Assessment and management of crab stocks under uncertainty of massive die-offs and rapid changes in survey catchability

Assessment and management of crab stocks under uncertainty of massive die-offs and rapid changes in survey catchability

J. Zheng and G.H. Kruse

Assessment and management of crab stocks under uncertainty of massive die-offs and rapid changes in survey catchabilityThis is part of Crabs in Cold Water Regions: Biology, Management, and Economics
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Description

Survey abundance of a stock can drop sharply from one year to the next. Typically, for fish stocks, stock assessment scientists assume that the sharp decrease in abundance is caused by changes in survey catchability, not natural mortality. However, for crab stocks, occasional sharp drops in abundance can be caused by an increase in natural mortality from massive die-offs, which are substantiated by subsequent annual surveys. Lacking auxiliary data on changes in catchability and mortality, stock assessment modelers are challenged to provide fishery management advice for the year in which the shift in survey abundance occurs. In this study, we used the 1999 stock assessment of St. Matthew Island blue king crabs (Paralithodes platypus) as an example to demonstrate our approach. First, we searchedf or evidence of an increase in natural mortality and decrease in survey catchability. The available evidence was not conclusive enough to rule out either cause. Second, we developed a four-stage catch-survey model to assess the stock under different assumptions of natural mortality in 1999. Finally, we used the model to evaluate the consequences of different natural mortality assumptions on the stock by projecting the stock abundance into the near future. We suggest that a conservative management approach should be taken when the stock-recruitment relationship may be depensatory or when the handling mortality rate of crab bycatch may be high.

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