Invertebrate drift and feeding habits of juvenile chinook salmon in the upper Sacramento River, California

The diurnal feeding habits of juvenile chinook salmon, Oncorhynchus tshawyt scha, rearing in the upper Sacramento River from April to June 1996 were ex amined in relation to food availability, Daily mean drift densities in the Sacramento River ranged between 211 and 2,100 organisms/100 m(3). Aquatic t axa, especially Chironomidae (Diptera) and Baetidae (Ephemeroptera), domina ted drift samples and salmon stomach contents. Selection of prey by juvenil e chinook salmon appeared to be based on size, abundance, or conspicuousnes s of organisms, as indicated by linear selection indices. The gape width of juvenile salmon increased predictably as the fish grew in length, but the sizes of prey consumed fell considerably short of gape potential due to the relative rarity of large prey in the environment. Mean stomach fullness wa s 2.4% of fish weight, indicating reasonable feeding opportunity during spr ing of wet years similar to 1996.