Return of adult anadromous allis shad (Alosa alosa L.) in the river Seine, France: a sign of river recovery?

In the past, the diadromous fish fauna of the river Seine comprised 11 native species. From the second part of the 19th century, the progressive modification of the river for navigation purposes and the increase of domestic and industrial pollution led to a general decline of migratory stocks and the extinction of several species. Among them, allis shad (Alosa alosa) was previously widespread in the Seine basin, but disappeared at the beginning of the 20th century. In July 2004, several individuals of shad were observed in the river Seine upstream of Paris (410 km from the sea), one of them was identified as an allis shad that had clearly spawned (thin fish and large scale erosion). A second allis shad was caught in a more downstream location (180 km from the sea) in November 2007 and a spawning mark was found on scales demonstrating clear evidence of reproduction. Recent occurrences of this species are discussed in relation to the present conditions of water quality and migration in the Seine watershed and recent evolution of the species' distribution in the northwestern part of France.