Blue shark（Chiba prefecture）
The blue shark belongs to the order Carcharhiniformes, the family Carcharhinidae, and the genus prionace. Its scientific name is Prionace glauca. The maximum body length (pre-caudal fin length) and body weight of the blue shark in the North Pacific is 290 cm/251 kg for females and 243 cm/168 kg for males (Fujinami et al. 2019). Body shape is slender and streamlined, the snout is long and conical, and the eyes are relatively large. The body color on the dorsal side is bright indigo and the ventral side is white (Compagno 1984).
Compagno, L.J.V. (1984) FAO Fish. Synop. 125(4/2):251-655. Rome: FAO.
Fujinami, Y., Semba, Y., and Tanaka, S. (2019) Fish. Bull., 117: 107-120.
Distribution
This is a species of pelagic shark that is widely distributed from the tropical to temperate zones across the ocean, including the Southern and Northern Pacific (Compagno 1984). They are known to be densely distributed particularly in the temperate zone (Nakano 1996), and especially densely distributed around the Emperor Seamounts Chain (hot spot) (Kai et al. 2017). They mainly inhabit the open ocean but are also seen in coastal areas. Blue sharks are known to move across depths, often swimming in deep waters during the day and in shallow water at night (Stevens et al. 2010).
Compagno, L.J.V. (1984) FAO Fish. Synop. 125(4/2):251-655. Rome: FAO.
Kai, M., Thorson, J.T., Piner, K.R., Maunder, M.N. (2017) Fish. Oceanogra. 26, 569-582.
Nakano, H. (1996) Monthly Kaiyo, 28: 407-415.
Stevens, J.D., Bradford, R.W., and West, G.J. (2010) Mar. Biol. 157: 575-591
Ecology
The age of sexual maturity begins is five years for males and six years for females, with lifespans exceeding 20 years (Fujinami et al. 2017a, 2019). Feeding grounds are the tropical and temperate zones. Blue sharks prey primarily on fish and cephalopods, while juveniles are preyed upon by larger sharks or marine mammals (Fujinami et al. 2017b). The breeding season is early summer, the birth season is from May to June, and the mating season is from June to August. It is known that the breeding grounds are in the waters at latitude 30 - 40 degrees north, and vary according to sex and growth stages (Nakano, 1994).
Fujinami, Y., Semba, Y., Okamoto, H., Ohshimo, S., and Tanaka, S. (2017a) Mar. Freshwater Res., 68: 2018-2027.
Fujinami, Y., Nakatsuka, S., and Ohshimo, S. (2017b) Pac. Sci., 72(1).
Fujinami, Y., Semba, Y., and Tanaka, S. (2019) Fish. Bull., 117: 107-120.
Nakano, H. (1994) Bull. Nat. Res. Inst. Far. Seas. Fish. 31: 141-256.
Use Form
Blue shark meat is processed into paste products while fins are dried, skins and vertebrae made into crafts, medicines and food additives (Kai and Fujinami 2020).
Kai, M. and Fujinami, Y. (2020) 35 Blue Shark, Prionace glauca, 2019 Current status of international fishery resources, Fisheries Agency, Japan Fisheries Research and Education Agency.
Fishery
This species is mainly caught as bycatch for tuna longline fishing vessels on the high seas or coastal areas of fishing countries, but some are caught as target species. In coastal areas of Japan, they are bycaught by small longliners, drift net fishing vessels or fixed fishing nets, etc. More than 90% of the total volume of landings in Japan come from Kesennuma, Miyagi Prefecture, but some are also caught in Shiogama and other areas. The volume of landings from longline fishing accounts for 80% to 90% of the volume of landings at Kesennuma, and those from drift net fishing accounts for 10 to 20% (Fisheries Research Agency 2019).
Fisheries Research Agency(ed.) (2019) 2016-2017 Survey results on tuna, marlin, and sharks at landing sites. Japan Fisheries Research and Education Agency.
Your comprehensive evaluation
Resource Status
Based on the stock assessment results or future forecasts using the stock assessment model, current or future stock statuses can be determined to be sound with no problems. However, it is desirable to determine reference points and formulate fishery management rules within the framework of the international community as soon as possible. On the other hand, the Kesennuma Fleet, which lands a lot of this species in Japan, voluntarily manages and regulates stocks. This is a desirable initiative from the perspective of keeping stocks sustainable.
Marine Environment and Ecosystem
Understanding the impact of blue shark fisheries on the ecosystem (in the North Pacific) requires the acquisition and monitoring of information. More specifically, the ecosystem model in the western and central Pacific is analyzed to obtain information on bycatch by longlines fishing and the like. Surveys by research vessels are conducted irregularly on the larvae and juveniles of tropical tunas and skipjack tuna. Zooplankton collection and marine environment surveys are also conducted. A scientific observer program was introduced in 2008, and a system has been established to acquire records of catch and bycatch by longlines and purse seine and size information, allowing information on bycatches and catch compositions to be partially collected.
