Single- versus Multiple-Pass Boat Electrofishing for Assessing Smallmouth Bass Populations in Virginia Rivers

Abstract

A depletion electrofishing study was conducted on two Virginia rivers to estimate the density and biomass of smallmouth bass Micropterus dolomieu. Population estimates generated by a maximum likelihood analysis (MLM) were compared with those derived by the Leslie method. Also, population size structures from depletion samples were compared with those from single-pass surveys. Adult smallmouth bass were successfully depleted in three to five runs at most sites, and estimates averaged 386/km (SE = 132) and 47/ha (SE = 17) on the Rappahannock River and 265/km (SE = 90) and 38/ha (SE = 20) on the James River. Age-0 population estimates averaged 221/km (SE = 60) and 28/ha (SE = 14) on the Rappahannock River and 248/km (SE = 88) and 31/ha (SE = 14) on the James River. Capture probability was highest (mean = 0.40) for adult bass on the Rappahannock River and lowest (mean = 0.17) for age-0 bass on the James River. Population estimates based on the Leslie method, which regressed catch per effort on cumulative catch (lagged for one run), were greater than those generated by MLM, but the differences were relatively consistent and averaged 25%. Single-pass runs provided biased estimates of size structure at two of four Rappahannock River sites based on total length comparisons of stock size fish and structural indices (more large fish were captured by depletion electrofishing); however, little size selectivity bias existed between methods on the James River. This study suggests that smallmouth bass densities can be successfully estimated from sample reaches within large Virginia rivers and provides cautious optimism that single-pass electrofishing adequately describes smallmouth bass population size structure in some Virginia rivers.