Effect of population densities on survival, growth, and behavior of coho salmon and steelhead trout fry

The fry of the very similar salmonids, coho salmon (Oncorhynchus kisutch) and steelhead trout (Salmo gairdnerii), live in very close association with each other during their first year of life in fresh water. The present study was designed to measure the effects of competition between these species. Populations of different densities of coho and steelhead fry in four identical artificial stream-channels were studied. Observations were made on survival, growth rates, and some aspects of behavior. Among the various groups of fry, survival was apparently dependent upon interspecific factors; the presence of another similar species had no observable effect. Low-density populations survived well, even when another species was present at a high-density. Survival of the fish at high-densities was always depressed, even when the companion species was present at low density. Steelhead fry demonstrated a faster initial growth rate than the coho, enabling them to exceed the coho in growth despite the latter’s earlier hatching and consequent initial size advantage. Growth rates were inversely related to density, both inter- and intraspecific effects being noticeable. The two species tend to be spatially segregated, coho occupying positions in the middle and upper layers of the streams, and steelhead remaining close to the bottom. This stratification was reflected in their feeding behavior and diet. Emigration activity occurred to a greater degree among the coho than the steelhead. Emigrants of both species were observed, to undergo substantial weight loss subsequent to their disappearance from the stream-channels. It was concluded that coho and steelhead fry live in close association with one another without experiencing extensive interspecific competition. This is because of segregation of the species by having differing habitats, feeding habits, growth and survival rates, and consequent population dynamics.