Using density estimates, sex ratios and size structure to assess the status of a threatened Australian freshwater crayfish ( Euastacus armatus ) population

Managing and conserving wild animal populations relies on accurately describing changes in populations through time. Density estimates, sex ratios and size structures are used to monitor population trends, including sustainability assessments of populations subject to harvest exploitation. We used a spatially explicit capture–recapture model (SECR) to generate capture probabilities for a threatened freshwater crayfish [ Euastacus armatus (von Martens 1866)], within a 6.2 km reach of the Murray River, southeast Australia, previously exposed to harvest. Capture probabilities were critical to our understanding of density, sex ratios, size structure and movement patterns which are all key parameters driving population dynamics. Model outputs, comparing catch rates over a 4-year period, showed declining estimates of density, uneven sex ratios, an unbalanced population size structure and restricted movement, all with potentially severe evolutionary consequences. Through the analysis of multiple indices (density estimates, sex ratios and size structures, and changes therein), we inferred that the illegal harvest of crayfish was likely driving changes in population dynamics within the study population. SECR models are a valuable tool for generating density estimates for a range of taxa and, when combined with information on sex ratios and size structures, provide a powerful means for stock assessments.