Managing Atlantic Bluefin Tuna in the Midst of Uncertainty
History of the Stock-Recruitment Relationship of the Western Stock at ICCAT
The stock-recruitment relationship, or SRR, is the relationship between the number of spawning fish in a population and the number of young fish they produce. This relationship is used by fisheries scientists to project future population sizes, and to evaluate the status of the population relative to the established rebuilding target. In the case of the western population of Atlantic bluefin tuna, scientists use two divergent SRRs in their projections: the long-standing high recruitment scenario, or HRS, which predicts that recruitment will increase as the number of adults increases up to a point; and the more recently introduced low recruitment scenario, or LRS, which suggests something changed in the environment that permanently reduced the number of young that recruit into the fishery even if the number of adults increases. While no evidence has been found of an environmental change, or regime shift, that would have led to a decreased carrying capacity and lower potential recruitment for bluefin tuna, because the spawning stock biomass and therefore recruitment have been low in recent years, the scientific body of the International Commission for the Conservation of Atlantic Tunas, or ICCAT, considers the high and low recruitment scenarios “equally plausible.” This has posed a significant challenge for managers because the two scenarios produce opposing assessments of stock status and conflicting results related to appropriate quotas and rebuilding targets necessary to achieve a healthy population.
This paper tracks the history of the debate regarding which of these two SRRs is more applicable to western Atlantic bluefin. Given the implications of quota setting for the health of the western bluefin population, it is important that fisheries managers have a clear understanding of the evolution of these two scenarios and the risks associated with setting quotas based on the wrong SRR. In light of the severely depleted status of western Atlantic bluefin and the lack of certainty regarding the appropriate SRR, a precautionary approach is warranted. Since the risks of exclusively managing under the LRS far exceed the risks of managing under the HRS, it is critical that ICCAT continue to follow the scientists' advice to maintain the quota at the current level or lower to allow the population to recover, to continue to grow to a size capable of withstanding greater fishing effort in the future and to enable scientists to gain further insight into the true SRR of western Atlantic bluefin tuna.
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