Reaching management objectives given uncertainty: A management strategy evaluation of the eastern Bering Sea snow crab (Chionoecetes opilio) fishery
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The snow crab (<italic>Chionoecetes opilio</italic>) population in the eastern Bering Sea historically supported a lucrative fishery, but was declared overfished in 1999. The subsequent rebuilding plan did not rebuild the fishery in the time specified, and low recruitment over this period likely contributed to the failure. A key component of the rebuilding plan for snow crab was the stock assessment on which management advice was based. In common with all stock assessments, there are numerous potential sources of uncertainty associated with the snow crab assessment, but it is not clear which source is most influential. Consequently, it is unclear how research should be focused and how management should respond to these uncertainties. This dissertation: 1) examines the influence of uncertainty on the ability of the assessment method to estimate quantities important to management, 2) reviews existing information on possible environmental drivers of recruitment, 3) presents an `oscillating control' model that links recruitment to changes in the Pacific Decadal Oscillation (PDO), 4) evaluates management strategies given recruitment dynamics driven by the PDO and 5) explores the influence of spatial structure the snow crab population on assessment. The estimation ability of the assessment method was determined using management strategy evaluation methods. The results of the evaluation allowed biases that may have influenced the ability of the management strategy to rebuild the stock to be identified. When all assumptions are correct, the assessment method provides good estimates of management quantities, including mature male biomass and the overfishing level. Data to reduce uncertainty around growth and natural mortality would be most beneficial to improving confidence in assessment estimates of mature male biomass. The 1988/89 regime shift in the North Pacific appears to have shifted the drivers of recruitment dynamics from female spawning biomass to environmental influences related to the winter PDO (winds at Unimak pass and sea surface temperature were also important variables). This conceptual model was evaluated using cross-validation and retrospective analysis, both of which indicate that it is relatively robust to varying levels of information. A potential mechanism for this change in driver is the influence of temperature during regimes on the timing of the ice retreat and its associated consequences (e.g. food availability, length of the pelagic stage in snow crab, and size of cold pool) for the survival of pelagic larvae and juvenile crab. A management strategy evaluation framework using three scenarios (status-quo, regime-based and `oscillating control') for future recruitment was used to evaluate the performance of harvest control rules, including the status-quo, a regime-based approach, constant proportion strategies, and Hilborn's `hold-steady' method. In regime-driven systems, regime-based harvest control rules increase yield and decrease variability at the cost of a higher probability of overfishing. In non-regime-based systems, regime-based harvest control rules slightly decrease yield (with no change in variability), and increase the probability of overfishing under the definitions imposed by US legislation. Finally, the spatial assessment developed to estimate spatially explicit recruitment, fishing mortality and movement was unable to accurately estimate movement parameters, but reasonably good fits to the data were achieved. The fitted spatial population dynamics model was used to simulate data sets to test the ability of a non-spatially structured (aggregate) assessment method to estimate trends in abundance and fishing pressure in the presence of spatial structure. The aggregate assessment method performed well if survey selectivity was well known, but was unable to reliably capture the dynamics of the population without data on survey selectivity. Additional data on movement from tagging studies or winter surveys would be useful to improve the understanding of the influence of spatial structure on snow crab in the eastern Bering Sea.
- Fisheries