Multiple human impacts by the City of Paris on fish communities in the Seine river basin, France

The Seine, which flows through greater Paris whose population is nearly 10 million, is one of the most severely damaged European rivers. Considering s everal published studies, this paper examines the consequences for the fish communities of the development of Paris. Three major impacts are studied: river modifications for navigational purposes, control of river flow by sto rage reservoirs and degradation of water quality downstream of the City. At the basin scale, river modifications to improve navigation greatly affect the species composition of the fauna. Construction of weirs and locks along all the wider reaches of the river and its major tributaries have progress ively caused the extinction of seven migratory species. Conversely, links w ith other catchments through waterways have favoured a colonization of the Seine river basin by several species. Locally, at the reach scale, channell ing leads to homogenization of habitat conditions, which results in a gener al decrease of specialized species. Several storage reservoirs regulate the upper Seine basin to control floods and maintain minimum low-water flow in the lower reaches. Their overall effects on fish communities are still poo rly documented. However, a detailed study reveals their negative consequenc es for the pike populations. Under natural flow conditions, one year out of two was suitable for pike reproduction. Since the development of flow regu lation, suitable conditions for reproduction are only observed in one year out of five. Because of its very large population, the city of Paris dramat ically affects the water quality of the Seine. In the last 30 years, organi c pollution has been strongly reduced. However, at present, water quality d egradation still causes noticeable decrease in fish species richness downst ream from Paris. In summer low-flow conditions, storm events lead to sudden decreases in dissolved oxygen, causing massive fish mortality downstream f rom the city. In such situations, tributaries, side-channels, sandpits and areas just below weirs, constitute valuable shelters for many species.