In many decapod fisheries, claws are removed and the animal is returned to the sea with the assumption that there is little impact on the fitness and welfare of the animal, or on the productivity of the population. Here, the impact of claw loss, by two methods of claw removal, is examined during competition between males for access to females in the crab Cancer pagurus. Males induced to autotomize a claw showed little reduction in their competitive ability; however, those subject to the fishery practice of manual declawing showed a marked decrease in their competitive ability. Compared with autotomized males, these declawed crabs displayed activities that suggest an awareness of the wound caused by the appendage being twisted off and the data are consistent with an impaired welfare for these animals. They were also less likely to display to their opponent compared with autotomized crabs. Intact males showed high aggression toward declawed males, which showed low aggression in return. Further, declawed crabs showed particularly high levels of submissive acts. The declawed crabs thus rarely gained the female compared with autotomized crabs. The present study demonstrates that manual declawing has a major detrimental impact on fitness and welfare of edible crabs and we suggest that this method of harvesting should be replaced with induced autotomy of a single claw.