BLUEFIN TUNA SPECIES
There are at least three different species of bluefin tuna: the Atlantic (the largest and most endangered), the Pacific, and the Southern. Some say there are four: the Pacific, the Southern, and two distinct species on the Atlantic (See Below).
It was long thought that that there were seven Thunnus species, and that Atlantic bluefin tuna and Pacific bluefin tuna were subspecies of a single species. In 1999, it was established, based on both molecular and morphological considerations, the Atlantic bluefin tuna and Pacific bluefin tuna are in fact distinct species.
The southern blue fin is found in warm waters of the Southern Hemisphere. It swims in waters off Australia and New Zealand and elsewhere and spawns in the Timor Sea south of Java, Indonesia. Many hang out in the Australian Blight or in waters off Tasmania and migrate northward to the Timor Sea via western Australia.
Websites and Resources: “Netting Billions 2020: A Global Tuna Valuation, Pew Charitable Trusts”, October 6, 2020 pewtrusts.org; Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems
See Separate Articles: TUNA: CHARACTERISTICS, SPEED, BEHAVIOR, FEEDING ioa.factsanddetails.com ; BLUEFIN TUNA: CHARACTERISTICS, BEHAVIOR, HUNTING AND MATING ioa.factsanddetails.com ; BLUEFIN TUNA COMMERCIAL FISHING ioa.factsanddetails.com ; BLUEFIN TUNA AND SUSHI ioa.factsanddetails.com ; BLUEFIN TUNA FISH FARMING ioa.factsanddetails.com ; LOW BLUEFIN TUNA NUMBERS: OVERFISHING, ORGANIZATIONS, QUOTAS AND PROTESTS ioa.factsanddetails.com
Pacific Bluefin Tuna
Pacific bluefin (scientific name: Thunnus orientalis) tuna are slightly smaller than their Atlantic cousins. They are closely related to the Atlantic bluefin and for years were considered to be the same subspecies. Also known as northern bluefin tuna, Pacific bluefin tuna have silvery white bottom half and blue and green on top half and back, with grayish-green iridescence and silver or gray spots on their bellies. Bluefin tuna fish have small yellow fins edged in black from second dorsal to tail fin. They have relatively small pectoral fins, and small eyes relative to other tuna species. They have a homocercal, lunate (crescent-shaped) tail that allows for great speed over long distances. [Source: NOAA, Matt Zbroinski, Animal Diversity Web (ADW) /=]
Adult males and females vary in length from 100 centimeters to 300 centimeters. It takes individuals five years to mature, generally averaging about 150 centimeters in length with a weight of around 60 kilograms. The largest Pacific bluefin tuna recorded reached 450 kilograms and 300 centimeters in length. Pacific bluefin tuna never stop growing, but growth slows over time. Males and females have about the same growth rate. /=\
Pacific bluefin have relatively small eyes compared to other species of tuna. A distinguishing characteristic of Pacific bluefin tuna is that the tips of the pectoral fins do not reach the front of the second dorsal fin. The tuna reach sexual maturity at approximately five years of age and can live up to 26 years, although the average lifespan is about 15 years. Adults are approximately 1.5 meters (4 feet 11 inches) long and weigh about 60 kilograms (130 pounds). The maximum reported length and weight for Pacific bluefin tuna is 3 meters (9.8 feet) in length and 450 kilograms (990 pounds).
Bluefin Tuna Habitat and Where They Are Found
Pacific bluefin tuna are native to the Pacific Ocean and Indian Ocean and are found mostly in temperate ocean waters but also in the tropics and cooler coastal regions. They mainly inhabit cooler waters of the North Pacific Ocean between a latitude of 45- and 5-degrees north. Some inhabit tropical waters of New Zealand and French Polynesia.[Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]
Of the tunas, Pacific bluefin tuna have the largest geographic range. They range from the east Pacific, to the Japanese mainland and its islands, to the western coast of the United States and Canada. Most of the U.S. catch of Pacific bluefin tuna is within about 100 nautical miles of the California coast. Tagging studies have revealed that some bluefin spend their entire lives in the Western Pacific Ocean, while others migrate to the Eastern Pacific Ocean. The trans-Pacific journey can take as little as 55 days..
