Elevated Water Temperature, Crowding, and Food Deprivation Accelerate Fin Erosion in Juvenile Steelhead

Abstract

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 (10°C or 15°C) 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 10°C 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.