We briefly reviewed the decision rules currently used for managing two major high-latitude crab stocks, snow crab (Chionoecetes opilio) and Dungeness crab (Cancer magister), in the United States and Canada and compared them with model-based reference points, harvest rate, and biomass proportion relative to virgin biomass, developed using species- and area-specific parameters. The model followed a size-based approach, which incorporated BevertonHolt and Ricker stockrecruitment models and estimated mean and median reference points. The recruitment was also perturbed to generate distributions of reference points. The BevertonHolt stockrecruitment model provided a lower harvest rate than that of the Ricker model. Harvest rates were lower for combined sexes spawning biomass than for female-only spawning biomass. Increasing the minimum size at first capture and decreasing the handling mortality resulted in increased harvest rates. Changes in fishery duration and timing of fishery open date did not change the harvest rate appreciably. The harvest rates for the Canadian snow and Dungeness crabs were mostly higher than those estimated for the Bering Sea and Southeast Alaska stocks. Reliable estimates of a number of life history parameters are lacking for both species, and hence, the results of this exercise need to be treated in a precautionary manner.
Length-based models were developed for the male Dungeness crab (Cancer magister) population on the Fraser delta near Vancouver, British Columbia. The models incorporate the probability of moulting, moult increments, natural mortality during moulting and non-moulting periods, direct fishing mortality, and handling mortality that occurs when sublegal-sized crabs are caught and released. The models were used to investigate how long-term yield might be affected by the combination of handling mortality and an intensive fishery. The models were calibrated to survey data, and key biological parameters were estimated. The probability of moulting is near one for male crabs in the 130-to 150-mm carapace width range and decreases as crabs get larger. There is a 70.1% probability a crab will survive the 1-month period beginning with a moult. The non-moulting natural mortality rate is 0.97 year -1 . When handling mortality is incorporated into the model, yield per recruit increases with the exploitation rate until it reaches approximately 94%. F 0.1 is equivalent to 70%. An approach was developed to calculate the threshold ratio of discarded to retained crabs beyond which fishing would reduce the long-term yield.Résumé : Nous avons mis au point des modèles basés sur la longueur que nous avons appliqués à une population de crabes dormeurs mâles (Cancer magister) du delta du Fraser, près de Vancouver. Les modèles tiennent compte de la probabilité de la mue, de la croissance à la mue, de la mortalité naturelle durant les périodes de mue et en dehors de ces périodes, de la mortalité directe due à la pêche et de la mortalité due à la manipulation, lorsque des crabes de taille inférieure à la taille réglementaire sont capturés et relâchés. Les modèles ont servi à déterminer comment le rendement à long terme peut être affecté par la combinaison de la mortalité due à la manipulation et d'une intensité de pêche élevée. Nous avons calibré les modèles aux données d'inventaire et nous avons estimé les paramètres biologiques. La probabilité de la mue chez les crabes de largeur de carapace 130-150 mm est près de 1 et elle diminue en fonction de la taille chez les crabes plus grands. La probabilité qu'un crabe survive à la période d'un mois qui suit la mue est de 70,1 %. La mortalité naturelle est de 0,97·année -1 en l'absence de mue. Si la mortalité due à la manipulation est ajoutée au modèle, le rendement par recrue augmente en fonction du taux d'exploitation jusqu'à atteindre approximativement 94 %. F 0,1 équivaut à 70 %. Nous avons développé une méthodologie pour calculer le seuil du rapport des crabes rejetés sur les crabes retenus au-dessus duquel la pêche réduit le rendement à long terme.[Traduit par la Rédaction] Zhang et al. 2134
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