Quantitative sampling of stream fish assemblages: Single- vs multiple-pass electrofishing

Stream fish assemblages were sampled by multiple-pass electrofishing and supplementary seine netting in 31 sites in the Johnstone River, north Queensland and 28 sites in the Mary River, southeastern Queensland to determine the sampling effort required to adequately describe the assemblages in terms of fish abundances, species composition and assemblage structure. A significantly greater proportion of the total number of fishes present at each site was collected by the first electrofishing pass in the Mary River (46%) than in the Johnstone River (37%) and this difference was suggested to be due to higher water conductivity in the former river. The mean proportion of the total species richness detected by the first pass was also significantly higher in the Mary River than in the Johnstone River (89 % and 82%, respectively). Multivariate comparisons of fish assemblage structure revealed that data collected by the first electrofishing pass poorly estimated the actual assemblage structure within a site and that up to three passes were required for estimates of assemblage structure to stabilize. This effect was evident for comparisons based on both absolute abundance and relative abundance data and was particularly marked for comparisons based on presence/absence data. This latter result suggests that, even though most species were detected on the first pass, the addition of rare species by subsequent passes had an important effect on the resultant description of assemblage structure. Supplementary seine netting had a greater effect on the determination of assemblage structure in the Mary River than in the Johnstone River. The results are discussed with reference to sampling design in studies of stream fish assemblages and a sampling protocol is recommended that enables the accurate determination of abundance, richness and assemblage structure in small- to medium-sized streams.