Single-Pass versus Two-Pass Boat Electrofishing for Characterizing River Fish Assemblages: Species Richness Estimates and Sampling Distance

—Determining adequate sampling effort for characterizing fish assemblage structure in nonwadeable rivers remains a critical issue in river biomonitoring. Two-pass boat electrofishing data collected from 500–1,000-m-long river reaches as part of the U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program were analyzed to assess the efficacy of singlepass boat electrofishing. True fish species richness was estimated by use of a two-pass removal model and nonparametric jackknife estimation for 157 sampled reaches across the United States. Compared with estimates made with a relatively unbiased nonparametric estimator, estimates of true species richness based on the removal model may be biased, particularly when true species richness is greater than 10. Based on jackknife estimation, the mean percent of estimated true species richness collected in the first electrofishing pass ( ) for all 157 reaches was 65.5%. The p̂j,s1 effectiveness of single-pass boat electrofishing may be greatest when the expected species richness is relatively low (.10 species). The second pass produced additional species (1–13) in 89.2% of sampled reaches. Of these additional species, centrarchids were collected in 50.3% of reaches and cyprinids were collected in 45.9% of reaches. Examination of relations between channel width ratio (reach length divided by wetted channel width) and values provided no clear recomp̂j,s1 mendation for sampling distances based on channel width ratios. Increasing sampling effort through an extension of the sampled reach distance can increase the percent species richness obtained from single-pass boat electrofishing. When single-pass boat electrofishing is used to characterize fish assemblage structure, determination of the sampling distance should take into account such factors as species richness and patchiness, the presence of species with relatively low probabilities of detection, and human alterations to the channel. Compared with the biological assessment of wadeable streams, assessment of nonwadeable riverine fish assemblages has lagged (Reash 1999). Criticism of the development of assessment techniques for nonwadeable rivers has largely focused on issues concerning stability and consistency of sampling (Simon and Sanders 1999). Among the complex sampling issues to be considered is how to determine the appropriate level of sampling effort. Angermeier and Smogor (1995) noted that determination of the appropriate level of sampling effort needed to characterize fish species richness is difficult. Too little sampling may negatively influence the reliability of conclusions, whereas too much sampling may be unnecessarily expensive. A thorough sampling effort occurs when measurement of the attribute of interest (such as species richness) approaches an asymptotic level and additional sampling yields comparatively little new information (Lyons 1992; Paller 1995). Determination of the appropriate level of sam* E-mail: [email protected] Received May 20, 2003; accepted June 23, 2004 pling effort in nonwadeable rivers is confounded by the consideration of study purpose and objectives and the size of the area to be sampled. Though the sampling method chosen for nonwadeable rivers is often boat electrofishing, the sampling volume can range from any navigable river (regardless of channel width or drainage area) to surface waters described as ‘‘large’’ rivers (drainage area 5 2,590–5,180 km2) or ‘‘great’’ rivers (.5,180 km2; Simon 1992). Although standardized sampling of fish assemblage structure on a large geographic scale has many benefits (Bonar and Hubert 2002), protocols for nonwadeable sampling effort vary. In some cases, sampling is conducted for a fixed distance, whereas in other cases sampling is conducted for a distance proportional to channel width. Gammon (1976) and Yoder and Smith (1999) indicated that a single boat-electrofishing pass for a reach length of 500 m was sufficient to give consistent data for assessment of fish assemblage structure. Lyons et al. (2001) suggested that asymptotic species richness, or about 95% of ‘‘true’’ species richness, could be achieved with a single electrofishing pass along a 1,600-m (1 mi) reach for warmwater rivers