Optimal Effort Intensity in Backpack Electrofishing Surveys

We evaluated the effect of backpack electrofishing effort intensity on the precision of estimates of fish density in a southern Ontario stream. Single-pass electrofishing was conducted on 30 sites at three electrofishing effort intensities (5, 10 and 15s/m(2)). We found that an effort of 5s/m(2) yielded an average catch that was 78% of the 15-s/m(2) effort. An asymptotic model effectively described the catch-effort relationship for most fish taxa in the stream (i.e., Rainbow Trout Oncorhynchus mykiss, Brown Trout Salmo trutta, sculpin Cottus sp., dace Rhinichthys sp., and darter Etheostoma sp.). Using the catchability parameters of this model, we evaluated the trade-off between sampling many sites at low intensity or fewer sites at higher intensity. The survey design that maximizes precision of fish-density estimates depends on the average time spent traveling between sites and the average area of sites. For southern Ontario streams, where sample sites were approximately 300m(2) and travel time between sites was 75min, the optimum electrofishing effort intensity was approximately 5s/m(2). The applicability of these results to other systems was demonstrated by showing how this optimum intensity was affected by differences in catchability rates of fish and travel distances. These findings will be used to standardize single pass electrofishing catches in nonshield areas of Ontario, and the approach may prove useful in other areas where this fishing technique is effective.