Consumption, Growth, and Potential Competition Between Juvenile Chinook Salmon and American Shad in Columbia River Food Webs

The Columbia River has been colonized by an ever increasing number of nonnative species raising concerns that competition for food may be limiting the growth of juvenile Salmon, many of which are listed under the ESA. In particular, fall Chinook Salmon (Oncorhynchus tshawytscha) that migrate through the Columbia River later in the season face potential competition from many planktivores, including nonnative juvenile American Shad (Alosa sapidissima), which are very  abundant and also migrate seaward in mid- to late- summer. For competition to exist between Shad and Salmon, food must be limiting, that is, at least one of the competitors needs to display decreased fitness. We analyzed the stomach contents of juvenile Salmon and Shad during mid-July and early August, 2013 in John Day Reservoir to assess prey importance, the degree of prey overlap, and to estimate consumption by juvenile Salmon from empirical field trials conducted in John Day Reservoir, Columbia River. Lastly, we used a Salmon bioenergetics model to estimate juvenile Salmon growth during these two periods using empirical prey percentages and consumption estimates derived from Salmon migrating through John Day Reservoir. Our data suggest that food may be limiting the growth of later migrating salmonids in the Columbia River.