Skipjack tuna（Kanagawa prefecture）
Skipjack tuna belong to the order Perciformes, suborder Scombridei, family Scombridae, and genusKatsuwonus. Its scientific name is Katsuwonus pelamis. The body is spindle-shaped and its cross-section is round. They have teeth on both upper and lower jaws. The two dorsal fins are slightly separated, and scales locate at the backs of the eyes, corselet, and lateral lines. The dorsal side is bluish purple in color and silvery white on the belly, and there are four to six vertical black stripes on the flanks.
Distribution
The distribution of skipjack tuna is wider from south to north in the western Pacific and narrower in the east, in line with the species’ preferred temperature range.
Ecology
This species spawns widely in the area where surface temperatures are above 24°C and specially year-round in tropical seas. The approximate fork length of this species is reported up to 44 cm at one year old, and 62 cm at two. It is reported that their body length reaches 100 cm in adulthood. Total life-span is estimated to be six years or more. This species preys mostly on fish, crustaceans and cephalopods. Their natural predators include tuna, marlin, Spanish mackerel, sharks, and seabirds.
Use Form
In addition to their use in canned and dried foods, skipjack tuna is eaten fresh as sashimi or lightly roasted (tataki).
Fishery
According to the breakdown of catches by fishing method in 2017, amount of catch by purse seine was 1,280 thousand tons, or 79 % of the total; by pole-and-line 120 thousand tons, or 8% of the total, and by “Other” commercial methods 220 thousand tons, or 13 % of the total.
By country, the USA, Korea, Taiwan, and Japan, accounted for 50 to 60% of total purse seine catches in recent years. Other major contributors were Papua New Guinea, Indonesia, and Philippines. In regard to pole ana line, although Japan had occupied about 60% of catches until 2005, catches by this method gradually decreased and Indonesia took the lead from 2006. In recent years, catches by Japan amount to around 40 to 50% of the total.
Your comprehensive evaluation
Resource Status
Skipjack tuna are one of the most important fishery resources. Its population size are estimated by catch, fishing effort, body length composition, and mark recapture method data by Multifan-CL model every three years. The data necessary for these analyses are surveyed and updated annually by the Japan Fisheries Research and Education Agency and relevant prefectures as a government-commissioned project. The spawning stock biomass of skipjack tuna has not been overfished and fishing has not exceeded overfishing level since 2010. The Western and Central Pacific Fisheries Commission (WCPFC) has implemented conservation and management measures based on the results of stock assessments carried out by the Pacific Community (SPC).
Marine Environment and Ecosystem
Regarding data on ecosystem impacts and feasibility of monitoring, ecosystem and bycatch issues, ecosystem model analyses, and bycatch data from longline fishery in the western and central Pacific Ocean are available. Research on larvae and juveniles of tropical tuna and skipjack tuna, zooplankton, and marine environments are conducted on an irregular basis. Owing to the scientific observer program, which was established in 2008, collection of information about fish catches in purse seine has been partially enabled.
Regarding the impact of fishing for target fish on other species, skipjack, one of usable bycatch species, there is a negligible impact on its stock status. Unusable bycatch species included rainbow runners (Elagatis bipinnulata), silky sharks, ocean triggerfish (Canthidermis Maculata), mackerel scads (Decapterus macarellus), mahi mahi (Coryphaena hippurus), and others. Assessments of the Productivity and Susceptibility Analyses (PSA) in the eastern Pacific revealed that impact risks were minor, except for silky sharks which were judged to be at moderate risk. Among the endangered species designated by the Ministry of the Environment, the risks were evaluated as moderate among loggerhead turtles (Caretta caretta), green turtles (Chelonia mydas), and hawksbill turtles (Eretmochelys imbricata).
Subsequently, the indirect impact of skipjack tuna fishing on the food web and the impact of fisheries on the environment will be discussed. Among predators of skipjack tuna are swordfish, Indo-Pacific blue marlin (Makaira mazara), Striped marlin (Kajikia audax), mako sharks (Isurus oxyrinchus), blue sharks (Prionace glauca), silky sharks, Oceanic whitetip sharks (Carcharhinus longimanus), and larger bigeye and yellowfin tuna. According to the mixed trophic impact of Ecopath (an ecosystem model for the western and central Pacific surface), while negative impacts to marlin and sharks are insignificant, moderate impacts to yellowfin and skipjack tuna themselves have been detected. Carnivorous skipjack tuna, which have limited selectivity, are regarded as more opportunistic in their feeding habits. The analysis with the above ecosystem model shows that the negative impact on bait organisms, fish, crustaceans and cephalopods, is insignificant. Analyses of predatory fish with roughly the same trophic level by the above ecosystem model showed negative impacts on yellowfin tuna. Since the mean trophic level of catch (MTLc) has been increasing since around 1980 with existing stock numbers and diversity declining, there are concerns over partial changes in the characteristics of the ecosystem.
This project found no reports of pollution or waste dumping violations caused by Japanese fishing vessels within the WCPFC convention area. Emissions (t-CO2/t) required for catch per unit effort were relatively low with medium- and large-scale purse seiners for Thunnus including skipjack among fishing vessels in Japan, so their effect on the atmospheric environment is regarded to be insignificant.
