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Stock Status and Conservation Information
(From ISC26 Plenary Report)

Albacore

Click here to see the ISC25 stock status and conservation information.

Stock Status

Estimated summary biomass (males and females at age-1+) declined at the beginning of the time series until 2004 (Figure 1). Subsequently, the summary biomass fluctuated without a clear trend until 2018, after which the biomass increased to values higher than those at the beginning of the model time period. It should be noted that the high summary biomass estimates since 2018 also had increasing uncertainty (Figure 1). These high summary biomass estimates can be largely attributed to several strong recruitment estimates in 2017 (∼297 million fish; 95% CI: 215– 381million fish), 2019 (∼292 million fish; 95% CI: 187 to 396 million fish), and 2022 (∼314 million fish; 95% CI: 93-536 million fish) (Figure 1). However, it should be noted that the recruitment estimates in the last 5 years (2020-2024) were highly uncertain and should be treated with caution. Estimated female SSB exhibited a similar trend to the summary biomass, albeit with a lag of several years, and showed an initial decline until 2007 followed by fluctuations without a clear trend through 2020 (Figure 1).

ISC26_alb_Figure8-1
Figure 1. Maximum likelihood estimates of (a) age-1+ biomass, (b) female spawning biomass (SSB), and (c) age-0 recruitment of north Pacific albacore tuna (Thunnus alalunga). Dashed lines indicate 95% confidence intervals.

The average fishing intensity during 2021 – 2023 was estimated to be F66%SPR (95% CI: F74%SPR – F57%SPR), which was relatively moderate and resulted in a population with an SPR of approximately 66%. Instantaneous fishing mortality at age (F-at-age) was similar in both sexes through age-5, peaking at age-4 and declining to a low at age-6, after which males experienced higher F-at-age than females up to age 14 (Figure 2). Juvenile albacore aged 2 to 4 years comprised approximately 76% of the annual catch-at-age in numbers between 1994 and 2024 (Figure 3) due to the larger fishery impact of surface fisheries (primarily troll, pole-and-line), which remove juvenile fish, relative to longline fisheries, which primarily remove adult fish (Figure 4).

ISC26_alb_Figure8-2
Figure 2. Estimated sex-specific instantaneous fishing mortality at age (F-at-age) for the 2026 base case model, averaged across 2021-2023.
ISC26_alb_Figure8-3
Figure 3. Historical catch at age of north Pacific albacore estimated by the 2026 base case model. Panels are arranged for the total catch and ages 0-5 (top), 6-10 (middle), and 11-15 (bottom). Note the difference in y-axis scale between the top panel and middle and bottom panels.
ISC26_alb_Figure8-4
Figure 4. Fishery impact analysis on north Pacific albacore showing female spawning biomass (SSB; red) estimated by the 2026 base case model as a percentage of dynamic, unfished female SSB (SSB_(current,F=0)). Colored areas show the relative proportion of fishing impact attributed to longline (green) and surface (blue) fisheries (primarily troll and pole-and-line gear, but including all other gears except longline).

Stock status is depicted in relation to the target (F45%SPR), threshold (30%SSBcurrent,F=0), and limit (14%SSBcurrent,F=0) reference points (Figure 1; Table 1). The estimated female SSB has never fallen below the threshold and limit reference points since 1994, albeit with large uncertainty in the terminal year (2024) estimates. The historical fishing intensity was estimated to have exceeded the target reference point for most years between 1997-2018. However, the estimated fishing intensity has been declining since 2019 and has been below the target reference point for the last six years. Even when alternative hypotheses about key model uncertainties such as growth were evaluated, the point estimate of female SSB in 2024 (SSB2024) did not fall below the threshold and limit reference points (Figure 5). Estimated average fishing intensity during 2021-2023 (F2021-2023) did not exceed the target reference point under the alternative hypotheses (Figure 5; Table 1). Average fishing intensity from 2021-2023 also did not exceed the average fishing intensity during 2002-2004 (Table 1).

Table 1. Estimates of maximum sustainable yield (MSY), female spawning stock biomass (SSB), fishing intensity (F), and reference point ratios for north Pacific albacore tuna for: 1) the base case model; 2) one important sensitivity model due to uncertainty in growth parameters; 3) a model representing an update of the 2023 base case model to 2026 data; and 4) a model representing uncertainty in the specification of initial fishing mortality.SSB0, SSBcurrent,F=0, and SSBMSY are the expected female SSB of a population in the equilibrium, unfished state; in the current, dynamic, unfished state; and at MSY, respectively. The Fs in this table are indicators of fishing intensity based on spawning potential ratio (SPR) and calculated as %SPR. SPR is the ratio of the equilibrium SSB per recruit that would result from the estimated F-at-age relative to that of an unfished population. Depletion is calculated as the proportion of the age-1+ biomass during the specified period relative to an unfished age-1+ equilibrium biomass. The model representing an update of the 2023 base case model is similar to but not identical to the 2026 base case model due to changes in data preparation and model structure. †A value of ›1 for the depletion ratio indicates higher age-1+ biomass in 2024 relative to the 2006–2015 period. §Higher %SPR values indicate lower fishing intensity levels. ¶Values of ›1 for ratios of F%SPR to F%SPR-based reference points indicate fishing intensity levels lower than the reference points.
ISC26_alb_Table1
ISC26_alb_Figure8-5
Figure 5. (a) Stock status phase plot showing the status of the north Pacific albacore (Thunnus alalunga) stock relative to the biomass-based threshold (30%SSBcurrent,F=0) and limit (14%SSBcurrent,F=0) reference points, and fishing intensity-based target reference point (F45%SPR) over the modeling period (1994 – 2024). Blue triangle indicates the start year (1994) and black circle with 95% confidence intervals indicates the terminal year (2024). (B) Stock status plot showing current stock status and 95% confidence intervals of the base case model (black circle), a sensitivity run an important sensitivity run of CV = 0.06 for Linf in the growth model (gray), an important sensitivity run with two initial fishing mortality parameters (red), and a model representing an update of the 2023 base case model to 2026 data (purple). Red zones in both panels indicate female SSBs falling below the limit reference point while the orange zones indicate female SSBs between the threshold and limit reference points. Green zones indicate female SSBs above the threshold reference point and fishing intensity levels below the target reference point. Yellow areas indicate female SSBs above the threshold reference point and fishing intensity levels above the target reference point. The Fs in this figure are indicators of fishing intensity based on spawning potential ratio (SPR) and calculated as %SPR. SPR is the ratio of the equilibrium SSB per recruit that would result from the estimated F-at-age relative to that of an unfished population. A higher %SPR indicates lower fishing intensity. Current fishing intensity values and SSB/SSBcurrent,F=0 ratios in (B) were calculated as the average during 2021-2023 (F%SPR, 2021-2023) and 2024 (SSB2024/SSBcurrent, F=0), respectively. The model representing an update of the 2023 base case model is similar to but not identical to the 2023 base case model due to changes in data preparation and model structure.

