We studied the effects of water temperature, density as measured by density index (DI), and food deprivation (alternating cycles of daily feeding and fasting) on dorsal fin erosion of steelhead Oncorhynchus mykiss through a factorial experiment lasting 22 weeks. Dorsal fin index (DFI) correlated inversely with age, length, and weight of fish and was significantly affected by crowding (i.e., DI). Increasing DI (from 0.0 to 0.5) also improved feed conversions and slightly depressed survival, but DI had no significant effects on weight gain, condition (K ), or carcass composition. Water temperature (10ЊC or 15ЊC) had a strong and significant effect on DFI; colder water improved DFI while slowing body growth. Low temperature treatments improved survival slightly and altered carcass composition by increasing moisture at the expense of fat and protein.The effects of density and temperature were also strongly interactive. Food deprivation slightly improved final DFI at low DIs, but greatly reduced the average size of fish affected by severe fin erosion. Food-deprived fish exhibited carcass composition trends similar to fish reared in 10ЊC water. Feed conversion improved significantly with alternating fasting and feeding cycles. Histological evaluation proved inconclusive for physical fin nipping evidence or for the presence of detrimental microorganisms.

Abstract. Dorsal fin condition was evaluated in two groups of hatchery‐reared steelhead trout, Oncorhynchus mykiss(Walbaum), and compared to wild fish. Hatchery fish were reared either in nursery tanks with baffles or in isolation (1 fish/rearing space). Evaluations were conducted to determine differences in dorsal fin condition between these groups and to monitor the progression and severity of fin damage. Isolated hatchery and wild steelhead trout maintained perfect dorsal fins with characteristic white margins. Hatchery fish reared in nursery tanks with baffles experienced dorsal fin erosion that started when the fish were about 40mm total length (30 days after being on feed). This erosion continued until dorsal fin length averaged only 2·3 mm in fish 161–200 mm total length in contrast to an average dorsal fin length of 20·9mm in wild fish of the same size.

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