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Stock Status and Conservation Information
(From ISC18 Plenary Report)

Western and Central North Pacific Swordfish

Stock Status

Estimates of total stock biomass show a relatively stable population, with a slight decline until the mid-1990s followed by a slight increase since 2000. Population biomass (age-1 and older) averaged roughly 97,919 t in 1974-1978, the first 5 years of the assessment time frame, and has declined by only 20% to 71,979 t in 2016 (Figure 1). Female spawning stock biomass was estimated to be 29,403 t in 2016, or about 90% above SSBMSY (Table 1 and Table 2). Fishing mortality on the stock (average F, ages 1  E10) averaged roughly F = 0.08 yr-1 during 2013-2015, or about 45% below FMSY. The estimated SPR (the predicted spawning output at the current F as a fraction of unfished spawning output) is currently SPR2016 = 45%. Annual recruitment averaged about 717,000 recruits during 2012-2016, and no long-term trend in recruitment was apparent. Overall, the time series of spawning stock biomass and recruitment estimates indicate a stable spawning stock biomass and suggest a fluctuating pattern without trend for recruitment (Figure 1). The Kobe plot depicts the stock status relative to MSY-based reference points for the base case model (Figure 2) and shows that spawning stock biomass declined to almost the MSY level in the mid-1990s, but SSB has remained above SSBMSY throughout the time series (Figure 1B).

For this 2018 benchmark assessment, note that biomass status is based on female spawning stock biomass, whereas for the 2014 update assessment, biomass status was based on exploitable biomass (effectively age-2+ biomass). It is also important to note that there are no currently agreed upon reference points for the WCNPO swordfish stock and that retrospective analyses show that the assessment model appears to underestimate spawning stock biomass in recent years.

Based on these findings, the following information on the status of the WCNPO SWO stock is provided:

  1. The WCNPO swordfish stock has produced annual yields of around 10,200 t per year since 2012, or about 2/3 of the MSY catch amount.
  2. There is no evidence of excess fishing mortality above FMSY (F2013-2015 is 45% of FMSY) or substantial depletion of spawning potential (SSB2016 is 87% above SSBMSY).
  3. Overall, the WCNPO swordfish stock is not likely overfished and is not likely experiencing overfishing relative to MSY-based or 20% of unfished spawning biomass-based reference points.
ISC18_Swordfish_Figure3
Figure 1. Time series of estimates of
  • (a) population biomass (age 1+) (first point in time series represents unfished biomass),
  • (b) spawning biomass,
  • (c) recruitment (age-0 fish), and
  • (d) instantaneous fishing mortality (average for ages 1 to 10, yr-1)
for WCNPO swordfish (Xiphias gladius) derived from the 2018 stock assessment.

The solid circles are the maximum likelihood estimates by year for each quantity and the error bars represent the uncertainty of the estimates (80% confidence intervals), green dashed lines indicate BMSY, equilibrium recruitment, and FMSY except for the population biomass time series.

