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Keywords:

  • abundance;
  • dams;
  • diadromous fishes;
  • distribution modelling;
  • human population density

Summary

1. Twenty-eight diadromous fish species occurred in Europe, North Africa and the Middle East in historical times. Their current distributions were assessed in terms of abundance classes (missing, rare, common and abundant) in 196 basins ranging from Morocco to northern Norway and from Greenland to Iran.

2. Their current distributions were modelled using abiotic, biotic, climatic and anthropogenic (regional anthropogenic pressures) variables. Anthropogenic variables were derived from characteristics of large dams (height, distance from the outlet, percentage of main stem river available downstream of dam) and human population density. These data were taken from the EEA Eldred 2.08 (European Lakes, Dams and Reservoirs Database) that deals comprehensively with large European dams and includes all obstacles of this type. To deal with ordinal response variables, we applied proportional odds models.

3. Twenty-two species-specific models were successfully built according to the reduction of deviance and the validation process, of which eight included one or more anthropogenic variables. No model could be established for six endemic or highly endangered species such as Acipenser sturio and Coregonus oxyrinchus.

4. Most response curves were easily interpretable since they were related to specific aspects of species’ ecology. Anthropogenic variables related to large dams impacted negatively on the distribution of diadromous fishes through the perturbation of river discharge patterns, the loss of river connectivity and the accessibility to essential habitats, particularly for Salmonid species that spawn in headwater streams. However, one species which can complete its life cycle using only the most downstream part of the basin, Liza ramada, was found to be favoured by the changes in hydrological regime. The bell-shaped curves obtained from human population density for three diadromous species were connected on one side to a common settlement history of human and animal populations and on the other side to negative impacts of human activities arising at high population density.

5. Our approach can provide the basis for identifying special areas of conservation prior to planning restoration programmes at country or basin scales, as well as for more specialized studies focusing on one species only or at the local scale.