According to to Animal Diversity Web: Some migration occurs as seasons change and water temperatures change, as they are mostly found in the waters between 18° to 20°C (64° to 68°F). In autumn, some travel to the northern shores of California, away from cooler Pacific waters with harsher windstorms. They also migrate further south during winter, as lower water temperatures decrease predator populations.
Pacific Bluefin Tuna Migrations
Many Pacific bluefin are born in the Sea of Japan and then migrate thousands of miles to east. Pacific bluefin tuna that breed and spawn in seas off Taiwan migrate northwards towards Japan or to the west coast of the United States. Tagged Pacific bluefins have been observed migrating between Japan and California and back and swimming to depths of 300 meters (1,000 feet).
Before the census started, the migration of the Pacific bluefin tuna had not been monitored much. But by tagging a 15-kilogram tuna, scientists found that it crossed the Pacific three times in just 600 days, according to Stanford University’s Barbara Block. Bluefin tuna that were first tagged in Tokyo Bay were spotted in Hokkaido three month later and off California six months after that. Another that was monitored for years swam from Baja California to Japan and back, logging more than 18,000 nautical miles. One tagged tuna swam back and forth across the Pacific Ocean three times in a year.
Some Pacific bluefin spawns in the waters between the Philippines and southern Japan, then migrates more than 6,000 miles to Baja. Peter Bridson, aquaculture research manager at the Monterey Bay Aquarium, told the Los Angeles Times, "The tuna come over to Mexico when they're 1 to 2 years old, and then when they're 5 to 6 years old they start migrating back over toward the other side of the Pacific to reproduce. [Source: Adam Yamaguchi and Zach Slobig, Los Angeles Times, July 21, 2011]
After Fukushima nuclear power plant disaster in 2011 waters around the plant absorbed radioactive materials and these radionuclides appeared in Pacific bluefin tuna caught in California. This was viewed as a further evidence of migrations between Japan and the U.S. West Coast. According to a Stanford University report: Pacific bluefin tuna are born several miles off the shores of Japan and surrounding countries. They spend their first year foraging in those waters, and then either remain in the western Pacific or migrate eastward to California. Although the movements of adult tuna have been revealed through electronic tagging programs, electronic tagging of juvenile bluefin in Japan has been difficult, and so certain details of this early-life migration remain unknown. [Source: Stanford Report, March 4, 2013]
Daniel Madigan, who works in the Micheli Lab at Stanford's Hopkins Marine Station in Monterey, Calif., worked with collaborators at the National Oceanic and Atmospheric Administration and Stony Brook University to sample 50 Pacific bluefin tuna that were between 1 and 4 years old. Because the fish are born near Japan, the smallest fish were certain to have swum in Japanese waters only in the year following the Fukushima accident. The presence of cesium in these fish would support the claim that they are still carrying radiation from Fukushima eastward, and that this measure can be used to trace bluefin migrations.
