Dietary overlap of juvenile fall- and spring-run chinook salmon, Oncorhynchus tshawytscha, in Coos Bay, Oregon

During the summer of 1987 in Coos Bay, Oregon, dietary overlap (Schoener index) between juvenile fall-run chinook salmon, Oncorhynchus tshawytscha, and an introduced stock of juvenile hatchery-reared spring-run chinook salmon was high (0.82), indicating the potential for competition for food between these two groups in times of food scarcity. Both groups consumed a variety of prey, including fishes, adult insects, algae, barnacle molts, gammarid and caprellid amphipods, and juvenile decapods. Diets of both salmon groups varied with fish size and capture location. Overlap was low (0.25-0.55) between the smallest juvenile fall chinook salmon (less than or equal to 80 mm FL), for which insects were the predominant prey (26% by weight), and all other length groups of both fall and spring chinook salmon, for which fish were the predominant prey (49%-94% by weight). Dietary overlap between both salmon groups was high in the lower bay (0.82), where fish prey predominated in the diets, and was also high in the mid bay (0.75), where algae and barnacle molts predominated in the diets. Three pieces of evidence suggest that the introduced hatchery-reared spring chinook salmon did not outcompete fall chinook. salmon for food: 1) both the median stomach fullness and the percentage of stomachs containing food was higher for fall chinook salmon than for spring chinook salmon, 2) the median stomach fullness of fall chinook salmon was as high in the period following releases of spring chinook salmon into the bay as in the period prior to the releases, and 3) food of high caloric density (i.e. fish prey) formed an equally high proportion of the diets of both salmon groups, indicating that the quality of food eaten by both was similar.