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An evaluation of the current minimum legal size limit for the Dungeness crab (Cancer magister Dana) fishery near Tofino, British Columbia Smith, Barry Douglas


The Dungeness crab (Cancer magister Dana) trap fishery in British Columbia has been regulated by a minimum legal size limit of 165 mm spine-to-spine carapace width (CW, 154 mm notch-to-notch CW) since early this century. Evaluation of the size limit by yield- and eggs-per-recruit analyses has been precluded by a lack of information on basic population variables such as growth, mortality, movement and mating behavior. Information on these variables was obtained in an intensive two year study of the Dungeness crab fishery near Tofino, B.C. from April 1985 until March 1987. A cohort of pre-recruit males and females was followed as the males entered the fishery and were subsequently exploited, yielding information on moult increments-at-size , length of intermoult periods, size-at-maturity, size of mating pairs, mortality, movement, commercial fishing success and exploitation rates. Male C .magister entered the fishery during their fourth year after settlement. Size frequency analysis revealed that nearly all legal-sized (>154 mm notch-to-notch CW) males are within the 155.0±11.2 mm instar (≈50% of this instar). Sublegal-sized males in this instar have a high annual natural mortality rate (M=2.8-4.5), hence a low probability (<10%) of surviving to legal size. Legal-sized males have a high annual fishing mortality rate (F=5.1-6.9). Consequently, a small component of the commercial catch is composed of males in larger instars. Despite females not being landed, the negative slope of female catch curves from four regional fisheries was steepest for the most heavily fished region and shallowest for the most lightly fished region. Females mate after moulting while accompanied by a larger male, and in a heavily fished population larger males may be absent. Thus, the catch curves suggested the possibility of reduced mating and moulting, and consequently fewer large females, in heavily fished populations. Yield-per-recruit analysis suggests landings might be markedly improved by lowering the minimum legal size limit. However, eggs-per-recruit analysis, which modeled the possible consequences of removing large males, suggests that for a heavily fished population the current size limit may result in up to a 50% decline in population egg production relative to an unfished population. The historical presumption that the current minimum legal size limit for males does not impact on population egg production should be reconsidered. To facilitate calibration of trap samples biased by differences in soak times, parameters for models describing changes in bait effectiveness over time, and agonistic interactions between crabs within a trap and those attempting to enter that trap, were estimated. Parameter estimates were obtained by simulating the entry and exit of crabs into and out of traps based on experiments which (1) emphasized the above two processes, and (2) estimated the daily probabilities of crabs of different sizes escaping traps.

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