How and Why Pacific Bluefin Tuna Can Migrate So Far
According to Animal Diversity Web: Pacific bluefin tuna migrate seasonally due to water temperature changes. Between June and August, when waters are warmer they migrate to breeding waters just west of Japan or between Japan and the Philippines. Some juveniles stay in the western Pacific; others, at about a year of age, migrate either south to Australia or east to the west coast of the United States. They remain in these areas (moving north and south along the coast of the United States and Mexico) for several years before returning to Japan to spawn. NOAA estimated the time to migrate from the U.S. western coast back to Japan (about 8,000 kilometers) as 55 days (or more; Boustany et al., 2010 reported a trip time of 66 days), and possibly more than half of those fish between one and three years will survive this trip. They tolerate water temperatures as low as 9°C during this migration. [Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]
Migration is highly dependent on food availability and water temperature. In order to swim to designated places, they rely on their visual and olfactory senses to guide them. They do not have very good vision, and so they migrate during the day. As they get older, their swimming patterns also change; immediately after they hatch, Pacific bluefin tuna swim closer together and at a faster rate. After 26 to 30 days, tuna speed decreases and space between two schooling fish increases. /=\
As endotherms (an unusual trait for fish), these tuna can tolerate a wide range of water temperatures and exhibit a high cardiac output. They can swim at fast speeds, clocked as high as 48 kilometers per hour in short sprints while hunting (generally less than 20 seconds at this speed). Migration distances can average 145 kilometers a day. /=\
Pacific Bluefin Tuna Feeding and Mating
Pacific bluefin tuna Pacific bluefin tunas are predatory and mainly eat squids and fish, such as sardines and anchovies, saury, herring, pompanos, mackerel, hake, other tunas, and occasionally red crabs and krill.. Research in 1971 indicated that the squid, Abraliopsis felis, comprised 89 percent of the diet of bluefin tuna in Californian and Mexican waters. Other studies, anchovies and squid were major parts of their diet. Another study found that about 11 percent of the diet of one group was Benoit's lanternfish (Hygophum benoiti). A major determining factor in regard to diet was the range and availability of food. In some cases when when food is limited, aggression increases and sometimes small bluefin tuna larvae was cannibalized.
The Pacific bluefin tuna breeding season is from April to July in Ryukyu Islands south of Japan and June to August in the Sea of Japan. The number of eggs ranges from 780,000 eggs to 35,000,000. The time to hatching ranges from 26 to 48 hours, with the average time being 38 hours. Young are born independent, On average males and females reach sexual maturity at age five years. Egg spawning occurs daily between April to July. It occurs at this time because egg spawning is temperature-sensitive, requiring water temperatures of 23.5 to 29.5°C. Like most bony fish, fertilization occurs externally. Mating can occur between more than one female or male. Larger females lay significantly more eggs than smaller females.[Source: Matt Zbroinski, Animal Diversity Web (ADW) /=]
Endangered Pacific Bluefin Tuna and Conservation Efforts
Though the Atlantic species is under the greatest threat, concerns also exist for the Pacific bluefin. Pacific-wide populations of Pacific bluefin tuna are well below target levels. Rebuilding measures are in place for U.S. fishermen. The fishing rate has been reduced to end overfishing. According to the 2020 stock assessment, Pacific bluefin tuna are overfished and subject to overfishing. NOAA Fisheries first determined the Pacific bluefin tuna stock to be overfished in 2013. The 2020 assessment completed by the International Scientific Committee for Tuna and Tuna-Like Species in the North Pacific Ocean found the stock is still overfished. [Source: NOAA]
Management of highly migratory species, such as Pacific bluefin tuna, is complicated because they migrate thousands of miles across oceans and international borders and are fished by many nations. Effective conservation and management of these resources requires international cooperation as well as strong domestic management. The United States continues to encourage harvest levels internationally that end overfishing and rebuild the population.
Two international organizations, the Inter-American Tropical Tuna Commission (IATTC) and the Western and Central Pacific Fisheries Commission (WCPFC) coordinate management of this fishery across jurisdictions of member and cooperating nations. Working with the U.S. Department of State, NOAA Fisheries implements the IATTC and WCPFC conservation and management measures as regulations for U.S. fleets.
According to Animal Diversity Web: Conservation efforts of Pacific bluefin tuna are in place, but are location-specific. Emphasis has been placed on reducing the commercial intake of young (less than three years old) bluefin tuna in multiple areas — especially in Japan and Korea. An upper limit of metric tons per year (5,000 in 2014) has been set in Mexico and the United States. Reported catches in the U.S. have been lower in recent years. Only after heading south into waters near Mexico have catch numbers increased. Recreational fishing in California has also been restricted. Since 2015, the catch limit has been reduced from 10 to two per day. [Source: NOAA, Matt Zbroinski, Animal Diversity Web (ADW) /=]
Two Atlantic Bluefin Tuna Species
There two separate Atlantic species (stocks) of bluefin tuna that are genetically distinct: one harvested mainly by European nations that roams the eastern Atlantic and breeds in the Mediterranean; and another that is harvested mainly by the United States, Canada and Japan that roams the western Atlantic and breeds in the Gulf of Mexico. About 20 percent of fished bluefins have traditionally come from the Mediterranean.
