Capture Efficiency of a Boat Electrofisher

Abstract

Models for fish capture efficiency (catchability) using a boat-mounted AC electrofishing sampling protocol were estimated for warmwater fishes in Illinois lakes through a calibration process. Catchabilities were determined for 37 inshore zones in blocked enclosures or ponds and 5 large water bodies during fall or spring. The abundances of vulnerable fish populations were determined by census following draining or by treatment using rotenone or primacord of known catchability. Inshore catchabilities, based on a zone 0-13 m from shore, were strongly dependent on fish length (as a unimodal function), fish taxa, mean depth, and surface macrophyte cover. Under average environmental conditions, maximum catchabilities by taxon ranged from 0.0018 to 0.14 and ranked (highest to lowest) as follows: largemouth bass ? Micropterus salmoides, common carp Cyprinus carpio, crappies Pomoxis spp. in spring, shad Dorosoma spp., bluegill Lepomis macrochirus, green sunfish L. cyanellus, crappies in fall, freshwater drum Aplodinotus grunniens, suckers (Catostomidae), and catfish (Ictaluridae). Catchabilities for common carp and shad were significantly lower in large water bodies, indicating that significant numbers were outside the inshore zone. However, the results for other species, including large samples of largemouth bass and bluegills, indicated that populations in similar whole lakes could be estimated from the predicted inshore electrofishing catchabilities in fall or spring. Strong biases in relative population density when inferred by catch per unit effort were demonstrated under differing environmental conditions. Also, estimates of ratios related to fish size, such as mortality rates, were seriously biased when based on the size structure of uncorrected catches. Therefore, catchability models are considered essential for assessing the absolute and relative attributes of fish populations across water bodies.