Capture Efficiency of Blue Catfish Electrofishing and the Effects of Temperature, Habitat, and Reservoir Location on Electrofishing-Derived Length Structure Indices and Relative Abundance

Current sampling methods for blue catfish Ictalurus furcatus are suspected of being strongly biased against preferred-length fish (>= 762 mm in total length [TL]), making it difficult to accurately determine the species' population density and size structure. To understand this potential bias with respect to electrofishing, we conducted seasonal and habitat-specific sampling on three Oklahoma reservoirs using 15-pulse/s DC at the 100-1,000 V setting (the percent of range being adjusted to achieve 4-A output). Temperature, habitat, and reservoir section were analyzed to determine which variables were associated with the highest total catch per unit effort (CPUE(Total)), the CPUE of preferred-length blue catfish (CPUE(762)), and the relative stock density of preferred-length fish (RSD(762)). Total CPUE and CPUE(762) were significantly higher when the water temperature was 18 degrees C or more, but the variability increased as the temperature exceeded 28 degrees C. Catch rates were significantly higher in the upper reservoir section for all length-groups, and no differences in CPUE Total were detected among habitats (channels, points, or flats). Both CPUE(762) and RSD(762) were highest in channel habitats, but the high variability and low catch rates of these larger fish limit the utility of habitat-specific sampling based on these findings. Additionally, we evaluated the capture efficiency of electrofishing by creating a population with a known length-frequency distribution. This population was sampled on three separate dates to determine which length-groups were more vulnerable to electrofishing. No significant differences in catch rate were detected among the length-groups, and the mean total catch from each sample was always less than 10% of the total population. Our results indicate that low-frequency electrofishing is not size selective and provides representative samples of blue catfish between 200 and 1,000 mm TL. We recommend that sampling be conducted at temperatures between 18 degrees C and 28 degrees C and that standard sampling protocols adopt a stratified design that incorporates reservoir section.