Paddlefish as Potential Acoustic Targets for Abundance Estimates

Abstract

Underwater acoustics is a noninvasive sampling technique that potentially reduces expense and injury to target species, but this method may be underutilized for sampling large freshwater fishes. We measured target strength (TS), developed anatomically based backscatter models, and conducted gill-net and acoustic surveys of paddlefish Polyodon spathula to explore the potential use of acoustic surveys for estimating the abundance of large freshwater fishes. Mean TS measured from two size-groups of paddlefish at 200 kHz was −37.14 decibels (dB; SD = −2.36) for age-0 fish (353–406 mm) and −27.25 dB (SD = −2.21) for adult fish (1,018–1,284 mm), indicating that TS could differentiate these size-groups. Backscatter models identified strong contributions of the swim bladder to TS and revealed the sensitivity of acoustic backscatter to paddlefish length, aspect, and acoustic carrier frequency. Model results were generally within one SD of measured means from individual fish of each size-group. Target strength results were used to count two populations of adult paddlefish in mobile surveys using an echo sounder with a 200-kHz, 6° split-beam transducer. One population was stocked in 1.6-ha Hebron Pond, where no large fish were previously present. The other population resided in 28-ha Horseshoe Lake, an Ohio River backwater. Twenty-one paddlefish stocked in Hebron Pond were accurately counted during the first of six side-looking surveys, but subsequent surveys only counted between two and seven fish. Depletion gillnetting results in Horseshoe Lake provided an estimated baseline of 130 ± 55 paddlefish for comparison with abundance estimates from side-looking and down-looking acoustic surveys during day and night. Acoustic abundance estimates ranged from 187–313 fish (side-looking) to 3,464–13,489 fish (down-looking) depending on survey time (day or night) and the approach to analysis. Ratio estimates and cluster estimates provided similar results, and the coefficient of variation of the mean (100·SE/mean) ranged from 20% to 50%. Acoustic estimates were either greater or more variable than those derived from depletion gillnetting, yet acoustic surveys required only 6 man-hours compared to 180 man-hours for the gillnetting estimate. Our study is the first to indicate that TS can be used to count adult paddlefish and that, upon refinement of survey techniques, TS can be used to estimate paddlefish abundance. The benefits of acoustic surveys may be realized sampling other large freshwater fishes when the target species can be differentiated with TS and considerations are made for transducer selection.