Effect of limb damage on the survival and burial time of discarded spanner crabs Ranina ranina (Linnaeus)

Abstract

Spanner crabs (Ranina ranina) often lose legs or segments of legs when being disentangled from fishing nets. Two field experiments investigated the effect of this on survival of discarded undersize crabs. To examine mortality directly attributable to limb damage, 100 crabs were each subjected to one of five treatments: no damage, or removal of one dactylus, three dactyli, one leg or one cheliped. Limb damage had a significant effect on survival, mean mortalities being, respectively, 5%, 20%, 25%, 55% and 90%. Thus, present fishing methods may lead to high mortality among discarded crabs.
To examine whether limb damage leads to increased exposure to predation, 40 crabs were released after each was subjected to one of the above treatments. There was no significant effect of limb damage on the time it took crabs to bury themselves. Crabs sank at a mean rate of 0.26 m s-1, and on reaching the sea floor most crabs were motionless for up to 20 min before becoming active and immediately burying themselves; time to burial ranged from 6 s to 20 min, with 65% burying themselves within 68 s of reaching the sea floor.