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Stock Status and Conservation Information
(From ISC19 Plenary Report)

Western and Central North Pacific Striped Marlin

Click here to see the ISC18 stock status and conservation information.

Stock Status

Biomass (age 1 and older) for the WCNPO MLS stock decreased from 17,000 t in 1975 to 6,000 t in 2017. Estimated fishing mortality averaged F=0.97 yr-1 during the 1975-1994 period with a range of 0.60 to 1.59 yr-1, peaked at F=1.71 year-1 in 2001, and declined sharply to F=0.64 yr-1 in the most recent years (2015-2017). Fishing mortality has fluctuated around FMSY since 2013. Compared to MSY-based reference points, the current spawning biomass (average for 2015-2017) was 76% below SSBMSY and the current fishing mortality (average for ages 3  E12 in 2015-2017) was 7% above FMSY.

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

  1. There are no established reference points for WCNPO MLS;
  2. Results from the base case assessment model show that under current conditions the WCNPO MLS stock is overfished and is subject to overfishing relative to MSYbased reference points (Table 1, Table 2, and Figure 1).
Table 1. Reported catch (t) used in the stock assessment along with annual estimates of population biomass (age-1 and older, t), female spawning biomass (t), relative female spawning biomass (SSB/SSBMSY), recruitment (thousands of age-0 fish), fishing mortality (average F, ages-3  E12), relative fishing mortality (F/FMSY), and spawning potential ratio of WCNPO MLS.
Year 2011 2012 2013 2014 2015 2016 20172 Mean1 Min1 Max1
Reported Catch 2,690 2,757 2,534 1,879 2,072 1,892 2,487 5,643 1,879 10,862
Population Biomass 5,874 6,057 4,937 6,241 5,745 5,832 6,196 12,153 4,509 22,303
Spawning Biomass 618 809 743 864 1,073 1,185 981 1,765 618 3,999
Relative Spawning
Biomass		 0.24 0.31 0.29 0.33 0.41 0.46 0.38 0.68 0.24 1.54
Recruitment(age0) 196,590 87,956 330,550 77,274 185,438 195,069 354,391 396,218 77,274 1,049,460
Fishing Mortality 1.11 1.06 0.86 0.63 0.62 0.51 0.80 1.06 0.51 1.71
Relative Fishing
Mortality		 1.85 1.76 1.42 1.05 1.03 0.85 1.33 1.76 0.85 2.85
Spawning
Potential Ratio		 9% 11% 11% 16% 17% 20% 14% 12% 20% 6%
1During 1975-2017
2Recruitment in 2017 is estimated from the stock recruitment curve.
Table 2. Estimates of biological reference points along with estimates of fishing mortality (F), spawning stock biomass (SSB), recent average yield (C), and spawning potential ratio (SPR) of WCNPO MLS, derived from the base case model assessment model, where “MSY Eindicates reference points based on maximum sustainable yield.
Reference Point Estimate
FMSY (age 3-12) 0.60
F2017(age 3-12) 0.80
F20%SSB(F=0) 0.47
SSBMSY 2,604 t
SSB2017 981 t
20%SSB0 3,610 t
MSY 4,946 t
C2015-2017 2,151 t
SPRMSY 18%
SPR2017 14%
SPR20%SSB(F=0) 23%
ISC19_Striped_Marlin_Figure1
Figure 1. Time series of estimates of (a) population biomass (age 1+), (b) spawning biomass, (c) recruitment (age-0 fish), and (d) instantaneous fishing mortality (average for age 3-12, year-1) for WCNPO MLS (derived from the 2019 stock assessment. The circles represent the maximum likelihood estimates by year for each quantity and the error bars represent the uncertainty of the estimates (95% confidence intervals), green dashed lines indicate SSBMSY and FMSY.

Conservation Information

The status of the WCNPO MLS stock shows evidence of substantial depletion of spawning potential (SSB2017 is 62% below SSBMSY), however fishing mortality has fluctuated around FMSY in the last four years. The WCNPO MLS stock has produced average annual yields of around 2,100 t per year since 2012, or about 40% of the MSY catch amount. However the majority of the catch are likely immature fish. All of the projections show an increasing trend in spawning stock biomass during the 2018-2020 period, with the exception of the high F scenario under the short-term recruitment scenario. This increasing trend in SSB is due to the 2017 year class, which is estimated from the stock-recruitment curve and is more than twice as large as recent average recruitment.

