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

Abstract

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. Received October 7, 2016; accepted February 2, 2017 Published online April 24, 2017

Evaluation of Unbaited Hoop Nets for Simultaneously Assessing Channel Catfish and Flathead Catfish populations in the Minnesota River

“…; and Tyszko et al. for studies related to catchability). Inaccurate abundance indices, varying catchability, and insufficient precision result in type I (rejection of a true null hypothesis) and type II (failure to reject a false null hypothesis) errors when comparing fish populations, regardless of the magnitude of CPUE.…”

“…Fisheries managers regularly assess differences in fish populations over time and among waterbodies with indices of relative abundance and size structure (e.g., CPUE and proportional stock density) calculated from catches in sampling gears (e.g., gill nets and electrofishing; Bonar et al 2009;Pope et al 2010). Sampling gears with the greatest efficiency (i.e., greatest CPUE or catch per person-hour) often are selected without consideration of accuracy, precision, or variation in catchability (i.e., fraction of the fish stock collected per unit of effort) related to relative abundance estimates (see Hangsleben et al 2013;Gwinn et al 2016;and Tyszko et al 2017 for studies related to catchability). Inaccurate abundance indices, varying catchability, and insufficient precision result in type I (rejection of a true null hypothesis) and type II (failure to reject a false null hypothesis) errors when comparing fish populations, regardless of the magnitude of CPUE.…”

Comparing Otoliths and Scales as Structures Used to Estimate Ages of Largemouth Bass: Consequences of Biased Age Estimates

“…As an alternative to relying on management surveys to obtain samples of trophy Largemouth Bass, otoliths can be collected by taxidermists to provide age and growth data (Horton and Gilliland 1993;Bulak and Crane 2002;Crawford et al 2002). This method not only avoids the public relations issue of sacrificing large fish but could provide better access to large fish that may be less vulnerable to standard survey gear than other segments of the population (Bayley and Austen 2002;Tyszko et al 2017) and that may be more vulnerable to angling than to standard survey gear (Pope et al 2005).…”

Variation in Reproductive Investment among Ohio Reservoir Largemouth Bass Populations

“…We determined fecundity, the total number of mature eggs per female, and ovary energy density (J/g) for Largemouth Bass populations in 19 Ohio reservoirs during 2012–2015. Each population sample was collected via standard boat electrofishing (see Tyszko et al 2017), with six, twelve, or eighteen 15‐min runs conducted at randomly selected sites. The number of sites sampled depended on reservoir size.…”

“…The number of sites sampled depended on reservoir size. Sampling occurred during mid‐April to mid‐May, immediately before or at the start of the spawning period (Carlander 1977), and targeted water temperatures between 15°C and 20°C (Tyszko et al 2017). Of the 19 populations, 12 were sampled in only 1 year, 1 was sampled in 2 years, and 6 were sampled in 3 years, yielding a total of 32 reservoir × year combinations.…”