Spatial, Seasonal, and Size-dependent Variation in the Diet of Sacramento Pikeminnow in the Eel River, Northwestern California

Weexamined the food habits of 2,077 Sacramento pikeminnow (46550 mm standard length [SL]) collected from the mainstem Eel River and South Fork Eel River, California in 1995-1997. Fifty-eight percentofthe fish contained prey in the upperdigestive tract. Sacramento pikeminnow consumed awidevarietvof orev: thediversitvofindividualdiets was hiaher from~ebruary-~aycoipa;edid~uly-~ctoderand higherinthe~outhiork Eel Riverthan in the mainstem Eel River. The proportion offish in thediet increased with Sacramento pikeminnow length but dietdiversity did no t Sacramento~ikeminnowcollectedfrom theSouth ForkEelRiver inthelate season contained the highest proportion offish. Predator and prey size were also positively related. In general, Sacramento pikeminnow preyed on lamprey and fishes in proportion to their availability, as estimated by electrofishing collections, buttendedtoavoidcannibalism. Sixpercentof thesacramento pikeminnow containingfood in the upper digestive tract had consumed juvenile salmonids. Sacramento pikeminnow were not more abundant where salmonids were present. Predation risk for salmonids may be most significant where the species occur together during low, clear water conditions. For example, 44% of 43 Sacramento pikeminnow>250 mm SLfrom our upstream-most collecting siteon the South Fork Eel River in August 1995contained salmonids. In mostof our studyarea, predation riskfor outmigrating salmonids may be reduced by hiah turbiditvand the availabilitv of alternative Drev. However. strona c~nclusion~bouttheabilitvof &cramento ~ ikemi inow in the Eel ~ i v e i to reduce salmonid populahons will require'information on food habits when juvenilesalmonids are abundant, and population estimates for the predator.