Diet and size-selective feeding by escaped hatchery rainbow trout Oncorhynchus mykiss (Walbaum)

Escaped hatchery rainbow trout, Oncorhynchus mykiss (Walbaum), at post-smolt (120–340 g) and adult stages (800–3400 g) adapted differently to natural marine prey after escaping from two fish farms in northern Norway. About 1 month after escape (July), more than 57% of the post-smolt fed actively on fish larvae, which contributed 63–75% of the diet by weight. Surface insects were consumed by more than half the post-smolts and represented 24–48% of the diet during the 3-month period of sampling (June–August). One month after escaping, forage ratios (weight stomach/weight fish × 100) exceeded 1, similar to ratios recorded for other wild anadromous salmonid species in the area. Post-smolt weight increased during the sampling period and the condition factor was stable. In contrast, the condition factor of escaped adult fish reduced significantly and the forage ratios were consistently low (0.05–0.77) during the 15 months of sampling (March–August) following their escapement. These fish fed primarily on a variety of different indigestible items (especially particles of seaweed and small pieces of wood) that contributed about 70% of the stomach content weight. They took fish larvae only in July. Although generally contributing little to their overall diet, marine prey of great variety was consumed by the adult fish. The results indicate that young domestic rainbow trout more easily adjust to natural feeding after escape than the older, larger fish, which often fed on indigestible items similar in shape to the commercial pellets to which they were accustomed.