A Comparison of Day and Night Electrofishing: Size Structure and Catch per Unit Effort for Smallmouth Bass

Catch per hour of effort (CPUE) of all smallmouth bass Micropterus dolomieui; CPUE of small (< 180 mm long), intermediate (180–279 mm), and quality-length (> 280 mm) smallmouth bass; and proportional stock density (PSD = 100 [number of quality-length fish/number of stocklength fish]; stock length = 180 mm or longer) were compared between day and night electrofishing samples. Data were from catches of smallmouth bass in the Maquoketa River, Iowa, during spring 1978. The CPUE for smallmouth bass was significantly higher for nighttime electrofishing (P < 0.01) than for daytime electrofishing. The CPUE for sampling at night for all length ranges of smallmouth bass was also significantly higher (P < 0.01) than during the day, and the PSD calculated from the daytime catch was 27, whereas it was 33 for electrofishing at night, a 22% increase. Night electrofishing in rivers is recommended for this species to improve gear efficiency, reduce the time necessary to make population estimates, and increase sample size for determining length-frequency distributions and age structure.