Integrating fishing and conservation in a risk framework: A stochastic population model to guide the proactive management of a threatened freshwater crayfish

Abstract

Abstract Numerous freshwater species are recreationally harvested, yet conservation concerns are often ignored. This may lead to conflict between fisheries and conservation management for a species. Approximately one‐third of freshwater crayfish species globally are threatened with extinction, including the Murray crayfish, Euastacus armatus (von Martens, 1866) in south‐eastern Australia. There has been substantial commercial and recreational fishing of E. armatus since European settlement, as well as other threats such as poor water quality (e.g. hypoxic ‘blackwater events’ during floods). A stochastic population model was developed for three regional populations of male and female E. armatus, and was used to examine the effects of fishing and hypoxic blackwater disturbance on a Murray crayfish population in a lowland region. The results indicate that the Murray crayfish population in the lowland region faces increasing risks from increasing fishing pressure, together with low frequencies of hypoxic blackwater disturbance only. Testing various fishery regulations, the modelling demonstrated that recent changes to the ‘slot size’ in occipital carapace length of 10–12 cm for legally harvesting crayfish was a suitable protection measure given the background levels of hypoxic blackwater disturbance, balancing fishing and conservation concerns. Although higher slots (11–13 cm and 12–14 cm) were less sensitive to higher fishing pressures, the recreational harvest declined. The model was most sensitive to early life‐history survival rates; however, sensitivity analysis of model variability also indicated that E. armatus may be susceptible to highly variable environments (not to be confused with highly variable flows). This study demonstrates the successful integration of fishing and conservation in a flexible risk framework for this threatened freshwater crayfish species; furthermore, fisher expectation can be managed by assessing the impact on harvest of potential changes to fishing regulations. This approach is easily transferable and would improve the management of other recreationally fished species facing multiple threats.