Estimating natural mortality of king crabs from tag recapture data

Estimating natural mortality of king crabs from tag recapture data

M.S.M. Siddeek, L.J. Watson, S.F. Blau, and H. Moore

Estimating natural mortality of king crabs from tag recapture dataThis is part of Crabs in Cold Water Regions: Biology, Management, and Economics
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Natural mortality (M) is used as a surrogate for the maximum sustainable yield level of fishing mortality in determining limit and target reference points under precautionary management for the Bering Sea and Aleutian Islands (BSAI) crab stocks. Precise M estimates are needed to improve stock-assessment models as well as computer-simulation models for evaluating alternative harvest strategies. In the federal crab fishery management plan for the BSAI areas, an annual M of 0.3 for Tanner and snow crabs (Chionoecetes) and 0.2 for king crabs (Lithodes and Paralithodes) were adopted. We attempted to determine plausible M values for BSAI golden, red, and blue king crab stocks by employing an optimization routine that minimized the differences between observed and expected effective tagged populations based on 1990s tag-release-recapture data. The age-based-Virtual Population Analysis (VPA) M estimator produced an annual M of 0.38 for male golden king crabs in the Aleutian Islands and 0.54 for male red king crabs in Bristol Bay. The annual M estimated by the multinomial maximum likelihood method for St. Matthew Island combined sexes blue king crabs was 0.19. The length-based-VPA M estimator was annually variable and ranged from 0.02 to 1.62 for the Bristol Bay red king crab males. The red king crabs M estimates appeared high considering its longevity. The simplified assumptions made for the length-based analysis cast doubt on the variable M estimates. Type A error was high among all experiments,but was estimated from the models and hence did not affect M estimates.

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