Bluefin tuna science remains vague

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Science  17 Nov 2017:
Vol. 358, Issue 6365, pp. 879-880
DOI: 10.1126/science.aar3928

The International Commission for the Conservation of Atlantic Tunas (ICCAT) spent the past 8 years and more than US$15 million to improve scientists' understanding of Atlantic bluefin and their management (1). Unfortunately, this year's long-awaited stock assessment produced more questions than answers.

ICCAT's model for the eastern bluefin stock's assessment is unreliable because small tweaks to the input data result in substantial differences in quota advice. For example, adding just one year of data increased the model's recommended quotas by 70% (2). Adding data from a single fishery (i.e., one abundance index) increased the estimated sustainable quotas by 126%. Depending on the assumptions made about future productivity, the model results range from estimates indicating that the stock is recovered to estimates indicating that it is not recovered.

For ICCAT's western stock assessment, the biggest challenge was the inability to determine what constitutes a healthy stock level, due to uncertainty that remains about some biological questions, such as the relation between the number of adults and the number of young fish produced (2). The assessment does, however, conclude that the stock size is as low as 45% of the 1974 level and just 18% of what it was in 1950 (2). The science is also clear that abundance is expected to decline, even without increased fishing (3). Furthermore, westward migration of eastern bluefin has increased in recent years (4), to the extent that estimated increases in the western stock size could be entirely the result of fishermen catching (and scientists counting) eastern fish in western areas.

Instead of providing precautionary advice in light of this severe uncertainty, ICCAT's scientific body told policy-makers that substantial increases in quotas are sustainable over the short term, even though these quotas are projected to cause declines in both stocks (4). As chair of IUCN's Tuna and Billfish Specialist Group, I would instead recommend that when policy-makers meet in Morocco in November that they set quotas at levels that would allow both stocks—neither of which is considered recovered—to grow.


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