Can escape gaps in traps improve selectivity in freshwater crayfish fisheries?

Minimum size at catch is commonly used to avoid overfishing. In marine crustacean fisheries, the use of escape gaps has been successfully implemented to reduce the undersized catch. In Sweden, the fishery administration has introduced trap gaps to inland signal crayfish (Pacifastacus leniusculus) fisheries, with the aim of letting undersized signal crayfish escape. In this article, we evaluate the implementation of 28 mm trap gaps. A morphological difference between male and female signal crayfish was found to hamper the positive effect of the escape gaps. Female tails exceeded the limitation of the trap gap before the crayfish reached a total length of 100 mm. Male crayfish were not limited by carapace width until total length exceeded 110 mm. Increased escape rate over time for both types of traps was found as an effect of bait remaining at time of catch. Our study shows that escape gaps can contribute to a higher selectivity in freshwater crayfish fisheries in order to fulfil both size- and sex-selectivity, but not with their present construction. The results call for an adjustment to the construction, suggesting more elliptical escape gaps in order to fulfil the intent of the implementation.