Variation in Bluegill Catch Rates and Total Length Distributions among Four Sampling Gears Used in Two Wisconsin Lakes Dominated by Small Fish

Abstract

Many Bluegill Lepomis macrochirus populations are dominated by fish ≤125 mm total length (TL) that may be underrepresented when using standard sampling gears. To identify efficient sampling methods for these populations, we compared catch per unit effort (CPUE) and TL frequency distributions of Bluegill captured in cloverleaf traps, boat electrofishing, mini-fyke nets, and beach seine hauls from two northern Wisconsin lakes supporting populations dominated by fish ≤125 mm TL. Mean Bluegill CPUE ranged from 41 (SE = 11) fish per cloverleaf trap lift to 16 (SE = 8) fish per beach seine haul. Cloverleaf traps generally captured smaller Bluegill relative to other gears and were the only gear to consistently capture Bluegill ≤80 mm TL. Conversely, boat electrofishing captured the widest TL range of Bluegill, and fish ≥80 mm TL composed a greater proportion of catch (37%) relative to other gears. With few exceptions, the effort required to detect 10% or 25% changes in Bluegill CPUE was >100 units of effort regardless of lake, sampling gear, or month. Furthermore, there was no consistency between lakes or months in terms of which sampling gear required the fewest number of samples to detect a 50% change in CPUE. Estimated units of effort needed to detect 10% or 25% changes in mean Bluegill TL were ≤16 for all sampling gears on the lake with consistently higher CPUE (i.e., more fish to measure per unit). In the lake with lower CPUE, cloverleaf traps consistently required less effort to detect changes in mean TL. We note that comparing sample size requirements among gears is not straightforward because gears are sampling differing segments of the Bluegill population. Our study emphasizes the importance of evaluating gear biases and sampling efficiency so that fisheries managers can develop suitable sampling protocols. ACKNOWLEDGMENTSWe thank the numerous undergraduate research technicians from the University of Wisconsin-Stevens Point and the University of Wisconsin-Madison that helped complete field work. We thank M. Flammang for constructive comments on an earlier draft.

Resisting ecosystem transformation through an intensive whole‐lake fish removal experiment

“…Bluegill showed the largest declines through 2020, concurrent with an increase in adult walleye, but bluegill CPUE was higher in the most recent sampling year (2021; Figures 2 and 3). Cloverleaf traps, which sample smaller individuals (Sullivan et al, 2019a), showed the most variability (Figure 3). Cloverleaf catches initially declined ~89% from pre‐removal‐2020 CPUE (CPUE change from 75 to 8.5 ind/trap), but then increased ~241% from 2020 to 2021 CPUE (CPUE change from 8.5 to 29.1 ind/trap; Figure 3).…”

“…Figures 2 and 3). Cloverleaf traps, which sample smaller individuals (Sullivan et al, 2019a), showed the most variability (Figure 3).…”

Evaluation of Threatened, Endangered, and Rare Fish Species and Communities of the St. Lawrence River and Its Tributaries in the United States

“…We did not sample ponds, lakes, or wetlands disconnected from the lotic drainage network. Our gear has been shown to be effective in each of the habitats where they were applied, using our procedures (Jensen 1986, Korman and Yard 2017, McKenna and Johnson 2005, McKenna et al 2012, Sullivan et al 2019). However, one of the reasons that we used multiple gear types is that sampling biases exist for every type of gear, and efficiencies of collection by any given gear vary by species and habitat conditions (e.g., turbidity, flow, etc.)…”

Relative Effectiveness of D‐Frame Dip Nets, Quatrefoil Light Traps, and Towed Ichthyoplankton Nets for Larval Muskellunge

“…Photographs of a quatrefoil light trap (left), a D-frame dip net (middle), and a towed ichthyoplankton net (right) being deployed to capture larval Muskellunge in and around structurally complex habitat in Snipe Lake, Wisconsin, during 2020. traps, fish/10 min for D-frame dip nets, and fish/100 m 3 for ichthyoplankton tows). Measures of sampling precision (e.g., coefficients of variation) and power-based analyses are frequently used as criteria for selecting sampling gears when gears operate in different manners or have disparate units of effort (e.g., Allen et al 1998;Doyle et al 2008;Sullivan et al 2019). However, given the paucity of information on sampling gears for larval Muskellunge and the expectation that catches would be low, with a high frequency of zero catches (e.g., Murry and Farrell 2006;Crane and Farrell 2015;Sheffer 2019), we placed greater importance on identifying a gear or gears with a high probability of capturing larval Muskellunge in general (but with potentially lower precision) than identifying a gear that consistently captured few or no fish (i.e., high precision).…”