Effects of an introduced piscivorous fish on native benthic fishes in a coastal river

1. We used field surveys to compare the density and mesohabitat-scale distribution of the native coastrange sculpin (Cottus aleuticus) and the prickly sculpin (C. asper) in coastal rivers in north-western California, U.S.A., with and without an introduced piscivorous fish, the Sacramento pikeminnow, Ptychocheilus grandis. We also measured mortality of tethered prickly sculpin in a field experiment including river, habitat type (pools versus riffles) and cover as factors. 2. Average sculpin density (C. aleuticus and C. asper combined) in two rivers without pikeminnow was 21 times higher than the average density in two rivers in a drainage with introduced pikeminnow. In riffles, differences in the density of sculpins among rivers could be linked to differences in cover. However, riffles in rivers without pikeminnow had an average sculpin density 77 times higher than rivers with pikeminnow, yet only nine times more cover. In pools, cover availability did not differ among rivers, but the density of sculpins in rivers without pikeminnow was 11 times higher than rivers with pikeminnow. 3. In the field experiment, mortality of tethered sculpin varied substantially among treatments and ANOVA indicated a significant River x Habitat x Cover interaction (P < 0.001). Overall, tethered prickly sculpin suffered 40% mortality over 24 h in rivers with pikeminnow and 2% mortality in rivers without pikeminnow, suggesting that predation is the mechanism by which the pikeminnow affects sculpins. 4. The apparent reduction in sculpin abundance by introduced pikeminnow has probably significantly altered food webs and nutrient transport processes, and increased the probability of extinction of coastrange and prickly sculpins in the Eel River drainage.