The Western Atlantic bluefin tuna (Scientific name: Thunnus thynnus) are the largest of the tuna species, reaching up to four meters (13 feet) in length and 907 kilograms (2,000 pounds). They have dark blue-green-black on their backs and upper sides and are silvery white on the lower sides and belly, with colorless lines alternating with rows of colorless spots on their lower sides. Bluefin tuna have small yellow fins from second dorsal to tail fin. The dorsal fin — the second fin on the back — of the Western Atlantic bluefin is reddish brown. The fish also has short pectoral fins. These characteristics separate this species from other members of the tuna genus, Thunnus. [Source: NOAA]
Also known as tuna, bluefin tuna, toro, maguro, giant bluefin and northern bluefin tuna, the Western Atlantic bluefin tuna have large, torpedo-shaped bodies that are nearly circular in cross-section. Bluefin tuna grow more slowly than other tuna. They have a long lifespan, up to 20 years or more and generally don’t spawn until they are about 8 years old.
Bluefin tuna are top predators. Juveniles eat fish, squid, and crustaceans, and adults feed mainly on baitfish such as herring, bluefish, and mackerel. Sharks, marine mammals (including killer whales and pilot whales), and large fish feed on bluefin tuna. Bluefish and seabirds also prey upon juvenile bluefin tuna.. Western Atlantic bluefin tuna spawn from mid-April to June, mainly in the Gulf of Mexico. Females can produce up to 10 million eggs a year. The eggs are fertilized in the water column and hatch in about two days.
In the western Atlantic, bluefin tuna are found from Newfoundland to the Gulf of Mexico. Atlantic bluefin tuna live near the surface in temperate waters but frequently dive to depths of 500 to 1,000 meters. They are a highly migratory species. They can migrate thousands of miles across an entire ocean. Tagging studies have indicated that bluefin tuna move across the east/west boundary in the Atlantic. They tend to spawn in the same areas in the Gulf of Mexico.. In the U.S. they can be found off New England, Mid-Atlantic and the Southeast.
Migrations of Atlantic Bluefin Tuna
Many Atlantic bluefin, migrate about 6,000 kilometers between North America and Europe. Fish of both Atlantic species migrate regularly across the Atlantic, traversing the entire ocean in as little as a month and a half. For much of the year the two species mix before heading off to separate spawning areas. They do not interbreed. The mixing had led some to believe that there was one vast Atlantic stock rather than an eastern Atlantic stock and western Atlantic stock. It has also led some to believe the Mediterranean stocks are in better shape than they actually are.
Migration patterns of Atlantic bluefin can be quite complex. Two fish tagged within a couple of minutes of each other off the coast of Ireland were 3,000 mile apart eight month later. One was northeast of Cuba and the other was off the coast of Portugal. One bluefin tuna tagged off the Bahamas was captured 50 days later in Norway after making a journey of 6,200 miles. Around breeding season many bluefins gather in the middle of the Atlantic between Bermuda and the Azores.
Most of the Atlantic bluefins studied migrated between their breeding ground in the Gulf of Mexico and feeding grounds in the central and eastern Atlantic. One thing that researchers found is that tuna that traditionally migrated to places where fishing regulations were strict and quotas were low also migrated across the ocean to places where regulations were ignored and quotas are high. In breeding grounds in the Gulf of Mexico where bluefin fishing is not allowed bluefin are caught by fisherman going after other species of tuna.
