Comparison of Catch Rate, Length Distribution, and Precision of Six Gears Used to Sample Reservoir Shad Populations

Abstract

Hydroacoustics, trawling, gillnetting, electrofishing, shoreline seining, and cove rotenoning were used concurrently in August 1991 at Lake Texoma, Texas–Oklahoma, to compare sampling efficiency for gizzard shad Dorosoma cepedianum and threadfin shad D. petenense. A simple random-sampling design was used at nearshore and offshore stations in each of three 400–2,000-ha sites in the reservoir. Most gears provided similar evidence of spatial patterns of shad abundance among sites, but length distributions and sampling precision varied among methods. At offshore transects, catch-per-unit-effort (CPUE) data for hydroacoustics, trawling, and gillnetting were positively correlated (r = 0.45–0.80; P < 0.05) when data from all sites were combined. Gears differed in proportions of small (age-0) shad versus larger (age-1 and older) fish. Gears that collected mainly age-0 shad were trawls, surface-set gill nets, and seine, whereas catches of age-1 and older shad were greater with bottom-set gill nets, electrofishing, and rotenone, Sampling precision, as measured by the coefficient of variation (CVX = SD/mean) among replicate samples for each species and site, ranged from 0.2 to 0.7 for hydroacoustics, surface-set gill nets (both species), trawl (threadfin shad), and rotenone (gizzard shad). Poorer precision was obtained with the seine (1.0–3.2, both species), electrofishing (0.7–1.8, both species), bottom-set gill nets (0.4–1.7, both species), trawl (1.0–1.5, gizzard shad), and rotenone (0.8, threadfin shad). We quantified relative gear efficiency by calculating the number of replicate samples needed to estimate median CPUE within 25% of the true median at 95% probability and multiplying this by the hours of labor required to obtain one replicate, Most labor estimates for hydroacoustics, trawling, and surface-set gill nets were less than 120 h (one week of work for three people); requirements were usually much greater for rotenone (1,524–5,384 h), seining (114–954 h), electrofishing (171–973 h), and bottom-set gill nets (53–418 h). The number of replicate samples needed to attain the desired precision criteria was 14–25 for hydroacoustics, 9–79 for trawling, and 5–55 for surface-set gill nets.