Counting crayfish: Active searching and baited cameras trump conventional hoop netting in detecting Euastacus armatus

Accurate distribution and abundance estimates for rare and endangered species are necessary to ascertain extinction threats and take appropriate conservation measures. Traditional capture-based methods are imperfect for surveying elusive species such as freshwater crayfish in upland streams. We compared estimates of Murray River crayfish Euastacus armatus abundance made via direct visual assessments by snorkel, against baited remote underwater video surveys (BRUVS) and traditional hoop netting conducted in 2 montane river systems. Similar total abundances were recorded via visual survey and BRUVS across 4 sites within 1 river system where E. armatus was relatively common. In contrast, markedly lower values were obtained at these sites via conventional hoop netting methods. In another stream where E. armatus is particularly rare, the only detection of this species was via BRUVS. Average catch per unit effort (CPUE) was highest from active visual surveys (2.99 ind. h^-1), followed by BRUVS (0.63) and hoop netting (0.13). Extremely low sampling efficiency from hoop netting was attributed to the short time periods crayfish attended baits (mean ± SE, 387 ± 209 s) relative to the rate of net retrieval (hourly). We conclude that in systems of adequate water clarity, visual surveys and BRUVS provide reliable, non-invasive methods for estimating freshwater crayfish abundance and hold promise particularly for research and monitoring of threatened Euastacus species.