Comparison of Diets of Prickly Sculpin and Juvenile Fall-run Chinook Salmon in the Lower Mokelumne River, California

I compared diets of prickly sculpin, Cottus asper, and juvenile fall-run chinook salmon, Oncorhynchus tshawytscha, in the lower Mokelumne River, California, from January through June during 1998 and 1999. Prickly sculpin fed primarily on chironomid (Diptera) larvae and hydroptilid and hydropsychid (Trichoptera) larvae. Juvenile chinook salmon fed on zooplankton, plus chironomid, hydroptilid, and hydropsychid pupae. Both supplemented their diets with several other prey items, including larval Sacramento suckers, Catostomus occidentalis. Their diets did not overlap significantly any time during the study. Each species fed more as time progressed in both years. A significant relationship between prey item and fish size was observed for juvenile chinook salmon in 1998 and for both species in 1999. The pattern of growth for chinook salmon and prickly sculpin suggests increased feeding might be more related to water temperature than increase in size. I observed no predation by prickly sculpin on juvenile chinook salmon during this study, although sculpin eggs and larvae were infrequently observed in salmon stomachs. RESUMEN Compare las dietas del sculpin espinoso, Cottus asper, y del salmon chinook, Oncorhynchus tshawytscha, juvenil, de corridas de otono en el bajo Rio Mokelumne esde enero a junio de 1998 y 1999. El sculpin espinoso se alimentaba principalmente de larvas de chironomid Diptera, hytroptilid y hydropsychid Tricoptera