Table 1. Reported catch (t) used in the stock assessment along with annual estimates of population biomass (age-1 and older, mt), female spawning biomass (mt), relative female spawning biomass (SSB/SSBMSY), recruitment (thousands of age-0 fish), fishing mortality (average F, ages 1 to 10, yr-1), relative fishing mortality (F/FMSY), and spawning potential ratio of WCNPO swordfish.
Year 2010 2011 2012 2013 2014 2015 2016 Mean1 Min1 Max1
Reported Catch 12,716 9,971 10,608 9,241 9,211 11,672 10,068 12,863 9,211 17,793
Population Biomass 66,417 66,087 68,117 67,885 69,560 71,951 71,979 67,487 51,856 97,919
Spawning Biomass 26,136 26,448 26,569 27,546 28,580 28,865 29,404 24,442 17,191 44,100
Relative Spawning
 Biomass		 1.66 1.68 1.69 1.75 1.82 1.84 1.87 1.56 1.09 2.81
Recruitment (age 0) 789 565 671 710 683 742 781 761 401 1241
Fishing Mortality 0.10 0.08 0.09 0.07 0.07 0.09 0.07 0.12 0.07 0.18
Relative Fishing
 Mortality		 0.57 0.46 0.51 0.44 0.40 0.51 0.44 0.72 0.40 1.05
Spawning Potential
 Ratio		 38% 41% 39% 45% 47% 39% 45% 29% 17% 47%
1 During 1975-2016
Table 2. Estimates of biological reference points along with estimates of fishing mortality (F), spawning stock biomass (SSB), recent average yield (C), and SPR of WCNPO swordfish, derived from the base case model assessment model, where “MSY Eindicates reference points based on maximum sustainable yield.
Reference Point Estimate
FMSY 0.17 yr-1
F0.2*SSB(F=0) 0.16 yr-1
F2013-2015 0.08 yr-1
SSBMSY 15,702 mt
SSB2016 29,403 mt
SSBF=0 97,286 mt
MSY 14,941 mt
C2012-2016 10,160 mt
SPRMSY 18%
SPR2016 45%
ISC18_Swordfish_Figure4
Figure 2. Kobe plot of the time series of estimates of relative fishing mortality (average of ages 1-10) and relative spawning stock biomass of WCNPO swordfish (Xiphias gladius) during 1975-2016. The white circle denotes the first year (1975) and the yellow circle denotes the last year (2016) of the assessment time horizon. The dashed lines represent the 95% confidence intervals around the 2016 estimate.

Conservation Information

Stock projections were conducted using a two-gender projection model. The five stock projection scenarios were: (1) F status quo, (2) FMSY, (3) F at 0.2*SSB(F=0), (4) F20%, and (5) F50% (Figure 3). These projection scenarios were applied to the base case model results to evaluate the impact of alternative levels of fishing intensity on future spawning biomass and yield for swordfish in the Western and Central North Pacific Ocean. The projected recruitment pattern was generated by stochastically sampling the estimated stock-recruitment model from the base case model. The projection calculations employed model estimates for the multi-fleet, multi-season, size- and age-selectivity, and structural complexity in the assessment model to produce consistent results.

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

  1. The results show that projected female spawning biomasses is expected to increase under all of the harvest scenarios (Table 3 and Figure 3), with greater increases expected under lower fishing mortality rates.
  2. Similarly, projected catch is expected to increase under each of the five harvest scenarios, with greater increases expected under higher fishing mortality rates (Table 3 and Figure 3).
ISC18_Swordfish_Figure5
Figure 3. Historical and projected trajectories of
  • (a) spawning stock biomass and
  • (b) total catch from the WCNPO swordfish base case model.
Stock projection results are shown for
  • S1 = the status quo or average fishing intensity during 2013-2015 (F2013∁E015= F43%);
  • S2 = FMSY (F18%);
  • S3 = F to produce 20% of unfished spawning stock biomass or F0.2*SSB(F=0) (F22%);
  • S4 = the highest 3-year average F during 1975-2016 or High F (F20%);
  • S5 = Low F (F50%).
Table 3. Projected values of WCNPO swordfish spawning stock biomass (SSB, mt) and catch (mt) under five constant fishing mortality rate (F, yr-1) scenarios during 2017-2026.
Year 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026
Scenario 1: F = F2013-2015
SSB 32,118 33,207 34,599 35,476 36,270 37,082 37,951 38,967 40,083 41,087
Catch 8,851 9,135 9,407 9,599 9,794 10,022 10,275 10,595 11,053 11,142
Scenario 2: F = FMSY
SSB 28,267 23,963 21,443 19,458 18,303 17,618 17,293 17,197 17,253 17,263
catch 20,885 18,323 16,509 15,294 14,666 14,353 14,308 14,520 14,650 14,348
Scenario 3: F = F20%SSB(F=0)
SSB 28,425 24,384 21,800 19,735 18,530 17,874 17,496 17,586 17,818 17,779
Catch 20,691 18,122 16,454 15,261 14,653 14,361 14,319 14,554 14,665 14,384
Scenario 4: F = F20%
SSB 29,007 25,431 23,527 21,763 20,736 20,131 19,893 19,883 19,981 20,066
Catch 18,680 16,933 15,657 14,726 14,242 14,033 14,050 14,292 14,496 14,253
Scenario 5: F = F50%
SSB 32,559 34,334 36,290 37,666 38,836 39,984 41,148 42,490 44,049 45,625
Catch 7,556 7,973 8,343 8,605 8,847 9,101 9,366 9,692 10,087 10,223

