Impact de la salmoniculture marine sur l'environnement en rade de Cherbourg (1993-1995) 1. Eau et Pélagos

The environmental impact of a brown trout farm located in the Grande Rade de Cherbourg (Normandy) is reported. During observations the fish stock evolved within the range of 140 to 350 metric tons. Farm influence was researched on water column through hydrochemistry, phytoplankton, faecal bacteria and, on bottom, through grain size, organic matter, metals, macrobenthos. This report only deals with water column. Except ammonia, no hydrological parameter seems to be affected, even near to the net pens. Phytoplankton and coliform bacteria were not altered. An extra concentration of ammonia was detected at some tens meters from the raft, with favourable conditions. But generally it was impossible to observe it unless close to the farming structure, downward the current, the velocity of which was responsible of that (0.55 m/s max. in spring tide). Therefore the impact of this brown trout farm on the water column is very weak, because of the highly dispersive character of the site. Another report deals with the impact on the bottom. (OCR not controlled)