Regarding the impact of blue shark longline fishing on other fish species, the stocks of albacore, bigeye tuna, yellowfin tuna, and swordfish, considered bycaught and are used, were not a matter of concern. Regarding the impact on bycaught and unused species, PSA assessments revealed that several species had a medium to high risk potential. More specifically, risk is high for green turtles, loggerhead turtles, hawksbill turtles, and olive ridley turtles, and moderate for lampris guttatus (moonfish) and false killer whales. According to the PSA assessment for endangered species designated by the Ministry of the Environment, the risk of sea turtles is high.
Regarding the indirect impact of fishing on the food web, the prey organisms of the blue shark (which near the top of the food chain), are smaller fish. Blue sharks are said to exhibit opportunistic feeding habits rather than targeting specific species. Given the total stock of high-catch small pelagic fishing the northwestern Pacific as an abundance of prey organisms, the total stock tends to be flat, so no adverse effect was found. Competitors include albacore, bigeye tuna, yellowfin tuna, and swordfish, which are bycatch species of tuna longlines, are caught in numbers and prey on smaller fish, similar to blue sharks, stocks were not of concern.
Regarding the impact of fisheries on the whole ecosystem, there is concern that the total catch and the mean trophic level of catches have been declining in the southern Pacific region since 2014. The impact on water quality was determined to be minor. However, the impact of longline vessels, which have relatively high emissions, on the atmospheric environment was a concern.
Fishery Management
Offshore tuna longline fishing is a designated fishery licensed by the Minister of Agriculture, Forestry and Fisheries. A longline fishery management plan for sharks has been prepared, with an annual landing limit of 7,000 tons. Fishing vessels of offshore longline fishery set as assessment target fishery are limited to less than 120 tons. The longline fishery management plan prohibits the use of shark lines and requires that fins not be cut from bodies until landing. There are restrictions on fishing gear for conservation and management measures for sea turtles and seabirds, and the catching of silky sharks and oceanic whitetip sharks is prohibited. Related fishermen's groups participate in the Kesennuma City Marine Plastics Countermeasures Promotion Council, and are working to reduce fuel consumption by 10%. Offshore tuna longline fishing is under the jurisdiction of the Skipjack and Tuna Fisheries Office, International Affairs Division, Fisheries Agency. The introduction of management measures in Japan based on recent discussions on stock assessment and conservation measures at WCPFC, etc. was evaluated as measures equivalent to adaptive management. In recent years, fishermen related to this fishery have been working on controlling the catch of blue sharks during birthing periods in addition to the contents of the longline fishery management plan. The Kesennuma Fisheries Cooperative takes a lead in implementing the Kesennuma Regional Fisheries Reconstruction Project (a reconstruction plan for offshore tuna longline fisheries and two plans for utilizing existing vessels) to found a cooperative corporation. Furthermore, the National Offshore Skipjack and Tuna Fisheries Association takes the lead in demonstrating a planned and efficient introduction of stock management and fishing vessels for improving working environments. Special commissioners from various fields participate in the Stock Management Subcommittee of the Fisheries Policy Council. At the WCPFC, which was also attended by stakeholders, conservation and management measures were considered based on the stock assessment conducted at the International Scientific Subcommittee on Tuna in the North Pacific (ISC). In accordance with the conservation and management measures, the Stock Management Subcommittee is implementing a longline fishery management plan for sharks.
Regional Sustainability
The Pacific blue sharks landed in Japan are mostly caught by offshore tuna longline fishing in Miyagi Prefecture. The trend for fishery income is low, and the earning rate and fishery-related assets were also slightly low. Regarding the stability of management, the stability of income was rather low, and the stability of catches were moderate. The financial situation of the fishermen's organizations was generally rather high. Operational safety is high, and contributions to local employment are high. Regarding fairness of working conditions, there were no particular problems in the fishing industry. The Kesennuma market has a large volume of blue shark landings, the number of buyers in each market varies depending on volume, and the principle of competition is generally working through auction or bidding transactions. Hygiene management is thoroughly implemented in accordance with a wholesale market development project. The meat is processed before shipping, and shark fins are traded as high-end food items. There were no problems with the fairness of working conditions in processing and distribution. The sustainability of the processing and distribution industry was evaluated as high. Advanced technologies are introduced, disseminated, and instructed. A distribution system is in place. The income level of fishery-related workers is relatively high.
Health, Safety, and Security
The lipid of blue sharks contains EPA and DHA, while the cartilage contains functional components such as chondroitin sulfate. An actual blue shark season is unknown because they are mainly caught as bycatch in tuna longline fisheries. The points to keep in mind when consuming have mainly to do with selecting fresh specimens for consumption as soon as fishy odors are likely to occur due to ammonia and trimethylamine as freshness degrades. In addition, since blue sharks are more likely to accumulate methyl mercury than other fish species, pregnant women should adhere to the standard serving amounts publicized by the Ministry of Health, Labour and Welfare.
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