Field experiments on stranding in juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) during rapid flow decreases caused byhydropeaking

Field experiments showed that sudden reductions in river flow may cause hig h mortality of juvenile salmonids through stranding. A 75-m(2) enclosure in the drawdown zone of a regulated river was stocked with a known number of wild 0 + and/or 1 + wild Atlantic salmon (Salmo salar) and brown trout (Sal mo trutta). The number stranded was estimated by counting the surviving fis h collected in a bag as they left the enclosure. In general, a far higher incidence of fish stranding was found during winte r conditions ( < 4.5 degreesC) compared with the higher temperatures during late summer and early autumn. This is probably mainly because of lower fis h activity during the cold season and a substrate seeking behaviour especia lly during daytime. Stranding was lower at night, probably because of a pre dominant night active behaviour. Hatchery salmon behaved oppositely to wild fish, and studies based on cultivated fish may give wrong conclusions as t o the consequences of hydropeaking. Searching for fish in the substrate und erestimated the consequences of sudden flow reductions, as fish were diffic ult to find. Stranding is not equal to mortality, as fish were found to sur vive for several hours in the substrate after dewatering. Long shut down pr ocedures of the turbines during daytime, decreased stranding of Atlantic sa lmon (7-9 cm) drastically under spring conditions. Temperature, season and light conditions have the most pronounced effect on stranding of juvenile s almonids. It is possible to reduce stranding by taking into account these e cological considerations during hydropeaking operations. Copyright (C) 2001 John Wiley & Sons, Ltd.