Moving from multiple pass depletion to single pass timed electrofishing for fish community assessment in wadeable streams

Area-delineated multiple-pass depletion electrofishing (ADEF) can be resource-intensive. It may not capture fish community state when resource limitations mean that the number of sites sampled in a system is insufficient to account for ecological heterogeneity. Rapid assessment techniques such as single-pass timed electrofishing could be more efficient and support increased sample size, but it is important to understand how resulting catches will relate to existing ADEF data/time series. Paired ADEF (3-pass) and single-pass timed (10-min) electrofishing (TEF) samples were collected for sites in the River Barrow catchment (Ireland) in 2015. Paired samples were used to derive species-specific size-based conversion factors (CF) for extrapolating TEF catch up to a predicted ADEF Pass 1 catch. Applying these CF to a set of independent 'validation' TEF samples (2008-2013) produced fish catch estimates similar to observed ADEF Pass 1 catches (2008-2015). Species size-distributions in extrapolated TEF data were also similar to those in paired ADEF Pass 1 samples. Pass 2 and Pass 3 catches by species were then predicted from extrapolated validation TEF catches using a regression model. Cumulative catch curves fitted to these predicted catches were similar to those fitted to observed ADEF catch data for most species. TEF samples provide estimates of fish species catch and size-distribution that can be extrapolated following a simple protocol and compared with ADEF data for small streams measuring up to 10 m in width.