QUANTITATIVE SAMPLING OF STREAM FISH ASSEMBLAGES - SINGLE-PASS VS MULTIPLE-PASS ELECTROFISHING

Stream fish assemblages were sampled by multiple-pass electrofishing a nd supplementary seine netting in 31 sites in the Johnstone River, nor th Queensland and 28 sites in the Mary River, southeastern Queensland to determine the sampling effort required to adequately describe the a ssemblages in terms of fish abundances, species composition and assemb lage structure. A significantly greater proportion of the total number of fishes present at each site was collected by the first electrofish ing 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). Multiv ariate comparisons of fish assemblage structure revealed that data col lected by the first electrofishing pass poorly estimated the actual as semblage structure within a site and that up to three passes were requ ired for estimates of assemblage structure to stabilize. This effect w as evident for comparisons based on both absolute abundance and relati ve 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 spe cies by subsequent passes had an important effect on the resultant des cription of assemblage structure. Supplementary seine netting had a gr eater effect on the determination of assemblage structure in the Mary River than in the Johnstone River. The results are discussed with refe rence to sampling design in studies of stream fish assemblages and a s ampling protocol is recommended that enables the accurate determinatio n of abundance, richness and assemblage structure in small- to medium- sized streams.