The importance of qualitative inventory sampling using electric fishing and nets in a large, tropical river (Brazil)

Qualitative fish sampling (single catch) using three methods (electric fish ing, gillnets, seine nets) was assessed at three sites in the Ivai River (P arana State, Brazil) to check their usefulness for quick inventory investig ations. Electric fishing at a constant effort (15 min per each) was conside red to be the best sampling technique. Taxon richness was calculated as the expected number of species using a rarefraction technique. Samples of 300- 325 individuals, and 6-10 repetitions in neighbouring segments were suffici ent. Even though electric fishing was conducted with the same effort in sim ilar segments at each site there were many differences in species abundance . This suggests that the estimated fish specimen number obtained with const ant effort is of limited validity on one sampling occasion and may be best considered as an `index of density' only. The differences may partially be caused by the segments being located along the right and left bank of the l arge river (problem of different habitats). Qualitative and quantitative di fferences between electric fishing, gill-netting and seine-netting samples were very high at a high significance level. Nevertheless, as 22.4% taxa we re caught only by gill or seine nets, these gears were important for comple menting the species list.