Three-Stage Catch-Survey Analysis Applied to Blue King Crabs

Three-Stage Catch-Survey Analysis Applied to Blue King Crabs

Jeremy S. Collie, Allison K. DeLong, and Gordon H. Kruse

Three-Stage Catch-Survey Analysis Applied to Blue King CrabsThis is part of Fisheries Assessment and Management in Data-Limited Situations
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Catch-survey analysis (CSA) is a stock assessment method that is applicable in data-limited situations. It requires time series of catch and relative abundance divided into two categories: recruits and post-recruits. This method is particularly useful for invertebrate populations (e.g., crabs) that cannot be aged but can be classified by their size distributions. In this study we extended the existing two-stage CSA to include a third stage, pre-recruits. An additional equation describes the transition of pre-recruits to the recruit stage as a function of survival and molt probability. We applied the three-stage CSA to two blue king crab (Paralithodes platypus) stocks: St. Matthew Island and the Pribilof Islands in the eastern Bering Sea. Sensitivity analyses and Monte Carlo tests were performed on data from the St. Matthew Island stock. The final version of the model includes bootstrapped confidence intervals on the estimated population abundances. Abundance of both stocks declined following high harvest rates in the early 1980s and the fisheries have been closed since 1999. Three-stage catch-survey analysis has several advantages over the two-stage CSA. It uses more data and therefore has a higher ratio of observations to parameters. Three-stage CSA also provides estimates of mature and legal male crabs, both of which are used for management. By accounting for observation errors for an additional stage, it provides more accurate estimation of abundance in the most recent year, which is critical for setting harvest guidelines.

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