Effects of a Limited Food Supply on Growth Rates of Coho Salmon and Steelhead Trout

Competition between coho salmon and steelhead trout was studied by comparing specific growth rates of the two species in a laboratory raceway. The raceway was partitioned into six test compartments, each 1.5 m long by 35.5 cm wide; each compartment had flowing water of about 200 liters per minute with temperature averaging 13.9 C and dissolved oxygen 6.5 ppm. Fingerling salmon and trout of similar size were compared, and tests were run on fingerlings 58 to 76 mm in length and also on fingerlings 91 to 105 mm long. The fish were fed the Oregon moist pellet from 5 October to 20 December 1967. Daily feeding rates were kept below optimum and ranged from 1.8% to 3.6% of body weight. When trout and salmon below 84 mm were held together in the same raceway, the trout grew at a significantly faster rate; in other words steelhead trout were dominant over coho salmon in utilizing food from a limited supply. Above 84 mm, the growth rate of the experimental trout was also greater than that of the salmon, although the difference was not significant at the 95% confidence level. Specific growth rates were dependent on the initial sizes of the fish.