Relationship between Discharge and Electrofishing Catch-per-unit-effort of Largemouth Bass in the Neuse River, North Carolina

Sportfish managers in coastal North Carolina are often challenged with interpreting estimates of largemouth bass (Micropterus salmoides) relative abundance (catch-per-unit-effort, CPUE) due in part to the influence of environmental factors on boat electrofishing techniques. To accurately assess population abundance using relative abundance indices, the effects of environmental variables on largemouth bass catch should be quantified. We sampled largemouth bass populations in Neuse River tributaries weekly in spring 2006 and 2007 to investigate the relationship between CPUE and streamflow. Catch-per-unit-effort appeared to be strongly related to discharge, but the relationship was not linear. Instead, we found that small increases in streamflow between 60 and 100 m3 sec-1 had a large effect on electrofishing CPUE. Above this streamflow threshold, CPUE was usually low and only exceeded 20 fish h-1 in 3 of 41 observations in 2006 and 2007. At lower streamflows, CPUE commonly exceeded 100 fish h-1, especially in 2007. In most cases, CPUE was higher when discharge was <85 m3 sec-1 in both years for all largemouth bass collected (P < 0.01), except for largemouth bass less than stock size (200 mm) in 2006 (P = 0.59). We recommend that spring sampling for largemouth bass on the Neuse River be limited to days when discharge is <85 m3 sec-1. Application of these methods could be applied on a broader scale to evaluate the influence of streamflows on the variability in CPUE of largemouth bass in other coastal rivers.

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