Comparing electrofishing and snorkelling for characterizing fish assemblages over time and space

Environmental processes and dispersal movements occurring over long distances (10 to 100 km) continually influence local stream fish assemblages. However, electrofishing protocols are classically implemented in short reaches (similar to 1 km) and are not suited for frequent characterization of assemblages over long distances. We developed a new sampling protocol (SPA: snorkelling point abundance) for characterizing fish assemblages over long distances, using series of sampling points, as often applied in electrofishing (PASE: point abundance samples by electrofishing). Nine pairs of PASE and SPA surveys, repeated in a narrow and in a wide stream, were compared. Greater species abundance, occurrence, and richness were found on PASE, but relative species abundance were comparable between protocols. Assemblages were highly variable over time (between-surveys) on both protocols. The spatial structure of point assemblages (within-survey) was consistent between protocols and related to species' habitat use (depth, current velocity). For several species, the longitudinal distribution of abundance along reaches was comparable between protocols when surveys were pooled. Overall, SPA could be an alternative to electrofishing for analysing spatial structure over long distances.