Stock Assessment and Future Projections of Blue Shark in the North Pacific Ocean through 2015

Note: this paper was included as a Working Paper at SC13 in 2017 and as an Information Paper at SC14 in 2018.

This document presents the results of the ISC SHARKWG’s assessment of blue shark in the North Pacific Ocean using a fully-integrated size-structured model. The last assessment was conducted in 2014. Time- series data updated through 2015 (catch, relative abundance, and sex-specific length composition from multiple fisheries), new biological information, and research into parameterization of a low-fecundity stock recruitment relationship (LFSR) enabled the development of an improved size-structured model. A Bayesian State-Space Surplus Production reference case model was also conducted to facilitate comparison with the 2014 assessment.
Stock Identification and Distribution
Blue shark (BSH) are widely distributed throughout temperate and tropical waters of the Pacific Ocean. The ISC SHARKWG recognizes two stocks in the North and South Pacific, respectively, based on biological and fishery evidence. Relatively few BSH are encountered in the tropical equatorial waters separating the two stocks. Tagging data demonstrate long distance movements and a high degree of mixing of BSH across the North Pacific, although there is evidence of spatial and temporal structure by size and sex.
Catch History
Catch records for BSH in the North Pacific are limited and, where lacking, have been estimated using statistical models and information from a combination of historical landings data, fishery logbooks, observer records and research surveys. In these analyses, estimated BSH catch data refer to total dead removals, which includes retained catch and dead discards. Estimated catch data in the North Pacific date back to 1971, although longline and driftnet fisheries targeting tunas and billfish earlier in the 20th century likely caught BSH. The nations catching most BSH in the North Pacific include Japan, Chinese Taipei, Mexico, and USA which account for more than 90% of the estimated catch (Figure 1E). Estimated catches of BSH were highest from 1976 to 1989 with a peak estimated catch of approximately 88,000 mt in 1981. Over the past decade BSH estimated catches in the North Pacific have shown a gradual decline from ~52,000 mt in 2005 to an average of ~35,000 mt annually in 2013-2015. While a variety of fishing gears catch BSH, most are caught in longline fisheries (Figure 2E).
Data and Assessment
Annual catch estimates were derived for a variety of fisheries by nation. Catch and size composition data were grouped into 18 fisheries for the period 1971 to 2015. Standardized catch-per-unit-effort (CPUE) data used to measure trends in relative abundance were provided by Japan, USA, Chinese Taipei, Mexico, and SPC (Figure 3E).
The north Pacific blue shark stock was assessed using an age-based statistical catch-at-length model, Stock Synthesis (SS), fit to time series of standardized CPUE and sex-specific size composition data. Sex-specific growth curves and natural mortality rates were used to account for the sexual dimorphism of adult blue sharks. A low fecundity stock recruitment (LFSR) relationship was used to characterize productivity of the stock based on plausible life history information available for north Pacific blue sharks. Models were fit to relative abundance indices and size composition data in a likelihood-based statistical framework. Maximum likelihood estimates of model parameters, derived outputs, and their variances were used to characterize stock status based on a reference case and to develop stock projections.
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