EFFECT OF STOCKING DENSITY ON THE SURVIVAL, GROWTH, AND DISPERSAL OF STEELHEAD TROUT FRY (SALMO-GAIRDNERI)

In a coastal British Columbia [Canada] stream a stocking density of between 0.3 and 0.7 fry/m2 maximized the production of steelhead trout (Salmo gairdneri) parr and smolts. A severe autumn flood doubled the mortality rates of fry stocked at densities of 0.7 fry/m2 or higher but had little effect on fry stocked at lower densities (< 0.15 fry/m2). Overall survival to smolts appeared to be lower than measured elsewhere for wild fish (2 vs. 4.5-18%). The proportion of (< 10%) of surviving fry found below the stocked sections was considered to be an indicator of potential displacement mortality in streams with no vacant downstream areas. This downstream dispersal was not density dependent and was small in comparison with the mortality of nondispersing fish. Although initial fry and parr sizes were density dependent, there were no detectable density effect on older parr or smolt sizes.