The SSB2024 was estimated to be approximately 60% (95% CI: 38 – 83%) of SSBcurrent,F=0 and 2.01 (95% CI: 1.26 – 2.76) times greater than the estimated threshold reference point (Table 1). The estimated current fishing intensity (F2021-2023) was estimated to be F66%SPR (95% CI: F74%SPR – F57%SPR) and was lower than both the F45%SPR target reference point and the average fishing intensity during 2002 – 2004 (Table 1).

    Based on these findings, the following information on the status of the north Pacific albacore stock is provided:

  1. The stock is virtually certain (>99%) not to be overfished relative to the threshold (30%SSBcurrent,F=0) and limit (14%SSBcurrent,F=0) reference points adopted by the WCPFC and IATTC, and
  2. The stock is virtually certain (>99%) to be not experiencing overfishing relative to the target reference point (F45%SPR).

Conservation Information

The constant fishing intensity scenario showed that the current fishing intensity (F2021-2023) is expected to result in female SSB increasing to 109,384 t (95% CI: 82,915 – 136,573 t) and a SSB/SSBcurrent,F=0 ratio of 0.83 by 2034. Over the next 10 years, there was: 1) a 100.0% probability of the female SSB remaining above the 14%SSBcurrent,F=0 LRP for all 10 years; 2) a 86.5% probability of the total biomass (age-1+) depletion being above the average of 2006 – 2015 for any year; and 3) a 95.3% probability of the fishing intensity remaining at or below the F45%SPR TRP for any year (Figure 6). For the second scenario, the period 2005–2019 was chosen because IATTC and WCPFC implemented the current management measures (WCPFC CMM 2019-03; IATTC Resolution C-05-02) that maintains albacore fishing effort at or below the average effort levels during 2002–2004 but before recent declines in F after 2020. If future fishing intensity is similar to the 2005 – 2019 period, it is expected to result in female SSB increasing to 67,340 t (95% CI: 47,082 – 87,597 t) and a SSB/SSBcurrent,F=0 ratio of 0.52 by 2034. Over the next 10 years, there was: 1) a 99.1 % probability of the female SSB remaining above the 14%SSBcurrent,F=0 LRP for all 10 years; 2) a 79.9 % probability of the total biomass (age-1+) depletion being above the average of 2006 – 2015 for any year; and 3) a 48.8% probability of the fishing intensity remaining at or below the F45%SPR TRP for any year (Figure 7).

ISC26_alb_Figure8-6
Figure 6. Future projection results under a constant fishing intensity (F2021-2023) harvest scenario. Solid lines indicate mean values, uncertainty ranges indicate 60% and 95% confidence intervals, and the dashed line is the reference point, respectively. (A) Annual changes in spawning biomass; (B) Interannual changes in fishing mortality (F%SPR); (C) Projected ratios to the limit reference point thresholds; and (D) Projected ratios to management targets for the total biomass.
ISC26_alb_Figure8-7
Figure 7. Future projection results under a randomly F (2005-2019) scenario. Solid lines indicate mean values, and uncertainty ranges indicate 60% and 95% confidence intervals, and the dashed line is the reference point, respectively. (A) Annual changes in spawning biomass; (B) Interannual changes in fishing mortality (F%SPR); (C) Projected ratios to the limit reference point thresholds; and (D) Projected ratios to management targets for the total biomass.

    Based on these findings, the following conservation information is provided:

  1. If fishing intensity over the next 10 years is maintained at the current fishing intensity (F2021-2023), then female SSB is expected to increase to around 83% of SSBcurrent,F=0 (109,384 t) by 2034, with a 100.0% probability that female SSB will remain above the 14%SSBcurrent,F=0 LRP for all 10 years and the harvest strategy management objectives of the IATTC and WCPFC (IATTC Resolution C-23-02; WCPFC Harvest Strategy 2023-01) will likely be met; and
  2. If fishing intensity over the next 10 years is similar to the 2005 – 2019 period, then female SSB is expected to decrease to around 52% of SSBcurrent,F=0 (67,340 t), with a 99.1 % probability that female SSB will remain above the 14%SSBcurrent,F=0 LRP for all 10 years and the harvest strategy management objectives of the IATTC and WCPFC (IATTC Resolution C-23-02; WCPFC Harvest Strategy 2023-01) will likely be met.