Estimating the population size of the endangered white-clawed crayfish austropotamobius pallipes using mark-recapture at three special areas of conservation (SACS)

The white-clawed crayfish, Austropotamobius pallipes, is the sole native freshwater crayfish species found in Ireland, with Irish populations being considered the most substantial and important in Europe. At present, the size of the majority of Irish crayfish populations is unknown. Recent outbreaks of the crayfish plague Aphanomyces astaci in Ireland amplify the need to acquire accurate population estimates. The Schnabel estimator and closed population model, the M_th model, were employed in this study to estimate the adult A. pallipes population sizes from three specific sites. All adult individuals captured using both hand-search and baited trap methods were marked by abla-tion and released back into the sampling area in close proximity to suitable refuges. Both methods provided similar population estimates, although the M_th model appears to be the most precise. The estimated adult crayfish population size using the M_th model method was 214 individuals at the River Abbert, 1392 at the River Manulla, 18,153 at Lough Owel Site 1 and 27,325 at Lough Owel Site 2. A significant relationship was found between baited trap CPUE and population estimate, indicating that with further research a CPUE value could be used to generate a rough estimate of population size. Mark-recapture data and population estimate results indicate abundant populations of A. pallipes are present at each site.