Population dynamics of juvenile steelhead trout in relation to density and habitat characteristics

Steelhead fry (Salmo gairdneri) were outplanted into a fishless stream in the spring of four years (1977-78 data from Reeves 1979; 1978-79 data from Cowles 1980; 1980-81 and 1981-82 data from this report). Initial population sizes were 5,000 fry (1.9-2.2 fry/m2) in 1977, 1978, and 1980, and 2,500 fry (0.8 fry/m2) in 1981. Fry emigration, survival, growth, production, and habitat selection was monitored each year. Comparative survival and growth of fry reared in a gravel and a non-gravel environment was investigated in 1980. Four major periods of fry emigration were observed in the East Fork of the North Fork, Mad River: 1) post-stocking or post-emergence, 2) early summer, 3) fall/winter, and 4) second-spring (yearling). Summer survival of steelhead fry was believed to be most influenced by the duration and the minimum level of the low-flow period. Winter survival was thought to be influenced by fish density in the fall. Fish growth and production was rapid in early summer, slowed to nearly zero over the low-flow period, increased immediately after fall rains (probably slowing at some point in mid to late winter), and increased rapidly during the second-spring. A significant (P 0.01) inverse relationship was found between fry density and stream width, instream cover, and water velocity. Stepwise regression models predicted 57% to 78% of the observed variation in fish populations. Fry that hatched and emerged from a gravel environment exhibited significantly (P 0.001) higher survival after one year of stream residence than did fry reared in a non-gravel environment. No difference in growth was observed between gravel and non-gravel reared fry.