Background information
Stock condition
Pacific bluefin tuna (PBF) is managed as a single, Pacific‑wide stock. The species spawns only in the western North Pacific, primarily around the Sea of Japan, and a portion of each year’s juveniles migrates across to the eastern Pacific to feed before most return west to mature and spawn. This “born in the west, feed in the east, return to spawn in the west” pattern is the backbone of how scientists and managers think about the stock and why the two Pacific Ocean tuna commissions (WCPFC and IATTC) coordinate on its management.
The 2024 assessment shows that, after decades of high fishing pressure, the stock fell to a historic low around 2010 and has since rebuilt strongly. By 2022, the breeding biomass (spawning stock biomass, or SSB) reached 23.2% of the unfished level (about 144,000 t) and the population had already met the second rebuilding target of 20% of the unfished level in 2021. The assessment attributes much of this recovery to sustained reductions in fishing on small, young fish (ages 0–3).
Current fishery status
PBF is harvested on both sides of the Pacific. In the western and central Pacific, the main fishing fleets come from Japan, the Republic of Korea, and Chinese Taipei; in the eastern Pacific, Mexico and the United States are the principal participants. Purse‑seine fleets catch most of the fish, historically with a strong focus on juveniles, while longline and various coastal gears catch smaller shares and generally target larger fish. Long time‑series of catches show pronounced swings: very high landings in the 1930s and 1950s, a low point around 1990, and then tightening rules from 2011 onward coinciding with stock rebuilding. In 2022, the data showed that about 83% of the stock impact came from western Pacific fleets and 17% from the east, with impacts stronger when more juveniles are taken.
Importance of the fishery
PBF is a high‑value species supporting industrial, coastal, and recreational fisheries, as well as ranching and farming operations, across the North Pacific. Its economic footprint, from vessels and ports to processors and export markets, helps explain the intense bilateral and multilateral attention to its management. Recognizing those stakes, the current WCPFC measure calls for stronger monitoring of both fisheries and farming and for developing a Catch Documentation Scheme (CDS) to keep illegal products out of markets. These are seen as practical steps to protect the value of legitimate fishing and the credibility of the supply chain.
Science
Stock status – what the latest assessment says
The 2024 benchmark assessment confirms that the rebuilding plan has worked better than expected. SSB exceeded the second rebuilding target in 2021 and was estimated at 23.2% of the unfished level in 2022 (point estimate 144,483 t), more than ten times the 2010 low. Recent fishing intensity (measured as an SPR‑based indicator of fishing mortality) has fallen markedly compared with the 2000s, and the stock is not overfished relative to the 20%SSB reference used for some other tunas and not subject to overfishing relative to common F‑based reference points. The science also notes uncertainty in very recent recruitment: modelled recruitment for 2019–2021 is on the low side, but a standardized juvenile index from a Japanese troll monitoring program indicates relatively strong recruitment in 2021–2023, consistent with the observed rise in biomass. Taken together, the trends reflect rapid rebuilding since 2011 driven by lower fishing mortality on small fish.
Scientific advice to managers
Scientists advise that some cautious catch increases are possible while keeping the stock above the 20% rebuilding target with about 60% probability, provided increases are carefully distributed across areas and sizes and current rules are fully implemented. The risk of dropping below the target rises as catch rises, especially if increases concentrate on juveniles. Continued protection of small fish is emphasized because taking many juveniles depresses future spawning capacity more than taking the same weight of adults. The assessment also flags practical needs: keep improving recruitment and adult abundance indices, maintain accurate catch and size data (including discards), and recognize that current projections assume full compliance with measures and do not explicitly model discard mortality, an issue that grows in importance as catches expand.
In 2025, the International Scientific Committee (ISC) PBF Working Group completed an extensive management strategy evaluation (MSE) that stress‑tests alternative harvest control rules (HCRs) across a range of biological “what‑ifs.” Sixteen HCRs were examined, tuned to two alternative balances of fishery impact between the western/central Pacific and the eastern Pacific (80:20 and 70:30). The MSE results show that many candidate HCRs meet safety objectives while differing in expected yield and how often catch limits would need to change. The exercise also clarified details like a ±25% cap on how much total allowable catch (TAC) can change between management periods, which smooths year‑to‑year shocks but can slow the response to rapid ups or downs in stock size. During finalization, the Working Group identified and excluded a small number of simulated assessment “outliers” that produced unrealistic biomass dips; importantly, removing those runs did not change the relative ranking of HCRs. Robustness tests suggested most HCRs handled scenarios like effort creep and doubled discards reasonably well; a steep, temporary recruitment drop predictably forced TAC cuts, with recovery following as recruitment improved.
