Fish catchability and comparison of four electrofishing crews in Mediterranean streams

The knowledge of capture efficiency and side effects of electrofishing is essential for research and monitoring of stream fish populations. Differences amongst electrofishing crews have hardly been investigated and are particularly important given the on-going implementation of the Water Framework Directive and wide-ranging exchange of data worldwide. We aimed to assess fish catchability in Mediterranean streams and to compare four electrofishing crews (with minor differences in gears used) and their short-term effects on fish populations. In eight different sites, we compared two adjacent stations, one sampled with conventional single-pass catch-effort data and the other closed with block nets and with four-pass removal estimates. We used a Williams’ cross-over design to estimate the independent effects of repeated sampling in four consecutive days, site and crew and also to assess a potential carry-over effect. We modelled capture probability and estimated population size using program MARK and an information-theoretic framework. Our results show that electrofishing was generally efficient in these reaches, with 50–100% of the species and of 40–60% of the individuals captured in a single pass. The CPUE was significantly higher at sites blocked with nets than at open sites, but observed richness was not significantly different. Capture probability was generally not constant along removal passes and increased with fish size. Observed fish richness and species composition did not depend on electrofishing crew and fishing day and there was no significant carryover effect. There were, however, significant differences in single-pass CPUE estimates amongst electrofishing crews, after accounting for other sources of variation. There was also a significant carry-over effect, surprisingly with increasing fish captured after fishing by specific crews. Overall, our results suggest that although capture probability depends heavily on a number of factors (such as species, size, and sampling site) and needs careful consideration, the effect of electrofishing crew is negligible for assessment of species richness and composition but considerable for fish abundance.