Rapid assessment of population dynamics and monitoring methods for invasive narrow clawed crayfish Pontastacus leptodactylus in a freshwater reservoir

Narrow-clawed crayfish (Pontastacus leptodactylus) are a data deficient invasive non-native species in the UK. Boshaw Whams (West Yorkshire, UK) contains the only known population of narrow-clawed crayfish in Yorkshire. The risk of further spread of these crayfish is high and it is important to establish the extent of the current invasion on the Generalised Invasion Curve to identify potential management options. We used a combination of methods over a 15-month period including trapping, Remote Underwater Video (RUV) and Baited RUV (BRUV) to establish the most efficient method for narrow clawed crayfish monitoring and determine annual population dynamics. There was no significant difference between the three methods in terms of detection efficiency thus we recommend a mixed approach in the future dependent on practitioner capacity. Significantly more males were observed through trapping than females and berried females were detected between February and April. A mark-recapture survey estimated the population to have a minimum size of 10,045 ± 5602 (95% CI) individuals in a waterbody spanning 50,000 m2. Boshaw Whams Reservoir should be considered as in the ‘Containment’ or ‘Asset Protection’ stage of the Generalised Invasion Curve, and action urgently required to prevent further spread.