A Square-Metre Electrofishing Sampler for Benthic Riffle Fishes

Abstract

We describe the design and field-testing of a pre-positioned 1-m² electrofishing sampler with a novel electrode configuration. The sampler produces relatively uniform voltage gradients (0.36-7.00 mean volts/cm: water conductivity = 100 μMHO, 153 VAC, 600 W) that are sufficient to stun fish and are largely restricted to the sampling frame. Voltage gradients averaged 0.1 mean volts/cm 10 cm outside the frame regardless of water conductivity and declined exponentially with increasing distance from the frame. We compared the effectiveness of our sampler with adjacent kick-seine samples in the upper, middle, and lower regions of a riffle in Green River, Kentucky, using stratified random sampling. Species rank concordances were statistically similar regardless of method or habitat; both methods yielded similar information on benthic species composition. Statistically similar estimates of benthic fish abundance and number of benthic fish species were derived regardless of habitat or method. The frame sampler was more precise than kick-seining for estimates of fish density only in the upper riffle, a habitat characterized by relatively shallow water and small, more uniform substrate than found in other sampled habitats. Fewer frame samples (9) than kick-seine samples (17) were required to characterize species richness of the benthic fish fauna. In each habitat, more benthic species were collected using electrofishers than with kick-seines; Shannon's species diversity and evenness were larger when estimated from electrofishing samples than kick-seine samples in each habitat. We conclude that our sampler allows repeatable, equal-effort sampling of benthic species from well-defined sampling areas, small enough to allow fine-grained measurement of microhabitat. Importantly, this is achieved without producing voltage gradients strong enough to bias adjacent samples.