Effects of fish size and ration level on the growth and food conversion efficiency of rainbow trout, Salmo gairdneri Richardson

The growth rates of rainbow trout, Salmo gairdneri, which ranged in size from 0.6 to 5.2 g and were provided rations of three different sizes, were measured during a 15‐day study. The relative maintenance rations (food/gram of fish at zero growth rate) decreased with increases of fish size. Consequently when fish were kept at low ration levels the growth rates and gross efficiencies of food conversion of ‘large’ fish were greater than those of ‘small’ fish. However, as ration level increased, the effects of fish size grew at the same rates or slower than small fish. With increases of ration to the highest levels (9–12%/day) gross efficiency values for large fish began to decline while those for small fish continued to increase.