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

  1. Projection results under the long-term recruitment scenario show that the stock has at least a 60% probability of rebuilding to 20%SSB0, the rebuilding target specified by NC14, by 2022 for all harvest scenarios, with the exception of the highest F scenario (Average F 1975-1977);
  2. However, if the stock continues to experience recruitment consistent with the shortterm recruitment scenario (2012-2016), catches must be reduced to 60% of the WCPFC catch quota from CMM 2010-01 (3,397 t) to 1,359 t in order to achieve a 60% probability of rebuilding to by 2022. This corresponds to a reduction of roughly 37% from the recent average yield of 2,151 t;
  3. For the constant catch projection scenarios that were tested, it was notable that all of the projections under the long-term recruitment scenario would be expected to achieve the spawning biomass target by 2020 with probabilities ranging from 61% to 73% and corresponding catch quotas ranging from 3,397 to 1,359 t (Table 3).
Table 3. Projected median values of WCNPO MLS spawning stock biomass (SSB, t), catch (t), and probability of reaching 20%SSB0 under five constant fishing mortality rate (F) and ten constant catch scenarios during 2018-2037. For scenarios which have a 60% probability of reaching the target of 20%SSBF=0, the year in which this occurs is provided; NA indicates projections that did not meet this criterion. Note that 20%SSBF=0 is 3,610 t and SSBMSY is 2,604 t.
Year 2018 2019 2020 2021 2022 2027 2037 Year when target
achieved with
60% probability
Scenario 1: Fstatus quo; Long-Term Recruitment
SSB 1931.3 2605.3 3591 4288.3 4639.4 4893.4 4884.4
Catch 2229.8 3089.8 3911.6 4412.8 4644.9 4797.2 4790.9
Probability of reaching
20% SSB		 0% 4% 44% 70% 79% 84% 84% 2021
Scenario 2: Fstatus quo; Short-Term Recruitment
SSB 1932.4 2556.5 3080 2786.9 2422.3 2071.4 2072.1
Catch 2224.6 2827 2871.7 2535.9 2260.7 2029.6 2030.4
Probability of reaching
20% SSB		 0% 4% 21% 9% 2% <0.5% <0.5% NA
Scenario 3: FMSY; Long-Term Recruitment
SSB 1935.1 2611.8 3650.5 4444 4860.6 5158.9 5203.5
Catch 2228.1 3092.7 3705.2 4241.6 4498.9 4666.4 4711.5
Probability of reaching
20% SSB		 0% 4% 47% 75% 83% 89% 89% 2021
Scenario 4: FMSY; Short-Term Recruitment
SSB 1932.9 2557.7 3126.3 2895.5 2552.2 2207 2197
Catch 2230.8 2829.6 2724.6 2450.7 2209.9 1994.1 1984.9
Probability of reaching
20% SSB		 0% 4% 23% 12% 4% <0.5% <0.5% NA
Scenario 5: F 20%SSBF=0; Long-Term Recruitment
SSB 1933.7 2611.9 3813.4 4943.7 5631 6358.1 6348.5
Catch 2227.6 3091.3 2996.4 3588.7 3933.2 4271.7 4266.7
Probability of reaching
20% SSB		 0% 4% 55% 85% 93% 97% 98% 2021
Scenario 6: F 20%SSBF=0; Short-Term Recruitment
SSB 1934 2560.5 3276.3 3274.8 3030.2 2697 2690.2
Catch 2224.9 2828.8 2211.6 2115.4 1969.7 1809.1 1804.7
Probability of reaching
20% SSB		 0% 4% 29% 28% 17% 6% 7% NA
Scenario 7: Highest F (Average F 1975-1977); Long-Term Recruitment
