Accuracy of Removal Electrofishing Estimates of Trout Abundance in Rocky Mountain Streams

Removal electrofishing is frequently used to estimate fish distribution and abundance in streams because it is simple and requires only one visit to a site. However, because the removal method usually overestimates capture efficiency and therefore underestimates fish abundance, some biologists have questioned its use in favor of less biased methods. In southern Idaho streams in the summers of 2006 and 2007, rainbow trout Oncorhynchus mykiss, cutthroat trout O. clarkii, and brook trout Salvelinus fontinalis were captured with backpack electrofishers using pulsed DC and marked and released in blocknetted sites; on the following day, four-pass electrofishing removals were conducted. Removal electrofishing underestimated the abundance of trout 10 cm and larger by 17% (four passes), 22% (three), and 25% (two); for trout less than 10 cm, the respective underestimates were 27, 27, and 37%. Removal estimates were biased in part because capture efficiency progressively decreased for fish 10 cm and larger from 58% in pass one to 37% (two passes), 30% (three), and 18% (four); a similar decline was not as evident for fish less than 10 cm. Increased channel complexity, in the form of boulder substrate, water depth, and stream shading, increased bias in removal estimates. Linear regression models incorporating these and other variables explained 44-67% of the variation in this bias. Visiting new sites in the summer of 2009 with a new field crew produced similar amounts of removal estimate bias, but predictions based on multiple-regression results did not correct the bias any more accurately than did using a mean correction rate from the original sites. Our results suggest that multiple-pass removal sampling in typical Rocky Mountain streams can produce population estimates that are consistently but not drastically biased and are, therefore, probably adequate for most basic fish population monitoring even without correction, especially if electrofisher settings and crew training balance the need to minimize injury with effective fish sampling.