Atlantic Bluefin and the ICCAT Line
On what is called the ICCAT line, Kenneth Brower wrote in National Geographic: ““The fisheries for the Atlantic bluefin tuna are managed by the International Commission for the Conservation of Atlantic Tunas. ICCAT stock assessment models make use of a dotted line dividing the North Atlantic vertically. Drawn in 1981, this demarcation follows the meridian at 45° west longitude and divides the western stock of Atlantic bluefin from the eastern. The lab’s pointillist maps show a curious thing. The positions of electronically tagged western bluefin, represented as reddish-orange circles, pack the Gulf of Mexico, the spawning grounds for this stock, and from there spill eastward into the Atlantic. They cross the ICCAT line with impunity and spread all the way to Portugal and Spain. The positions of tagged eastern-breeding bluefin, represented as white circles, pack the Mediterranean, spawning grounds for this stock, and from there spill westward, crossing the ICCAT line to fill the coastal waters of the United States and Canada. [Source: Kenneth Brower, National Geographic, March 2014]
“The ICCAT line, the maps testify, is a fiction. Scientists once believed that each stock stayed on its own side of the ocean, but no one believes that now. Everywhere in the Atlantic, all across the feeding grounds of this species, the eastern and western stocks mix. It seems that only in their respective spawning grounds are they separate. [Source: Kenneth Brower, National Geographic, March 2014]
“The fact of this mixing was well established by Block, other taggers, and DNA researchers more than a decade ago. It has yet to be incorporated in ICCAT models. The best estimates today are that around half of the bluefin caught off the eastern shores of North America originated in the Mediterranean, yet any of these tuna, if caught in the west, are still counted as fish of western origin. The ICCAT line is not just a dull management tool — it is no tool at all. The ICCAT model fails, as well, to make any allowance for illegal fishing, though studies indicate that the illegal take is large.
Endangered Western Atlantic Bluefin Tuna
The catch of blue fin tuna in the western Atlantic was 5,000 tons in 2001, one tenth the amount taken in the 1970s. Kenneth Brower wrote in National Geographic: “Bluefin tuna are among the most overfished species on Earth. The stock that spawns on the western side of the Atlantic has been reduced by 64 percent since 1970. The tonnara of Sicily — the mazes of net pens in which, for millennia, Sicilians have collected giant bluefin to kill in the ritualized climax called mattanza — have been folding one after another for decades, as have similar mazes, by different names, throughout the rest of the Mediterranean. [Source: Kenneth Brower, National Geographic, March 2014]
““Estimated Spawning Stock Atlantic Bluefin Tuna (1950-2008)” shows a graph of the spawning biomass of Gulf of Mexico breeders atop a similar graph for the Mediterranean breeders. Both populations are represented by lines in the shape of eels, and both eels are diving toward the bottom of their graphs. They have plunged past the dotted line representing sustainable yield and are headed for a spot where the kilotons of spawning biomass read zero.
For 2021-2023, the stock is being managed under ICCAT Recommendation 20-06 that is responsive to scientific advice. Regulations do not allow targeted fishing of bluefin tuna in the Gulf of Mexico, an important spawning area for the species. According to NOAA, the population level is unknown for bluefin tuna in the western Atlantic. The fishing rate is at recommended level. Fishing gear used to catch bluefin tuna rarely contacts the ocean floor and has minimal impact on habitat. Fishing gear used by U.S. fishermen to target schools of bluefin tuna is fairly selective, and allows for the live release of any unintentionally caught species. [Source: NOAA]
Some fisheries biologists believe that Atlantic bluefin populations, if allowed to recover, could grow to five times their present size. According to the 2020 stock assessment western Atlantic bluefin tuna stock is not subject to overfishing, but the overfished status is unknown. The most recent assessment by the International Commission for the Conservation of Atlantic Tunas (ICCAT) shows a spike in the eastern population, a result of lower catch quotas. But scientists say the true increase is likely smaller.
Image Source: Wikimedia Commons, NOAA, graphs and graphics from The Pew Charitable Trusts (“Netting Billions 2020: A Global Tuna Valuation, October 6, 2020 pewtrusts.org)
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated April 2023