Research Needs

The lack of sex-specific size composition data and the simplified treatment of the spatial structure of swordfish population dynamics remained as two important sources of uncertainty for this benchmark assessment.

ISC18_Swordfish_Figure1
Figure 4. Stock boundaries used for this assessment of North Pacific Ocean swordfish: purple lines indicate stock area divisions; stock area 1 was assessed as the WCNPO stock, stock area 2 contains the Eastern Pacific Ocean stock, the green line indicates Western Central Pacific Fisheries Commission convention area, blue dashed line indicates IATTC convention area.
ISC18_Swordfish_Figure2
Figure 5. Annual catch biomass (t) of WCNPO swordfish (Xiphias gladius) by country for Japan, Chinese Taipei, the U.S.A., and all other countries during 1975-2016.

Eastern Pacific Ocean Swordfish

Stock Status

For the EPO stock, exploitable biomass had a declining trend during 1969-1995 and increased from 31,000 t in 1995 to over 60,000 t in 2010, generally remaining above BMSY (exploitable biomass, age-2+). Harvest rates were initially low, have had a long-term increasing trend, and likely exceeded HMSY in 1998, 2002, 2003, as well as in 2012, the terminal year of the stock assessment.

Based on these findings, the following information on the status of the EPO SWO stock is provided:

  1. No target or limit reference points have been established for the EPO swordfish stock under the auspices of the IATTC
  2. The Kobe plot shows that overfishing likely occurred (>50%) in a few years relative to MSY-based reference points, but may have occurred (<50%) from 2010 to 2012.
  3. There was a 55% probability that overfishing occurred in 2012, but there was a less than 1% probability that the stock was overfished relative to MSY-based reference points.

Conservation Information

Stochastic projections for the EPO stock show that exploitable biomass will likely have a decreasing trajectory during 2014-2016 under the eight harvest scenarios examined. Under the high harvest rate scenarios (status quo catch, maximum observed harvest rate, 150% of HMSY), exploitable biomass was projected to decline to 31,170 t (BMSY) by 2016 with corresponding harvest rates above HMSY. In comparison, under the status quo harvest rate scenario, exploitable biomass was projected to decline to only 40,000 t by 2016, well above the BMSY level. Overall, the projections showed that if recent high catch levels (9,700 t) persist, exploitable biomass will decrease and a moderate risk (50%) of overfishing will continue to occur.

The risk analyses for harvesting a constant catch of EPO SWO during 2014-2016 showed that the probabilities of overfishing and becoming overfished increased as projected catch increased in the future. Maintaining the current (2010-2012) catch of EPO SWO of approximately 9,700 t would lead to a 50% probability of overfishing in 2016 and a less than 1% probability of the stock being overfished in 2016.

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

  1. For the EPO SWO stock, overfishing may have occurred (<50%) from 2010 to 2012, and the average yield of roughly 10,000 t in those years, or almost two times higher than the estimated MSY, is not likely to be sustainable in the long term.
  2. While biomass of the EPO stock appears to be nearly twice BMSY, any increases in catch above recent (3-year average 2010-2012) levels should consider the uncertainty in stock structure and unreported catch.