Management Framework
WCPFC’s current management framework
Within the WCPFC area, CMM 2024‑01 implements the PBF harvest strategy and sets the rules that anchor current fishing. Fishing effort north of 20°N must remain below the 2002–2004 average. Catch limits are specified by size class (<30 kg (“small”) and ≥30 kg (“large”)) for Japan, Korea, and Chinese Taipei. For example, Japan’s annual initial limits are 4,407 t of small fish and 8,421 t of large fish; Korea’s are 718 t small and 501 t large; and Chinese Taipei’s is 2,947 t of large fish. Members may carry over up to 17% of unused quota to the next year, and a 0.68 conversion factor allows limited transfer from small‑fish limits to large‑fish limits in the same year (not the other way), reinforcing the policy preference for catching larger fish. The measure requires comprehensive annual reporting across gears and size classes (including discards), encourages juvenile catch reduction, strengthens monitoring of fisheries and farming, and advances a catch documentation scheme (CDS). It also directs close coordination with the IATTC so rules work across the fish’s full range, and it schedules a 2026 review informed by the new stock assessment and the MSE‑based harvest strategy.
b. Management issues
Two strategic questions dominate. First, how to share fishing impacts between the western/central Pacific and the eastern Pacific. Science shows the heaviest impact comes from western Pacific fleets (~83% in 2022) so choosing and then holding to a fair WCPO:EPO balance (e.g., 80:20 or 70:30) is pivotal both for stock outcomes and for regional equity. The MSE evaluates HCRs tuned to each of these splits so managers can see the trade‑offs clearly. Second, how to treat juveniles versus adults. The biology is unequivocal: heavy juvenile harvest suppresses future spawning biomass more than adult harvest does, so rules that incentivize taking larger fish, such as the size‑based limits and the one‑way small‑to‑large transfer, are more consistent with long‑term stock health.
There are also practical implementation issues. The MSE exposed a few estimation outliers in simulated assessments. While they did not change the ranking of HCRs, they highlighted the need for robust diagnostics once an HCR is in place. The cap on changes to the total allowable catch (TAC) (±25%) helps industry plan, but it also means catches may lag real‑time stock swings, especially during abrupt recruitment changes. Finally, discard mortality remains uncertain and is not fully represented in projections. As catches increase, accounting for discards through observer programs, electronic monitoring, and clear rules becomes more important. Beyond fisheries specifics, the ISC notes that fully integrating climate variability and change into stock assessments and management advice will take additional analytical work and cross‑institutional coordination.
Improving future management
The data and trends point to a practical path. The two tuna commissions should adopt a long‑term harvest strategy built around a transparent harvest control rule and agreed reference points, tuned to a chosen WCPO:EPO impact ratio, and accompanied by predictable TAC‑change rules so industry can plan with confidence. Keeping juvenile controls tight and using permitted small‑to‑large transfers judiciously, aligns catches with the biology underpinning recovery. Strengthening data systems remains foundational, where timely catch and size reporting (including discards), reliable recruitment monitoring, and a robust adult index reduce uncertainty in the assessment and in the management procedure that will calculate TACs. Completing the CDS will protect compliant fishers and markets by reducing incentives for illegal catch. The MSE shows that well‑designed procedures are robust to many uncertainties, but it also normalizes the idea that if recruitment dips, short‑term TAC cuts are the right response to keep the stock on track, an expectation worth building into decision‑making now.
Summary and take‑home message
In plain terms, PBF has rebounded strongly under tighter rules. The 2024 assessment shows the breeding biomass above the second rebuilding target and fishing pressure at levels compatible with sustainable management, with much of the gain coming from reduced juvenile harvest. The science indicates that measured catch growth is feasible, but only if we lock in a durable harvest strategy that keeps the fishery within safe bounds and continues to protect small fish. The management focus for the next two years is to choose and implement that strategy. In other words, select an HCR, agree on the WCPO:EPO impact balance, settle sensible TAC‑change rules, and embed solid monitoring and a catch documentation scheme so data and markets stay trustworthy. If we do those things, the fishery can remain profitable and reliable while the stock stays healthy.