| Summary: | Minimum size is a common management regulation in noble crayfish (Astacus
astacus) fisheries. The most used catching method baited traps, catch crayfish
down to 50 mm total length (TL). In Norway the minimum size is 95 mm TL and this
regulation allow female crayfish to spawn a least once before being harvested. To reduce
the fraction of non-legal sized crayfish in trap catches, mesh-size were increased from
17.5 to 21 mm (knot to knot) in Lake Steinsfjorden (1983) and nationwide (2001). We
compared catch per unit effort data in traps with different mesh sizes from the two most
important noble crayfish fisheries in Norway to evaluate the effects of the regulation.
Traps with mesh size 21 mm catch significantly fewer crayfish compared to traps with 17.5
mm and 12 mm. As intended, traps with 21 mm caught significantly less non-legal sized
crayfish compared to 17.5 mm. Increased mesh size thus reduce the catch-induced mortality
of non-legal sized crayfish, resulting in a larger spawning population. Mesh size
regulations is an important management tool, especially for recruitment limited crayfish
population, and is rather easy to enforce compared to minimum size.
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