River fragmentation and fish population structure: a comparison of three Swiss midland rivers

Anthropogenic river fragmentation has the potential to alter fish population structure and diversity, but it can be difficult to infer whether observed effects reflect the obstruction of fish movement or population responses to altered flow regimes or habitat structure concomitant with fragmentation. We addressed the influence of habitat fragmentation on the European Chub (Squalius cephalus), a large-bodied fish with strong swimming ability that is a ubiquitous habitat generalist that occurs at high densities in lentic and lotic habitats. European Chub populations have never been stocked and are little affected by habitat alterations. Thus, they are good sentinels for the effects of fragmentation per se. We used microsatellite genotyping of 1726 fish from 38 sites to compare the genetic structure and diversity of European Chub populations from 3 Swiss midland rivers that are unequally affected by man-made fragmentation. The Thur is unfragmented, the adjacent Glatt is strongly fragmented by tall barriers, and the Broye is fragmented by a large number of low barriers that are individually traversable. European Chub from the Thur possessed the highest allelic diversity overall and showed no significant isolation-by-distance and only a weak nonsignificant upstream decline in genetic diversity. European Chub from the Glatt were less genetically diverse and showed significant isolation-by-distance that was explicable by existing barriers and the presence of a lake in the system and a steep upstream decline of allelic diversity. European Chub from the Broye showed no genetic substructure, suggesting that barriers of low height, even when numerous, had little effect on population connectivity. The detectable effects of anthropogenic fragmentation of rivers on a highly mobile fish like the European Chub suggests that less mobile species, such as habitat specialists or small benthic species, are potentially more vulnerable to isolation.