Understanding How Standardized Spring Electrofishing Surveys Index Reservoir Largemouth Bass Density and Size Structure

We estimated the catchability of Largemouth Bass with Ohio Division of Wildlife (DOW) standard electrofishing methods using mark-recapture experiments that encompassed a standardized time period and compared differences among length categories.  Sampling was conducted throughout spring (April-June), including the standard DOW assessment  period (mid-April-mid-May) in three Ohio reservoirs. We evaluated statistical power to detect a 50% difference in bass density with CPUE  and the rate of type I error using a resampling analysis as the number of sites increased from 2-10 using (1) samples collected during the standard period or (2) randomly sampled weeks from the entire spring period.  Catchability and CPUE were greatest and least variable from recapture events conducted inside the standardized time period.  Statistical power exceeded 0.80 and the rate of type I error was 0.03 or lower when samples were only taken inside the standardized time period and at least eight sites were sampled.  When sampling randomly during spring, statistical power never exceeded 0.60 and the rate of type I error exceeded 0.30.  Catchability was lower for Largemouth Bass below stock length and potentially lower for memorable length bass.  Our results show that spring electrofishing surveys can provide a valid index of Largemouth Bass density and size structure.