Fishery Management
The stock assessments implemented by SPC are not in agreement in WCPFC. Medium- and large-scale purse seine and pelagic and offshore skipjack tuna fishing by pole-and-line requires official permits from the minister of Japan’s Ministry of Agriculture, Forestry and Fisheries , and coastal pole and line fishing of skipjack tuna requires the approval of the Wide Sea-area Fisheries Adjustment Commission. Output controls are not implemented. Since the stock assessment results are not agreed, it is not possible to conclude whether the input controls were successful with regard to fishing pressures. As for the technical control, while the installation of fish aggregating devices (FADs) is regulated, prohibition period was shortened. Catching silky and oceanic white tip sharks using pelagic and longline methods for skipjack and other tuna are prohibited. Medium- and large-scale purse seine operations near whale shark are also prohibited. There were no particular problems regarding abandoned fishing gear for pole-and-line fishing. Tuna Fisheries Office of International Affairs Division o of Japan’s Fisheries Agency is cooperating with the WCPFC and the SPC. Medium- and large-scale purse seine that fish skipjack tunas are managed and supervised by the Tuna Fisheries Office of the International Affairs Division and the Fisheries and Resources Management Division of Fisheries Agency, and pole-and-line fishing is managed and supervised by the Tuna Fisheries Office of the International Affairs Division. Coastal pole-and-line fishing of skipjack tuna is subject to the approval by the Wide Sea-area Fisheries Adjustment Commission due to relation to the management of bluefin tuna. The management system is established and functioning. In medium- and large-scale purse seine and pelagic and coastal longline fishing, any vessel must have appropriate observers on board , when ordered by the Minister of Agriculture, Forestry and Fisheries. In Japan, procedures for prior confirmation of imported fish (which require certificates and related documents ensuring that they were caught by fishing vessels on the Positive List), have been compiled at the Fisheries Agency. Projects for strengthening tuna stock management skills under the international agreement at the WCPFC are being conducted as well. To revise domestic ministerial ordinances, reviewing resource management policies, according to management objectives, results of the stock assessments, and other management measures by the administration authority are evaluated to be measures equivalent to adaptable management. Under resource management policies, fishermen implement suspension of fishing on a voluntary basis. The Japan Far Seas Purse Seine Fishing Association, etc. are actively planning for the realization of effective control measures. Fishermen’s’ organizations are leading the reform plan and verification project. Coastal Fishery Cooperatives are promoting sales by establishing a skipjack tuna brand. Stakeholders participate in the Resource Management Subcommittee of the Fisheries Policy Council and NGOs participate in the WCPFC annual meetings and the Scientific Committee.
Regional Sustainability
Most skipjack tuna in the western and central Pacific Ocean are caught by medium- and large-scale pelagic tuna purse seines using one vessel (Miyagi, Tokyo, Kanagawa, Shizuoka, Mie, Niigata, Tottori, and Nagasaki), medium- and large-scale offshore tuna purse seine using one vessel (Shizuoka), pole-and-line pelagic skipjack tuna fishing (Miyagi, Shizuoka, Mie), pole-and-line offshore skipjack tuna fishing (Miyazaki), and pole-and-line coastal skipjack tuna fishing (Kochi). Fishing income has trended at the medium level. Profitability is low and assets related to fishery are at a medium level. Management stability is medium in terms of both income and stability of catches. Financial reports are not published by most of fishermen’s organizations. Operational safety is high, and contributions to regional employment are high. As for labor conditions, there was no significant problems in fisheries. Most skipjack tuna is landed on the base market and the number of buyers is proportional to the quantity of the catches in each market, and the principle of competitive bidding at auction is mostly working. Hygiene management is thoroughly managed along with the wholesale market development project. The product goes to the medium-to-luxury consumer market. There were no labor issues at the processing or distribution stages. Sustainability in the processing and distribution industry can be evaluated as high. The introduction and propagation of advanced technologies are accomplished through a well-prepared logistical system. Income levels of people working in the fishery industry are relatively high. Both pole-and-line and purse seine fishery are based on traditional methods, and while traditional processing and distribution technology are maintained, new usages are also being developed.
Health, Safety, and Security
Skipjack tuna contains various functional nutrients, such as niacin (the coenzyme of redox enzymes in the body), vitamin B1 (a contributor to cell metabolism), vitamin D (which promotes absorption of calcium and phosphates and is a principal component of bones), serene (which has antioxidative properties), selenoneine (reputed to provide various benefits, including detoxication of methylmercury), taurine (which has functions including prevention of arteriosclerosis and heart disease). Skipjack tuna fat is rich in both EPA, which prevents thrombus, etc. and DHA, which promotes brain development and prevents dementia, etc. Additionally, this species has high protein and the red muscle is rich in iron. The best time for harvesting is from spring to autumn. Spring skipjack tuna are called “hatsu-gatsuo” (“first skipjack”), which are good for eating and low in fat. Autumn skipjacks are called “modori-gatsuo” (“returning skipjack”), which are rich in fat and delicious. Care should be taken to avoid histamine poisoning and anisakis parasites when eating them raw. Histamine poisoning is caused by increases in histamine that are produced when bacteria break down histidine an amino acid which is contained in abundance in skipjack muscles. It is important that the fish be maintained at low temperatures while handling and processing and these low temperatures maintained even after thawing. Since anisakis parasites, live in the internal organs and invade the muscles after the death, they can be avoided if the fish is eaten while fresh or just after thawing and by never eating raw internal organs.
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