Abstract

Baited traps catch a large fraction of smallsized crayfish, and in Lake Steinsfjorden, S.E. Norway, we have registered an increased mortality of at least 10-20% due to handling stress, consumption and transplantations among non-legalsized crayfish {i.e. < 95 mm total length). Laboratory experiments with unbaited traps indicated that increased mesh size will reduce the fraction of non-legalsized crayfish in the trap catches. Simultaneous catches with traps of mesh size 17.5 mm and 21 mm in Lake Steinsfjorden showed small, but significant differences in length distribution. The relative selectivity of traps with mesh size 21 mm is high down to 85 mm total length, even though estimates from the relationship between total length and carapace circumference indicate that males smaller than 92 mm and females smaller than 97 mm can escape through the meshes. Thus, for some reason, a large proportion of small crayfish that easily can escape from the traps do not escape. Escapement do not increase when the traps are emptied later in the morning after the bait generally is consumed. Trap catches in 1982-85 showed a highly significant, 4% less fraction of non-legalsized crayfish in traps with mesh size 21 mm as compared to traps with mesh size 17.5 mm, and we therefore recommend a mesh size of 21 mm when the minimum legalsize of crayfish is 95 mm total length.

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