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

Field experiments showed that sudden reductions in river flow may cause high 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 (Salmo trutta). The number stranded was estimated by counting the surviving fish collected in a bag as they left the enclosure. In general, a far higher incidence of fish stranding was found during winter conditions (< 4.5°C) compared with the higher temperatures during late summer and early autumn. This is probably mainly because of lower fish activity during the cold season and a substrate seeking behaviour especially during daytime. Stranding was lower at night, probably because of a predominant night active behaviour. Hatchery salmon behaved oppositely to wild fish, and studies based on cultivated fish may give wrong conclusions as to the consequences of hydropeaking. Searching for fish in the substrate underestimated the consequences of sudden flow reductions, as fish were difficult to find. Stranding is not equal to mortality, as fish were found to survive for several hours in the substrate after dewatering. Long shut down procedures of the turbines during daytime, decreased stranding of Atlantic salmon (7-9 cm) drastically under spring conditions. Temperature, season and light conditions have the most pronounced effect on stranding of juvenile salmonids. It is possible to reduce stranding by taking into account these ecological considerations during hydropeaking operations.