Assessing Reservoir Largemouth Bass Standardized Boat Electrofishing: Effect of Catchability on Density and Size Structure Indices

摘要

Understanding catchability-the fraction of a stock caught by a defined unit of effort-is crucial to using fisheries assessment data to index abundance. We conducted mark-recapture experiments to estimate catchability and evaluate standard boat electrofishing methods for assessing populations of Largemouth Bass Micropterus salmoides. We then used a resampling analysis to test for differences in bass CPUE (fish/h and fish/km) between two high-density reservoirs and one low-density reservoir and among surveys within each reservoir. We compared scenarios using surveys conducted only during (1) the standard time period (mid-April to mid-May) and (2) the entire assessment period (early April to mid-June). We considered the percentage of significant differences in CPUE between the high-density and low-density reservoirs to represent statistical power (i.e., the ability to detect a difference in CPUE when a difference actually exists) and the percentage of significant differences in CPUE between surveys in the same reservoir to represent the false-positive rate (i.e., the detection of a difference in CPUE when no difference in density exists). Catchability and CPUE were greatest and least variable during recapture events conducted during the standard period. The mean catchability of sub-stock length Largemouth Bass (150-200 mm) and memorable-length bass (>= 510 mm) was significantly less than those for other length categories. Statistical power exceeded 80%, and the false-positive rate was generally less than 10% for sampling during the standard period at as few as six electrofishing sites. When including samples from outside the standard period, power was lower and the false-positive rate was as high as 60%. Power and false-positive rate were similar whether effort was measured in time or distance. Our results emphasize that standardized springtime boat electrofishing assessments validly index Largemouth Bass density and size structure.