Impact of Ocean Acidification on Recruitment and Yield of Bristol Bay Red King Crab
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The excess of anthropogenic carbon dioxide (CO2) produced since the industrial revolution is being absorbed by the oceans through the carbon cycle. Atmospheric carbon dioxide has increased about 40% since the preindustrial era, and the oceans have absorbed more than a third of these emissions. This has led to the release of H+ ions via seawater carbonate chemistry, and hence to a reduction in ocean pH, that is, ocean acidification. Ocean pH has been reduced by roughly 0.1 units, which is equivalent to an increase in H+ of roughly 30%, and about a 16% decrease in . Corrosive waters, the waters below the CaCO3 saturation horizon, are predicted to reach shallower depths more in the Northeast Pacific Ocean than in any other ocean basin. The saturation horizon is projected to reach the surface of the North Pacific Ocean during this century, and some regions of the Bering Sea are predicted to become carbon shell corrosive seasonally by the middle of this century, which will expose a wide range of North Pacific species, including Bristol Bay Red king crab, to corrosive waters. Bristol Bay Red king crab has been one of the most valuable fished stocks in the US. It is managed by the State of Alaska under federal guidelines defined in the Fishery Management Plan (FMP) for crab in the Bering Sea and Aleutian Islands. Current management rules are designed to handle short-term fluctuations in stock abundance mainly due to exploitation. The impact of ocean acidification on red king crab is predicted to lead to long-term changes to stock abundance, and for which management is currently unprepared. This thesis explores the impact of ocean acidification on recruitment and yield of Bristol Bay red king crab under a range of ocean acidification scenarios and management strategies. The management strategies include setting the exploitation rate for the directed fishery to that under the overfishing limit (OFL) rule, applying constant exploitation rates, and setting exploitation rates that maximize catch and discounted profit. Trends in recruitment to the first size-class in the stock assessment model are estimated using a pre-recruit model in which survival is parameterized based on experimental results from the NMFS Kodiak laboratory. Exploitation rates are estimated, and time series (2000-2100) for MMB, catch, and discounted profit are projected, for each management strategy for three levels of variable fishery costs and for the economic discount factor. The catch, biomass, and discounted profit equilibrate at non-zero values for the no-OA scenario, but are driven to zero for all exploitation rates in the OA scenarios. Lower constant exploitation rates lead to a longer time before the biomass is driven close to zero, but the total discounted profit is highest at the highest exploitation rate for the three OA scenarios. The OFL control rule performs better than the constant exploitation rate strategies in terms of conserving the resource, because this rule closes the fishery at low biomass levels, which are also unprofitable. Estimated total discounted profits for the strategies which maximize catch and discounted profit are about the same for the base no-OA scenario, while the strategy that maximizes profit leads to slightly higher discounted profit and it depletes the stock below the biomass threshold sooner than the strategy which maximizes catch. Catches are the same for the strategies which maximize catch for no-OA scenario, and are higher for the strategy which maximizes catch for the OA scenarios. Higher discount rates lead to higher biomasses and catches, and the fishery is closed earlier for higher costs (food, fuel, and bait costs) for the OA scenarios when exploitation rates are selected to maximize profit.
- Fisheries