Patterns in Catch Per Unit Effort of Native Prey Fish and Alien Piscivorous Fish in 7 Pacific Northwest USA Rivers

Abstract

ABSTRACTNonnative or alien invasive species are commonly accorded threats to native biological assemblages; however, it is difficult to separate the effects of aliens from other covarying disturbances. We evaluated the effect of alien piscivorous fish on native prey species in seven Pacific Northwest rivers through the use of a spatially balanced random sample of 20 sites on each river. The rivers lacked large main‐stem dams, and point sources, if any, met state and federal water quality standards. Individual sample sites were electrofished a distance equal to 50 times their mean wetted channel widths, and all fish were identified to species, measured, and returned to the rivers alive (except for museum voucher specimens). At nearly all sites in all seven rivers, we found that the catch per unit effort (CPUE) of native prey species varied inversely with the CPUE of alien piscivores. In the two rivers most dominated by alien piscivores, we collected native prey at only 20%–25% of the sites. We conclude that piscivorous alien fish are associated with reduced population sizes of native prey species, at least during the summer low‐flow period, and are potential threats to prey species persistence.