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Economics & Social Sciences Research Program

Bioeconomic Model for Estimating Maximum Economic Yield of North Pacific Crab Stocks

Two sets of linked biological and economic models were developed for the snow crab (Chionoecetes opilio) resource in the eastern Bering Sea (EBS snow crab). The first bioeconomic model is based on the population dynamics model on which the 2009 stock assessment was based. This population dynamics model accounts for sex, maturity state, shell condition and length, and is fitted to a variety of data sources including survey indices of male and female abundance and the length-compositions of survey and fishery catch and bycatch.

The economic component of this bioeconomic model relates fishing mortality in the directed pot fishery to sea days and estimates variable costs related to fuel, food, and bait (the latter by relating sea days to pot lifts). The second bioeconomic model is based on a simpler population dynamics model (males only with fewer size-classes) and the same economic model. This simplified model fits the available data as well as the more complicated model and is such that it could be used in dynamic optimization analyses.

The two bioeconomic models are compared for the case in which the population dynamics are deterministic (no process error about the Beverton-Holt stock-recruitment relationship). The estimates of maximum sustainable yield (MSY) and maximum economic yield (MEY), and the corresponding effort levels, EMSY and EMEY, are MSY: 44,100 t vs. 44,900 t; MEY: 156.9 million $ vs. 162.0 million $; EMSY: 4.745 vs. 7.140 sea days; and EMEY: 4,165 vs. 6,309 sea days.

The version of MEY used here is a static concept and does not account for the impact of the initial state of the resource as well as costs and revenues during transient periods. These factors are important for EBS snow crab because this stock was assessed to be below the biomass corresponding to MSY in 2009. The effort level which maximizes net present value (discount rate = 0.05) is 5,570 sea days (i.e., higher than EMSY). Sensitivity tests explored the impact of changes to major assumptions of the models, including the value assumed for natural mortality, M, the form of the stock-recruitment relationship (Beverton-Holt or Ricker), and whether account is taken of parameter uncertainty and recruitment fluctuations.

By Mike Dalton and Brian Garber-Yonts

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