An evaluation of the use of mating marks as an indicator of mating success in male Dungeness crabs

Dungeness crab (Cancer magister) fisheries of the eastern Pacific, from Alaska to central California, have a minimum size limit, measured as carapace width (CW), to ensure that all harvested males have had the opportunity to participate in at least one mating season as sublegal-sized crabs. Dependable indicators of female mating activity have been revealed, but mating activity of males has only been indirectly inferred through examination of so-called "mating marks", presumably created during the premating embrace between a male and female. Previous researchers found that mating marks were present on a higher percentage of sublegal-sized males than on legal-sized males, leading to implications that in a fished population large females may go unmated, and cause a reduction in overall egg production. I present results from laboratory experiments designed to discover the process of mating mark formation, along with at-sea observations documenting the occurrence of mating marks in a natural population in northern California. The experiments involved laboratory matings of male and female crabs to determine the factors that influence the formation of mating marks. Hypothetical factors that may affect mating mark formation include relative male and female sizes, mating frequency, competition, and substrate type. A total of 107 male crabs, some mated multiple times and in different conditions, were allowed to form a total of 155 premating embraces in the laboratory. No mating marks were produced in any of the controlled laboratory matings, however. Mate takeovers, when a competing male displaces the female mate of another male in a premating embrace, were observed, resulting in males forming premating embraces with each other. In addition, inter-male competitive behaviors were observed that may produce marks on the males' claws. At-sea observations revealed that as the mating season progressed, the frequency of mating mark occurrence on all male crabs increased. Throughout the 2004 mating season, sublegal crabs in northern California had a slightly higher frequency of mating marks than legal crabs on half of the collections while half of the collections yielded a slightly higher mating mark frequency on legal crabs. Reliable use of mating marks to indicate mating success would require that (1) every premating embrace, involving the smallest to largest male capable of mating, produces mating marks; (2) every premating embrace results in successful copulation and insemination; and (3) the timing and location of field samples are appropriate for estimating population-level mating activity. Data from laboratory matings and at-sea observations show that these assumptions cannot be reasonably accepted, and therefore it is misleading to use mating marks as an index of mating activity in male Dungeness crabs.