Understanding size bias in electrofishing samples of black bass

Objective Boat electrofishing is widely used to sample black bass Micropterus spp., but memorable-trophy-sized black bass often appear more frequently in angler catches than in electrofishing samples, suggesting potential sampling bias. Therefore, we conducted a study to determine if a size bias against these larger size-classes exists for electrofishing. Methods Using marked populations in two hatchery water storage reservoirs, we estimated size-specific catchability across eight spring and eight fall electrofishing trials. Capture probabilities were also estimated from radio-tagged fish detected in the electrofishing boat’s path. Length frequencies of fish captured by angling and electrofishing in our study reservoirs were compared to examine size distribution of both capture methods. Live-imaging sonar and radiotelemetry were used to examine behaviors influencing capture. Live-imaging sonar was also used to compare boat avoidance with the electrofisher powered on and off. Results Mean catchability (q = recaptures·marked fish−1·hours of effort−1) was lowest for memorable-­trophy-sized fish (q = 0.11), followed by stock-quality (q = 0.18), quality-preferred (q = 0.28), and preferred-memorable size-classes (q = 0.24). Similarly, we observed lower capture probability for larger radio-tagged fish (only fish ≥350 mm were radio-tagged, so stock-sized fish were not evaluated by radio tags). Length frequencies differed significantly between angling and electrofishing samples, with angling having a greater proportion of quality-sized and larger fish (smaller classes underrepresented in angling), with moderate to strong effect size. Live-imaging sonar observations indicated that fish escaped by reacting to the outer edge of the electric field before entering zones capable of immobilization. Fish escaped at greater distances when power was on versus off, suggesting that they were detecting and avoiding the electric field. Large individuals also tended to reside farther offshore or deeper, which could lead to a greater degree of escape. Conclusion We found that black bass of memorable size and larger are less catchable by electrofishing. This gear bias influences estimates of mortality and proportional size distribution. Therefore, we have provided a correction factor that should be considered for electrofishing length data before making such computations.