New Stock Assessment Models Suggest Bigeye Tuna May No Longer Be Overfished

posted in: October 2017 | 0
Distribution and magnitude of bigeye tuna catches for the most recent decade of the stock assessment (2006-2015) by 5◦ square and fishing gear: longline (green), pole-and-line (red), purse seine (blue) and miscellaneous (yellow), for the WCPO and part of the EPO. Overlayed are the regional boundaries for the stock assessment (2017 regional structure).


Every year, the Western and Central Pacific Fisheries Commission (WCPFC) holds its annual meeting in December mainly to hash out how to best manage tuna stocks that have been depleted over the years by purse seiners and longline vessels. With delegates often refusing to budge from positions that protect their countries’ economic interests, it often happens that they struggle for days to reach agreements on how to best relieve the pressure on stocks that are overfished and/or subject to overfishing, only to have nothing to show for it by the meeting’s end.

This year, there have been early indications that the participating commission members are still far apart on what they believe is the best way to manage the bigeye tuna stock targeted by Hawai‘i’s longline fleet. But, for now, the pressure to severely cut back on fishing effort may not be as great as it has been in the past.

At the WCPFC’s Scientific Committee meeting held in the Cook Islands this past August, scientists with the Oceanic Fisheries Program of The Pacific Community released their most recent stock assessment for bigeye throughout the area of the commission’s jurisdiction. They found that in most of their computer model runs, the current spawning biomass of the stock appeared to have improved since the last stock assessment in 2014, which determined that the spawning biomass was merely 16 percent of what it is calculated to have been when the stock was unfished.

Although the stock was still more depleted than most other tuna stocks, the scientists wrote, when new growth rates were factored in, all models indicated that the spawning biomass was well above 20 percent of the unfished spawning biomass. That suggests that the stock is no longer overfished under the commission’s definition, although it would still be subject to overfishing.

“If recommendations are to be made based solely on models with the new growth function, then the bigeye stock in the WCPO [Western and Central Pacific Ocean] appears not to be in an overfished state although several model runs are at the boundary of the designation of overfishing,” they wrote.

However, if models with the older growth rates are considered, stock status estimates are less clear-cut, they continued. In addition to using newer growth rates, the scientists revised the way they evaluated fishing impacts by region. The older growth models, when combined with the new regional structure, yielded more optimistic results than when evaluated under the 2014 regional structure. In the former case, the estimated spawning biomass level would indicate that the stock is no longer overfished; in the latter, it would still be considered overfished.

“The 2017 regional structure led to significantly more optimistic results, which can be attributed to the model redistributing the stock biomass from the more heavily exploited equatorial regions to the northern temperate regions, which are less exploited. Consequently, the depletion trajectories for individual regions for the 2014 and 2017 regional structures are relatively similar among regions (higher in equatorial regions, lower in northern temperate regions), though because more of the population is assumed to be present in the northern regions under the 2017 regional structure, the net depletion estimated for the WCPO is lower,” they wrote.

“Given the critical nature of the growth assumptions on the assessment, it may be worthwhile to consider further investment in research to improve the existing growth data set and its analysis,” they wrote, suggesting that more work be done to improve age distribution estimates and to better characterize any regional differences in growth between the WCPO and the Eastern Pacific.

Finally, the scientists cautioned against simply assuming that the stock was no longer overfished based on the model results, especially given that bigeye recruitment spiked relatively recently.

“The significance of the recent high recruitment events and the progression of these fish to the spawning potential component of the stock are encouraging, although whether this is a result of management measures for the fishery or beneficial environmental conditions is currently unclear,” they wrote.

They pointed out that similar recruitment events have also been estimated for skipjack and yellowfin tuna in the region, and bigeye in the Eastern Pacific, “which may give weight to the favorable environmental conditions hypothesis. Whether these trends are maintained in coming years will help tease these factors apart and will likely provide more certainty about the future trajectories of the stock.”

Bridging Measure

The WCPFC’s Conservation and Management Measure (CMM) for tropical tunas expires at the end of this year. Hoping to avoid repeating the lackluster results typical of previous annual commission meetings, representatives from several member countries met in Honolulu in late August to draft a new measure to be adopted when the full commission meets in Manila in early December.

With regard to longline catch limits for bigeye, the group identified three options:

• Maintain the current catch limits (for the United States, that’s 3,345 metric tons);

• Representatives from the United States proposed that catch limits apply only to fishing that occurs between 20 degrees North latitude and 20 degrees South. This approach would target the area where fishing pressure is heaviest and also allow the Hawai‘i longline fleet to fish unfettered across a large portion of its historic fishing grounds.

• In stark contrast to the U.S. proposal, Japan suggested setting hard caps for all participating members. Notably, while Japan proposed that its cap, as well as those for China, Taiwan, Korea, and Indonesia remain as they are, it recommended cutting the United States’ cap from 3,345 to 2,508 metric tons. The Hawai‘i longline fleet in recent years has consistently met or exceeded its annual catch limit, forcing it to enter into quota transfer agreements with Guam, American Samoa, and the Commonwealth of the Northern Mariana Islands. The fleet would likely hit an annual limit of 2,508 metric tons before mid-year considering its catch rates over the last few years.

Given the great divergence between the latter two options, it’s possible the commission — which tends to adopt only those measures that have full consensus — may again fail to reach an agreement or simply agree to maintain the status quo.

— Teresa Dawson

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