THE EFFECT OF DIET ON DORSAL FIN EROSION IN STEELHEAD TROUT (ONCORHYNCHUS-MYKISS)

A 2x2 factorial experiment of diet type (krill vs fish meal) and stero id supplementation (0 vs 30-mu-g 17-alpha-methyltestosterone kg(-1)) w as conducted to determine the effects on dorsal fin erosion in steelhe ad trout. Triplicate tanks of 250 fry were fed one of the four diets a t a rate calculated to produce 115 g fish in 34 weeks. Fish were trans ferred to larger tanks when mean density index reached 0.40. Dorsal fi n index (DFI, measure as mean dorsal fin height x 100/total fish lengt h) was greater (P less-than 0.001) among fish fed krill-based diets th an for fish fed fish-based diets at weeks 12, 22, and 34 of the trial. Added testosterone decreased (p=0.04) DFI among fish fed the krill di et at week 12 but otherwise has no effect on fin condition. Addition o f testosterone to either diet type decreased (P=0.02) critical thermal maximum, which is a measure of fish resistance to thermal stress. The results suggest that diet composition can influence the rate of dorsa l fin erosion in steelhead trout through a metabolic, behavioral, or c ombined change.