Treatment of Data and Model Uncertainties in the Assessment of Southern Bluefin Tuna Stocks
T. Polacheck, A. Preece, A. Betlehem, and N. Klaer
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Southern bluefin tuna (SBT) is a highly migratory pelagic species with reasonably complete catch and effort time series data for the entire history of its exploitation. The information base for assessing its status is rich compared to that available for most large pelagic fisheries. The current SBT stock assessment is based on information from catch per unit effort (CPUE), qualitative fishery indicators, and virtual population analyse (VPA) tuned to CPUE and tagging data using the ADAPT framework (Gavaris 1988). Within this framework conflicting results about the most recent trends and future projections occur, depending on hypotheses used regarding natural mortality rates, selectivity, and methods for modeling the older aged fish. A full cross of the parameter space representing hypotheses for the major uncertainties results in thousands of VPAs, which can no longer be individually assessed. Instead, a sample of the parameter space is taken and a weighted mean estimate of the status of the stock is produced. The approaches and methods for identifying, weighting, sampling, and incorporating uncertainties into the assessment process for SBT are described and discussed. The VPA estimates of the current status of the SBT spawning stock relative to historical levels exhibit a remarkable degree of robustness to the range of hypotheses while the estimates of absolute stock size exhibit a much wider range of variability. Not all hypotheses for the major uncertainties were equally compatible with the data and we explore the effects on the results of weighting them by an objective lack of fit index. We also look at the effects on the results of different prior weightings. The results emphasize the need for an integrated and balanced research strategy if substantial improvements to the assessment are to be achieved.
- Item number: AK-SG-98-01af
- Year: 1998
- DOI: https://doi.org/10.4027/fsam.1998.32