Effect of varying dietary protein level in different families of rainbow trout

Groups of rainbow trout (Salmo gairdneri) fingerlings from five different families and five inbred groups were fed for 153 days on each of four diets similar in energy content, but differing in their percentage of protein. The protein contents were 24, 33, 42 and 51%, respectively. Significant differences between fish families were found for growth, condition factor, chemical composition of the carcass, and digestibility of protein. At the end of the experiment, interaction between diet fed and family was found close to significance for weight and relative growth rate, and significant for length and condition factor. This interaction indicates that prospects for selectively breeding strains of rainbow trout specifically better able to utilize protein are promising.Fish growth rate increased with increasing dietary protein content. The corresponding condition factor, content of dry matter and energy in the carcass decreased. The high protein diets gave more protein in the carcass and a more discoloured liver than the low protein diets. Apparent protein digestibility increased with increasing protein level.