Results
The total catches of bluefin have fluctuated considerably during the last 50 years (Figure E-1).
|
Figure E-1: Retained catches of Pacific bluefin tuna, by gear, 1952-2020. GN: gillnet; LL: longline; LX: hook and line; OTR: other; PS: purse seine. |
The consecutive years of above-average catches (mid-1950s to mid-1960s) and below-average catches (early 1980s to early 1990s) could be due to consecutive years of above-average and below-average recruitments. The estimated impact of the fisheries on the bluefin population for the entire time period modeled (1952-2018) is substantial. The WPO fisheries have had a greater impact than the EPO fisheries, and their impact increased starting in 1980s only leveling off in 2000s (Figure E-2).
|
Figure E-2: Estimates of the impact on the Pacific bluefin tuna population of fisheries in the EPO and in the WPO (upper panel). The dashed line represents the estimated hypothetical unfished spawning biomass, and the solid line the estimated actual spawning biomass. The shaded areas indicate the impact attributed to each fishery. The lower panel presents the proportion of impact attributed to the EPO and WPO. (Figure from the draft Executive Summary of ISC 2022 stock assessment; subject to change and approval by the ISC Plenary.) |
The WPO fisheries have had a greater impact than the EPO fisheries, and their impact increased starting in 1980s only leveling off in 2000s.
A benchmark stock assessment was carried out by the Pacific Bluefin Working Group of the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC) in 2022. The assessment was conducted with Stock Synthesis 3, an integrated statistical age-structured stock assessment model. The base-case model results show that: (1) spawning stock biomass (SSB) fluctuated throughout the assessment period (fishing years 1952-2020); (2) the SSB steadily declined from 1996 to 2010; (3) the SSB has increased since 2011 resulting in the 2020 SSB being back to the 1996 level; (4) total biomass after 2011 continued to increase with an increase in young fish, creating the 2nd highest biomass peak in the assessed history in 2020; (5) fishing mortality (F%SPR), which declined to a level producing about 1% of SPR in 2004-2009, returned to a level producing 30.7% of SPR in 2018-2020; and (6) SSB in 2020 was 10.2% of SSB
F=0, an increase from the 5.6% of SSB
F=0 estimated for 2018 in the 2020 assessment (2018 was the last year of the 2020 assessment). Based on the model diagnostics, the estimated biomass trend for the last 40 years is considered robust although SSB prior to the 1980s is uncertain due to data limitations. The SSB in 2020 was estimated to be around 65,464 t, which is a 30,000 t increase from 2018 according to the base-case model. An increase of young fish (0-2 years old) biomass was observed in 2016-2020, likely resulting from low fishing mortality on those fish and is expected to accelerate the recovery of SSB in the future even further. Historical recruitment estimates have fluctuated since 1952 without an apparent trend. A substantial decrease in estimated
F is observed in ages 0-2 in 2018-2020 relative to the previous years (note that stricter management measures in WCPFC and IATTC have been in place since 2015).
The point estimate of the 2020 SSB was 10.2% of the SSB in the absence of fishing (10.2%SSB
F=0), and the recent (2018-2020) fishing mortality (
F) corresponds to F30.7%SPR. Because the harvest strategy includes catch limits, fishing mortality is expected to decline,
i.e.,
Fx%SPR will increase as biomass increases. No biomass-based limit or target reference points have been adopted to evaluate whether Pacific bluefin is overfished. However, the stock is overfished relative to common target reference points, but does not exceed the IATTC limit reference point used for tropical tunas. Also, no fishing intensity-based limit or target reference points have been adopted to evaluate whether overfishing of Pacific bluefin is occurring.
Resolution
C-18-02 states that the Commission recognizes that the management objective of the IATTC is to maintain or restore fish stocks at levels capable of producing MSY, and shall implement a provisional rebuilding plan by adopting an 1) initial (first) rebuilding target of SSB
med, 1952-2014 (the median point estimate for 1952-2014) to be achieved by 2024 with at least 60% probability and 2) a second rebuilding target of 20%SSB
F=0 to be achieved within 10 years of reaching the initial rebuilding target or by 2034, whichever is earlier, with at least 60% probability. The IATTC has adopted resolutions to restrict the catch of bluefin tuna in the EPO (e.g.
C-16-08,
C-18-01,
C-20-02,
C-21-05). Resolution
C-21-05 states that during 2021-2022, in the IATTC Convention Area, combined total commercial catches of Pacific bluefin tuna by all CPCs shall not exceed the catch limit of 7,295 metric tons. During 2023-2024, in the IATTC Convention Area, combined total commercial catches of Pacific bluefin tuna by all CPCs shall not exceed the catch limit of 7,990 metric tons. Resolution
C-18-02 also requires that no later than the IATTC meeting in 2020, taking into account the outcomes of the Joint IATTC-WCPFC NC Working Group, the Commission shall consider and develop candidate reference points and harvest control rules. These candidate reference points and harvest control rules will be forwarded to the Joint IATTC-WCPFC NC Working Group and ISC for consideration and potential inclusion in a management strategy evaluation to be completed by the ISC. This task has still not been accomplished.
The 2020 SSB was above the initial rebuilding target but remains below the second rebuilding target adopted by the WCPFC and IATTC. However, stock recovery is occurring at a faster rate than anticipated by managers when the Harvest Strategy to foster rebuilding were implemented in 2014. Projections based on the base-case model under several harvest scenarios and time schedules as requested by the RFMOs, including the harvest strategy proposed at the Joint WCPFC NC-IATTC WG meeting. Under all examined scenarios the second rebuilding target is reached, and the risk of SSB falling below the historical lowest SSB at least once in 10 years is negligible. Some scenarios had the future impact ratios between WPO and EPO specified by the RFMOs and were tuned to achieve the second rebuilding target with 60% probability, and result in larger catch increases.