Fish communities and river alteration in the Seine Basin and nearby coastal streams

Spatial variation of quality of fish communities in the whole Seine basin a nd nearby coastal streams were examined by the Index of Biotic Integrity (I BI). The relationship between quality of fish communities and river alterat ion was also studied. A trend of fish community degradation was found from the periphery to the centre of the basin and from upstream to downstream. T his trend is conform with the gradient of anthropic pressures observed in t he catchment. Variations in IBI scores were significantly related to water quality, diversity of physical habitat and substrate clogging. In headwater s, effects of water quality and substrate clogging were predominant, sugges ting that fish community quality is first linked to land use in the catchme nt. Downstream, IBI scores were mainly linked with diversity of habitat sug gesting that fish community quality principally depends on direct river bed alterations. Relationships between each metric of the IBI and the river fe atures tended to show that reduction of habitat heterogeneity first reduced species richness, whereas degradation of water quality first affected the functional structure of the fish community. These results provide useful el ements to develop a global rehabilitation of fish communities at the whole basin scale.