Abstract Harvesting regulations for crustacean decapods generally focus on total catch and minimum legal size of individuals. However, there are trade-offs between total catch and minimum size, and possibly also for maximum size. The outcome of various harvesting restrictions was assessed by model iterations for one lake based on a long time series (29 years) of crayfish population size and annual harvesting. The paper explores how this decline might have been avoided by alternative harvest regulations based on a strong decline in the surveyed population and use of a deterministic model with two stable equilibria. The results of the model simulations suggested that the decline may have been avoided by reducing the annual catches by 15% without changing the minimum size regulation of 95 mm. The decline may also have been avoided by protecting the largest (and most fecund) individuals, e.g. by allowing harvesting of crayfish between 50 and 98 mm without reducing the total catches. The latter strategy might also have counteracted a long-term evolution towards smaller crayfish caused by selective harvesting of the largest individuals.