SSB 1932.8 2611.8 2739.8 2299.1 2102 2028.4 2036.2
Catch 2226.4 3088.5 7520.7 6557.5 6184.4 6058 6084.1
Probability of reaching
20% SSB		 0% 4% 9% 4% 2% 1% 1% NA
Scenario 8: Highest F (Average F 1975-1977); Short-Term Recruitment
SSB 1933.5 2559.4 2289.2 1330.7 968.3 858.7 859.2
Catch 2225.9 2827.6 5362.9 3399.3 2751.6 2564.6 2570.9
Probability of reaching
20% SSB		 0% 3% 2% <0.5% 0% 0% 0% NA
Scenario 9: Low F (F30%); Long-Term Recruitment
SSB 1933.6 2612.5 4009.5 5603.2 6742.4 8287.5 8353
Catch 2228.6 3093.5 2117.6 2693.6 3075 3558.2 3577.8
Probability of reaching
20% SSB		 0% 4% 63% 93% 98% >99.5% >99.5% 2020
Scenario 10: Low F (F30%); Short-Term Recruitment
SSB 1932.5 2555.6 3453.8 3788.4 3747.4 3537.4 3525.3
Catch 2228.4 2832 1572.9 1623.8 1589 1515.8 1511.6
Probability of reaching
20% SSB		 0% 4% 37% 54% 54% 44% 42% NA
Scenario 11: Current Quota; Long-Term Recruitment
SSB 1946.7 2823 4141.1 5220.9 6074.7 8147.5 8715.3
Catch 2150.6 2150.6 3396.8 3396.7 3396.3 3396.1 3396.8
Probability of reaching
20% SSB		 <0.5% 17% 61% 76% 83% 93% 95% 2020
Scenario 12: Current Quota; Short-Term Recruitment
SSB 1948.8 2737.1 3279.8 2592.9 1781.9 524.2 436.7
Catch 2150.6 2150.6 3393.7 3377.1 3319.7 2954.7 2903
Probability of reaching
20% SSB		 <0.5% 15% 36% 20% 7% <0.5% <0.5% NA
Scenario 13: 10% Reduction; Long-Term Recruitment
SSB 1947.9 2826.1 4225.3 5467.3 6492.5 9096.5 9798.7
Catch 2150.6 2150.6 3057.1 3057.1 3056.8 3057.1 3057.1
Probability of reaching
20% SSB		 <0.5% 17% 63% 81% 87% 96% 97% 2020
Scenario 14: 10% Reduction; Short-Term Recruitment
SSB 1948.6 2738 3390.9 2886.8 2162.9 763 587
Catch 2150.6 2150.6 3054.6 3052.8 3032.5 2846.7 2780.1
Probability of reaching
20% SSB		 <0.5% 15% 40% 26% 12% <0.5% <0.5% NA
Scenario 15: 20% Reduction; Long-Term Recruitment
SSB 1949.9 2829.1 4317.7 5750.4 6954.1 9928.4 10806.2
Catch 2150.6 2150.6 2717.4 2717.4 2717.4 2717.4 2717.4
Probability of reaching
20% SSB		 <0.5% 18% 65% 84% 90% 98% 99% 2020
Scenario 16: 20% Reduction; Short-Term Recruitment
SSB 1949.3 2739.2 3495.1 3176.4 2570.8 1175.5 883.3
Catch 2150.6 2150.6 2716.8 2714.3 2710.8 2648.8 2610.7
Probability of reaching
20% SSB		 <0.5% 15% 43% 34% 19% 1% <0.5% NA
Scenario 17: 30% Reduction; Long-Term Recruitment
SSB 1947.6 2824.5 4381.5 5981.7 7356.2 10856.1 11783.5
Catch 2150.6 2150.6 2377.8 2377.8 2377.8 2377.8 2377.8
Probability of reaching
20% SSB		 <0.5% 17% 67% 87% 94% 99% >99.5% 2020
Scenario 18: 30% Reduction; Short-Term Recruitment
SSB 1947.4 2733.8 3594 3479.2 3018.1 1736.6 1383.5
Catch 2150.6 2150.6 2377.8 2377.1 2377.1 2365.6 2355.3
Probability of reaching
20% SSB		 <0.5% 15% 45% 42% 29% 5% 2% NA
Scenario 19: 40% Reduction; Long-Term Recruitment
SSB 1949.2 2831.8 4486.8 6295.8 7868.9 11749.2 12851.3
Catch 2150.6 2150.6 2038.1 2038.1 2038.1 2038.1 2038.1
Probability of reaching
20% SSB		 <0.5% 18% 70% 90% 95% >99.5% >99.5% 2020
Scenario 20: 40% Reduction; Short-Term Recruitment
SSB 1949.9 2737.3 3689.5 3756 3445.9 2444.2 2124.2
