Globally, overharvest contributes to population declines of fish and invertebrates. Quantifying the effects of fishing and associated harvest is essential to preventing such declines. Murray crayfish Euastacus armatus exhibit slow growth and late age at maturity, and were historically abundant in Australia's Murray-Darling Basin, but river regulation, pollution and overfishing have reduced their range and abundance. Diminished recreational fisheries still exist, but data to quantify catch, effort and harvest are not available to inform management decisions. We used a population model to assess the status of a harvested Murray crayfish population, informed by data collected from a fishery-dependant survey. Quantitative fisher surveys were undertaken in the Murrumbidgee River during the 2012 Murray crayfish open season (May-August inclusive). Shortly after the surveys, and in light of fishery-independent data that indicated a Murray crayfish decline, the crayfish recreational fishery was spatially reduced and shortened to a 3-month season (June-August), bag limits were reduced and size limits were changed from a minimum length limit of 90 mm to a harvest-slot length limit (HSLL) of 100-120 mm. Modelling of the Murray crayfish population indicated it was most likely to be relatively small with a mean population size of 2777 individuals and affected by a high rate of crayfish harvest (harvest probability 0.4), indicating vulnerability to further decline should fishing effort and harvest persist. The population model was used to assess the regulation change to a HSLL, with the results supporting the regulation change and indicating that implementation of the HSLL would stabilise the population, albeit over many years. This study demonstrates that empirical data and population modelling are crucial to inform fishery regulations for threatened species.