Assessing risks to threatened crayfish populations from sex-based harvesting and differential encounter rates: A new indicator for reproductive state

•A new indicator of reproductive state (Size at Functional Reproduction, SFR) in a threatened crayfish more adequately assesses risk from harvest pressure than the contemporary indicator (Size at Onset of Maturity, SOM)•Disparity in sex ratio between modelled and field data demonstrate differential encounter rates between female and male Euastacus armatus.•Variability in size and sex ratios and differential encounter rates indicate potential problems with using sex ratios as a measure of exploitation and recovery of Euastacus armatus.•Current harvestable slot limit length for Euastacus armatus should either be altered to account for our findings or that all female E. armatus be protected from harvest.•SOM should be replaced with the new ecological indicator SFR for monitoring, conservation and fisheries management of all decapods. Globally, freshwater crayfish are imperilled, particularly large species harvested for human consumption. Crustacean fisheries typically rely on size- and sex-specific harvest regulations to counterbalance the adverse effects of over-harvesting which manifests within populations as disparate size truncation, and uneven sex ratios. A stochastic population model was used to examine the risk from harvest pressure on a threatened crayfish Euastacus armatus in south-eastern Australia. We compared a new indicator, size at functional reproduction (SFR – females with eggs), with the contemporary index of size at onset of maturity (SOM) when applying the current regulations of no-take for females with eggs. SOM under estimates risk compared with SFR. This risk difference was amplified with increasing harvest pressure, proving SOM inadequate for characterizing risk. Variable and increasing adult sex ratios with size was thought to indicate past harvest pressure and signs of recovery from harvest over-exploitation. However, in conjunction with evidence of differential encounter rates between female and male crayfish, we show that high sex ratios in catch data were not due to harvest pressure, pointing to problems with using sex ratio as a measure of exploitation and recovery. Our modelled results were unable to produce high sex ratios in catch data without a differential encounter rate included. If differential encounter rates exist, of the scenarios explored, the greatest risk was associated with the current harvest regulations. With recent declines in distribution and abundance, we argue that either the harvestable slot limit length be al