An Evaluation of Multipass Electrofishing for Estimating the Abundance of Stream-Dwelling Salmonids

Abstract

Failure to estimate capture efficiency, defined as the probability of capturing individual fish, can introduce a systematic error or bias into estimates of fish abundance. We evaluated the efficacy of multipass electrofishing removal methods for estimating fish abundance by comparing estimates of capture efficiency from multipass removal estimates to capture efficiencies measured by the recapture of known numbers of marked individuals for bull trout Salvelinus confluentus and westslope cutthroat trout Oncorhynchus clarki lewisi. Electrofishing capture efficiency measured by the recapture of marked fish was greatest for westslope cutthroat trout and for the largest size‐classes of both species. Capture efficiency measured by the recapture of marked fish also was low for the first electrofishing pass (mean, 28%) and decreased considerably (mean, 1.71 times lower) with successive passes, which suggested that fish were responding to the electrofishing procedures. On average, the removal methods overestimated three‐pass capture efficiency by 39% and underestimated fish abundance by 88%, across both species and all size‐classes. The overestimates of efficiency were positively related to the cross‐sectional area of the stream and the amount of undercut banks and negatively related to the number of removal passes for bull trout, whereas for westslope cutthroat trout, the overestimates were positively related to the amount of cobble substrate. Three‐pass capture efficiency measured by the recapture of marked fish was related to the same stream habitat characteristics that influenced (biased) the removal estimates and did not appear to be influenced by our sampling procedures, including fish marking. Simulation modeling confirmed our field observations and indicated that underestimates of fish abundance by the removal method were negatively related to first‐pass sampling efficiency and the magnitude of the decrease in capture efficiency with successive passes. Our results, and those of other researchers, suggest that most electrofishing‐removal‐based estimates of fish abundance are likely to be biased and that these biases are related to stream characteristics, fish species, and size. We suggest that biologists regard electrofishing‐removal‐based estimates as biased indices and encourage them to measure and model the efficiency of their sampling methods to avoid introducing systematic errors into their data.

Evaluation of an Unsuccessful Brook Trout Electrofishing Removal Project in a Small Rocky Mountain Stream

“…Previous studies (Riley and Fausch 1992;Peterson et al 2004b;Rosenberger and Dunham 2005) have demonstrated that electrofishing removals can produce biased estimates of abundance because CP is lower than what is actually measured, producing abundance estimates that are too low, and in our case, removal efficiency estimates that were probably too high. We have no way of knowing true CP in this study.…”

“…Although CP from multipass removals averaged 0.78, we realized that this was probably an overestimate (Riley and Fausch 1992;Peterson et al 2004b;Rosenberger and Dunham 2005), which would in turn cause underestimation of population abundance and overestimation of removal efficiency (especially for age-0 fish). Estimates of abundance should therefore be considered minimum estimates, and estimates of CP and removal efficiency should be considered maximum estimates.…”

“…We have no way of knowing true CP in this study. Peterson et al (2004b) found that the removal method overestimated CP by 39% and underestimated abundance by 88%, but they used straight DC to minimize fish injury, which most likely reduced capture efficiency. In this study, pulsed DC with relatively high voltage and pulse rates was used to maximize CP, but nevertheless our associated removal efficiency estimates were probably still positively biased to some degree.…”

Polymorphic mountain whitefish (Prosopium williamsoni) in a coastal riverscape: size class assemblages, distribution, and habitat associations

“…Habitat size and complexity, fish species and size, density, and the intensity of sampling effort may affect the efficiency of snorkeling estimates of fish abundance (Riley and Fausch 1992, Rodgers et al 1992, Bayley and Dowling 1993, Peterson et al 2004). .…”

Modeling Management Scenarios and the Effects of an Introduced Apex Predator on a Coastal Riverine Fish Community

“…Removal estimates generally do not account for heterogeneity in capture probability, which can lead to an underestimate in population size (Peterson et al 2004). This underestimate in population size could lead to exaggerated effects of flathead catfish predation on native fish functional groups and imprecise or inaccurate predictions of possible management actions related to flathead catfish removal programs.…”