Stock Status and Conservation Information
(From ISC21 Plenary Report)
Pacific Striped Marlin
The Plenary reviewed and agreed to forward the same stock status information that was adopted by ISC20 unchanged, except for the omission of accompanying figures and tables and slight clarifying modifications. Furthermore, the Plenary agreed to an addition to the conservation information adopted by ISC20 in light of questions and clarifications regarding the rebuilding plan initiated by the WCPFC. The ISC Plenary also reiterates the concerns expressed by the BILLWG in their special comments about the stock assessment that are reproduced below.
Click here to see the ISC20 stock status and conservation information.
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 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. 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 .
Based on these findings, the following information on the status of the WCNPO MLS stock is provided:
- There are no established reference points for WCNPO MLS;
- 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 MSY-based reference points.
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 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 and the ISC conclusion on recruitment scenarios, the following conservation information is provided:
- In response to a request from NC15, both long-term and short-term recruitment scenarios were evaluated. The ISC 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;
- 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.
- The Interim Rebuilding Plan for North Pacific Striped Marlin (Attachment L, WCPFC16 Summary Report) was adopted by WCPFC16 in 2019. The ISC responded to several technical questions during ISC21 and based on these responses (see Section 3.3.1 and ISC/21/ANNEX/08) recommends that the next WCNPO MLS stock assessment be completed in 2022 followed by updating of the rebuilding plan using the latest scientific information. Further, the ISC seeks clarification prior to the assessment on whether or not the rebuilding target, 20%SSBF=0, is based on dynamic biomass. If the rebuilding target is based on dynamic biomass, then a target rebuilding date will be needed for the projections to support the rebuilding plan.
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.
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 there are some conflicts in the data (ISC/19/ANNEX/11). 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.