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

Abstract

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 altered to account for our SFR results or that all female E. armatus be protected until the numerous uncertainties outlined herein are resolved. Finally, as SFR directly represents the biology of a species and will not inappropriately characterize risk compared with SOM, the new indicator SFR must replace SOM for monitoring, conservation and fisheries management of all decapods.