and current velocity on days when migration was observed as "suitable conditions" for the species' upstream migration. Our findings for the 1963-2019 period revealed that trends in the frequency of suitable conditions for upstream migration were species-specific. For Atlantic salmon, particularly fall migrants, the frequency of suitable conditions has declined over time in certain areas. In contrast, the frequency of suitable migration conditions for both allis shad and sea lamprey has shown an overall increase across the basin. This study offers policy-makers and environmental managers valuable insights for developing effective, spatially explicit strategies to preserve diadromous fish populations amid ongoing climate change.