Dispersal, growth and mortality of brown trout (Salmo trutta L.) stocked in a regulated West Norwegian river

As part of a fishery enhancement scheme, 70 000 fingerlings of anadromous brown trout (Salmo trutta L.) were stocked in the regulated River Teigdal, western Norway. To evaluate the stocking programme 10 000 of the hatchery reared fish were marked and released at 10 different localities. The results showed that the hatchery reared fish were inferior to wild brown trout. During the first weeks densities were high in the areas of stocking, varying between 27 and 680 individuals per m2. Stocked fish colonizing adjacent areas were mostly found upstream. Very few fish were found to cross the river from the point of release. One week after stocking food consumption of the hatchery reared fish was low compared with wild trout. Stocked trout had a lower growth rate (0.05 mm d−1) than native fish (0.08 mm d−1). However, fish stocked in deeper, slower flowing areas grew faster than those stocked in shallow riffle areas. The mortality of the stocked trout was 99% from July to April, being highest during the winter. Wild fish survived better, having a mortality of 79% over the same period. The value of the stocking programme is questioned. As an alterative, methods of biotope adjustment are proposed to increase the carrying capacity of wild trout in the river.