Catch 2150.6 2150.6 2038.1 2038.1 2037.9 2037.6 2036.4
Probability of reaching
20% SSB		 <0.5% 15% 48% 49% 41% 16% 10% NA
Scenario 21: 50% Reduction; Long-Term Recruitment
SSB 1950.4 2829.7 4548.9 6512.1 8259.1 12654 13799.3
Catch 2150.6 2150.6 1698.4 1698.4 1698.4 1698.4 1698.4
Probability of reaching
20% SSB		 <0.5% 17% 71% 92% 97% >99.5% >99.5% 2020
Scenario 22: 50% Reduction; Short-Term Recruitment
SSB 1949.1 2737.4 3791.4 4065.7 3916.3 3214.4 3021.3
Catch 2150.6 2150.6 1698.4 1698.4 1698.4 1698.4 1698.4
Probability of reaching
20% SSB		 <0.5% 15% 51% 57% 53% 35% 29% NA
Scenario 23: 60% Reduction; Long-Term Recruitment
SSB 1949.9 2829.1 4631.3 6798.1 8741.1 13605.2 14857.1
Catch 2150.6 2150.6 1358.7 1358.7 1358.7 1358.7 1358.7
Probability of reaching
20% SSB		 <0.5% 18% 73% 94% 98% >99.5% >99.5% 2020
Scenario 24: 60% Reduction; Short-Term Recruitment
SSB 1948.6 2737.7 3888.1 4364.3 4396.6 4110.1 3970.5
Catch 2150.6 2150.6 1358.7 1358.7 1358.7 1358.7 1358.7
Probability of reaching
20% SSB		 <0.5% 15% 53% 65% 67% 63% 59% 2021*
Scenario 25: 70% Reduction; Short-Term Recruitment
SSB 1948.7 2736.4 3979.8 4667.7 4886 4960.9 4977
Catch 2150.6 2150.6 1019 1019 1019 1019 1019
Probability of reaching
20% SSB		 <0.5% 15% 56% 72% 78% 85% 86% 2021
Scenario 26: 80% Reduction; Short-Term Recruitment
SSB 1948.7 2736.2 4071.1 4971.3 5380.3 5909.1 5977.5
Catch 2150.6 2150.6 679.4 679.4 679.4 679.4 679.4
Probability of reaching
20% SSB		 <0.5% 15% 58% 79% 88% 97% 97% 2021
Scenario 27: 90% Reduction; Short-Term Recruitment
SSB 1950.6 2740.5 4170.3 5284.1 5881.7 6836.7 7009.4
Catch 2150.6 2150.6 339.7 339.7 339.7 339.7 339.7
Probability of reaching
20% SSB		 <0.5% 15% 61% 85% 94% >99.5% >99.5% 2020
* This scenario has a 60% probability of being at or above 20%SSBF=0 in 2020 but drops slightly below 60% starting in 2035.

It was also noted that retrospective analyses show that the assessment model appears to overestimate spawning potential in recent years, which may mean the projection results are ecologically optimistic.

Special Comments

The WG achieved a base-case model using the best available data and biological information. However, the WG recognized uncertainty in some assessment inputs including drift gillnet catches and initial catch amounts, life history parameters such as maturation and growth, and stock structure.

Overall, the base case model diagnostics and sensitivity runs show that that there are some conflicts in the data (ISC/19/ANNEX/11). When developing a conservation and management measure to rebuild the resource, it is recommended that these issues be recognized and carefully considered, because they affect the perceived stock status and the probabilities and time frame for rebuilding of the WCNPO MLS stock.

Research Needs

To improve the stock assessment, the WG recommends continuing model development work, to reduce data conflicts and modeling uncertainties, and reevaluating and improving input assessment data.