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Stock Status and Conservation Information
(From ISC17 Plenary Report)

North Pacific Blue shark

Stock Status

The assessment uses a fully integrated, sex-specific Stock Synthesis (SS) model. Model inputs have been greatly improved since the previous assessment. The main differences between the present assessment and the 2014 assessment are:

  1. use of SS with a thorough examination of the size composition data, and the relative weighting of CPUE and composition data
  2. improved life history information, such as growth and reproductive biology, and their contribution to productivity assumptions
  3. an improved understanding and parametrization of the low fecundity stock-recruit relationship (LFSR)
  4. catch, CPUE, and size time series updated through 2015;
  5. a suite of model diagnostics including implementation of an Age Structured Production Model implemented in SS

There remain some uncertainties in the time series based on the quality (observer versus logbook) and timespans of catch and relative abundance indices, limited size composition data for several fisheries, potential under-reporting of catch, and life history parameters.

Extensive model explorations showed that the reference run had the best model performance and showed fits most consistent with the data. The CPUE indices used in the reference case were considered most representative of the North Pacific blue shark stock due to their broader spatial temporal coverage in the core distribution of the stock and the statistical soundness of the standardizations. Alternate CPUE series for the latter part of the time series produced different stock trajectories depending upon the index used, but in each case, median SSB during the last three years exceeded SSBMSY. Using alternate assumptions on stock productivity (i.e., form of the stock recruitment relationship) also resulted in variation in the stock trajectories; assuming stock productivity lower than supported by current biological studies, resulted in lowered spawning stock biomass relative to MSY.

Results of the reference case model showed that the spawning stock biomass was near a time-series high in the late 1970s, declined to its lowest level between 1990 to 1995, subsequently increased gradually to reach the time-series high again in 2005, and has since shown small fluctuations close to the time-series high. Recruitment has fluctuated around 37,000,000 age-0 sharks annually with no apparent trend (Figure 1). Stock status is reported in relation to maximum sustainable yield (MSY) based reference points.

  1. Female spawning biomass in 2015 (SSB2015) was 69% higher than at MSY and estimated to be 295,774 t (Table 1; Figure 1).
  2. The recent annual fishing mortality (F2012-2014) was estimated to be well below FMSY at approximately 38% of FMSY (Table 1; Figure 1).
  3. The reference run produced terminal conditions that were predominately in the lower right quadrant of the Kobe plot (not overfished and overfishing not occurring) (Figure 2).
ISC17_SHARK_Figure1
Figure 1. Results of the SS stock assessment reference case model for the North Pacific BSH assessment:
  • (A) estimated age-0 recruits (circles) and 95% confidence intervals (vertical bars);
  • (B) estimated female spawning biomass and 95% confidence intervals (blue shaded area);
  • (C) estimated fishing mortality (sum of F’s across all fishing fleets).
Red solid lines indicate the estimates of SBMSY and FMSY in (B) and (C), respectively.
Table 1. Estimates of key management quantities for the North Pacific blue shark SS stock assessment reference case model and the range of values for 13 sensitivity runs.
Management
Quantity
Reference
Case Model
Range for
Sensitivity Runs
SSB1971 301,739 t 174,381 - 980,878 t
SSB2015 295,774 t 140,742 - 1,082,300 t
SSBMSY 175,401 t 100,984 - 482,638 t
F1971 0.15 0.01 - 0.15
F2012-2014 0.14 0.06 - 0.15
FMSY 0.36 0.26 - 0.66
SSB2015/SSBMSY 1.69 1.39 - 2.59
F2012-2014/FMSY 0.38 0.15 - 0.50
ISC17_SHARK_Figure2_A ISC17_SHARK_Figure2_B
Figure 2. Kobe plots of the trends in estimates of relative fishing mortality and biomass of North Pacific blue shark between 1971 and 2015 for the reference case of
  1. the SS stock assessment model
  2. the BSSPM stock assessment model [Explain CI bands in panel B.]

Conservation Information

Target and limit reference points have not yet been established for pelagic sharks by the WCPFC or the IATTC, the organizations responsible for management of pelagic sharks caught in international fisheries for tuna and tuna-like species in the Pacific Ocean.

The 2015 SB exceeds SBMSY and F2012-2014 is below FMSY. Future projections under different fishing mortality (F) harvest policies (status quo, +20%, -20%, FMSY) show that median BSH spawning biomass in the North Pacific will likely remain above SBMSY in the foreseeable future (Table 2, Figure 3). Other potential reference points were not considered in these evaluations.

Continued improvements in the monitoring of BSH catches, through carefully designed observer programs and species-specific logbooks, including recording the size and sex of sharks retained and discarded for all fisheries, as well as continued research into the biology and ecology of BSH in the North Pacific are recommended.

Table 2. Projected trajectory of spawning biomass (in metric tons) for alternative harvest scenarios.
Year Average F
+ 20%
FMSY Average F
- 20%
Average F
(2012-2014)
2015 295,774 295,774 295,774 295,774
2016 306,782 306,782 306,782 306,781
2017 316,169 312,081 318,183 317,173
2018 322,317 312,329 326,829 324,559
2019 324,795 310,499 332,111 328,419
2020 326,757 307,209 336,929 331,782
2021 328,323 303,909 341,210 334,675
2022 329,623 300,842 344,988 337,181
2023 330,719 298,091 348,286 339,349
2024 331,656 295,663 351,138 341,218
ISC17_SHARK_Figure3
Figure 3. Comparison of future projected North Pacific blue shark spawning biomass under different F harvest policies (status quo, +20%, -20%, and FMSY) using the SS reference case model. Status quo fishing mortality was based on the average from 2012-2014.