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Stock Status and Conservation Information
(From ISC20 Plenary Report)

Western and Central North Pacific Striped Marlin

Click here to see the ISC19 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 (Table 1; Figure 1). 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 yr-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 (Table 1; Figure 1). 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 – 12 in 2015-2017) was 7% above FMSY (Table 2)

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 – 12), relative fishing mortality (F/FMSY), and spawning potential ratio of WCNPO MLS.
ISC20_mls_Table1
  1. 1 During 1975-2017
    2 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” indicates reference points based on maximum sustainable yield.
ISC20_mls_Table2
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.
ISC20_mls_Figure1

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. In response to a request from NC15, the ISC evaluated both long-term and short-term recruitment scenarios and concluded that the short-term recruitment model was the most appropriate model to use for conducting stochastic stock projections for WCNPO MLS because the time trend in the recruitment is not captured by the long term recruitment scenario;
  2. If the stock continues to experience recruitment consistent with the short term recruitment scenario (2012-2016), then 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 20%SSB0=3,610 t by 2022. This change in catch corresponds to a reduction of roughly 37% from the recent average yield of 2,151 t.

It was also noted that retrospective analyses (ISC/19/ANNEX/11) 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.