Predation and dietary habits as possible explanations for poor return rates of stocked sea trout from the river Aurland

Abstract

The river Aurland was earlier known to be the best Norwegian river for sea trout fishing. Today, after regulation for hydroelectric purposes, the catch has fallen dramatically despite releases of hatcheryreared sea trout smolts and a remaining natural production. A field experiment in May–June 1991 indicated heavy predation on smolts leaving the river. In the river Aurland the water discharge during the smolt migration period is greatly reduced as compared to the situation prior to regulations. Other authors have shown that large water discharge during smolt migration correlates with a high return rate of adult salmon, presumably because of reduced predation pressure on the smolts. Results are also presented indicating poor feeding success of reared smolts compared to wild smolts, which may affect marine survival.