Temperature- and habitat- specific length bias and catch rate of electrofishing for blue catfish (Ictalurus furcatus)

Current sampling methods for blue catfish Ictalurus furcatus are suspected to be strongly size-biased, making it difficult to accurately access population density and size structure. To understand this potential bias for electrofishing, we conducted seasonal and habitat-specific sampling for 2 years beginning in June 2006. Kaw, Keystone, and Oologah Reservoirs in Oklahoma were sampled using 15-pps DC electricity at 1000 volts (4 amps). Temperature, habitat, and reservoir section were analyzed to determine which variables are associated with the highest total catch-per-unit-effort (CPUE Total) and CPUE of preferred length (≥ 762mm, TL CPUE762) blue catfish, which typically appear underrepresented in electrofishing samples. Total CPUE and CPUE762 for blue catfish was significantly (p < 0.05) higher for warmer water temperatures (≥ 18°C), but variability increased as temperatures exceeded 28°C. Catch rates were also significantly higher in the upper reservoir section. No difference was detected for microhabitat (channels, points, or flats). Additionally, we evaluated the actual length-specific bias of electrofishing using a population with a known length-frequency and size distribution. Approximately 300 blue catfish (approximately 25 fish from each 100-mm length class between 200 and 1000 mm) were collected and transported to a 12-ha impoundment where no blue catfish were previously present. This 'artificial population' was sampled to determine which length classes were more vulnerable to electrofishing. No difference in catch rate was detected for any length classes (P < 0.05) and mean total catch was less than 10% of the total population.