Snorkeling as an Alternative to Depletion Electrofishing for Estimating Abundance and Length-Class Frequencies of Trout in Small Streams

Abstract

Trout counts and length-class frequencies determined by snorkeling were compared with estimates determined by electrofishing in 25 reaches of small (≤11-m wetted width) streams of the Bighorn Mountains, Wyoming. The numbers of trout (brook trout Salvelinus fontinalis, rainbow trout Oncorhynchus mykiss, and cutthroat trout O. clarki) counted by snorkeling were highly correlated (r ² = 0. 90–0.99) to depletion estimates of abundance determined by electrofishing, averaging 65% of depletion estimates. Inclusion of instream cover and underwater visibility significantly improved predictions of all salmonid species combined in regression models that predicted depletion estimates by electrofishing from snorkeling counts. Length frequencies obtained by snorkeling and electrofishing were similar when frequencies of trout in three length-classes (0–100, 101–300, and >300 mm) were adjusted (observed length by snorkeling divided by 1.25) for underwater visual magnification. Snorkeling was an alternative to electrofishing for estimating the abundance and length-class frequencies of salmonids in small streams of the Bighorn Mountains because high underwater visibility allowed the snorkeler to view both banks from the center of the stream and because there was only a small amount of complex cover (woody debris or interstitial spaces among the substrate) in which fish could hide.