There’s hardly a fish species as unique and majestic as Bluefin Tuna. Big, smart, and fast, this pelagic predator is renowned for its fantastic game qualities and unmatched culinary value. For all its popularity, the Bluefin Tuna migration and conservation status were largely unknown until a few decades ago. This allowed Bluefins to be commercially fished to the brink of extinction.
Conservationists realized that the first step to saving these magnificent fish was to understand their behavior. This resulted in a huge international effort geared toward learning about Bluefin Tuna migration, spawning, and feeding habits. Today, we look into Bluefin Tuna migration and what it means for the species’ conservation efforts.
Why is Bluefin Tuna Important?
There are a number of things that make Bluefin Tuna special. Apart from Mackerel Sharks, Bluefins are the only species that can auto-regulate their body temperature. This allows them to be highly migratory and to tolerate a wide range of water temperatures. Bluefins are also tier-one predators. For this reason, their role in the ecosystems they inhabit is of vital importance.
For most people, Bluefin Tuna is special because of its delicious fatty meat. This, incidentally, is where the problem lies. But more on that later.
Before we get into the different types of Bluefins and their migratory habits, let’s take a look at some of the most distinctive Bluefin Tuna features:
- Bluefins travel vast distances – over 6,000 miles a year;
- They are extremely fast swimmers – they can reach speeds of almost 50 miles per hour;
- Bluefins often dive deeper than 3,000 feet;
- Their life expectancy is up to 40 years;
- Bluefins are unique in that they are warm blooded;
- They can withstand temperatures from 37–86°F;
- In order to breathe, Bluefins must constantly keep swimming;
- Bluefins boast fantastic vision – they are very efficient hunters by sight;
What do Bluefin Tuna Eat?
Because they constantly need to be on the go, Bluefins have an extremely fast metabolism. This causes them to be highly predatory. The bigger Bluefins get, the bigger the fish they hunt. Younger Bluefins eat crustaceans, fish, and cephalopods. As they mature, they switch to a diet of herring, anchovies, sardines, bluefish, and mackerel.
Bluefin Tuna are very smart about how they hunt. When searching for prey, they take water temperature, chlorophyll levels, and ocean currents into account. Whatever prey they’re after, Bluefins will remain in waters that allow them to digest their food efficiently. This means that they will sometimes refrain from entering a bountiful hunting ground if they sense that the temperature difference is too taxing for their metabolism.
For Bluefin Tuna living in the northern hemisphere, spawning usually occurs during spring, when waters start to warm.
How Big are Bluefin Tuna?
Thanks to their feeding prowess, Bluefins can reach impressive sizes. Atlantic Bluefin is the biggest, and can reach an astounding 2,000 pounds. They’re followed by Pacific Tuna, which top out at around a thousand pounds. Southern Bluefin Tuna are the smallest, and can reach 570 pounds.
The three Bluefin varieties exhibit different migratory habits unique to the environment they inhabit. Let’s take a look at each subspecies in more detail.
Atlantic Bluefin Tuna
Atlantic Bluefin Tuna (Thunnus thynnus) inhabit the waters of the Atlantic and the Mediterranean. These are the biggest Bluefins, which makes them particularly interesting for fishermen. A little too interesting, you might say. Atlantic Bluefin Tuna is one of the most critically endangered fish species in the world.
Depending on where they spawn, Atlantic Bluefin Tuna have two main stocks: western and eastern. Both are highly migratory, and often share the same feeding grounds. However, when the time comes to spawn, the eastern and western stocks go their separate ways. Research has shown that Bluefins always spawn in the same area that they themselves hatched in.
When do Atlantic Bluefin Tuna Spawn?
The western Bluefin stock spawn from mid-April to June, mainly in the Gulf of Mexico. The Eastern stock sees its reproductive season from May to July, in the Mediterranean Sea. As they spawn, eastern Bluefins try to keep up with the east-to-west water temperature increase. This causes the eastern Bluefins to move from the Levantine Sea to the Balearic Sea during this period.
According to the NOAA, the western Bluefin Tuna stock has a few spawning “sweet spots” in the Gulf of Mexico. These are areas that show favorable ocean circulation, as well as a significant presence of tracked Bluefin Tuna.
Atlantic Bluefin’s reproductive capabilities are very impressive. Each Tuna can lay as many as 540 million eggs each spawning season. However, as Atlantic Bluefins have a lifespan of 20 years or more, they usually don’t spawn before the age of eight. That, coupled with the fact that they spawn together in large schools, makes Bluefins extremely vulnerable to commercial fishers, especially in the Mediterranean.
Atlantic Bluefin Tuna Conservation
Year after year, Mediterranean commercial fishing vessels line up to scoop Bluefins up in their purse seines. A distressingly large number of these Bluefins are juveniles, fish who haven’t had a chance to spawn in their lifetime.
Over in the Gulf, surface longliners targeting Yellowfin and Swordfish regularly make Bluefins their bycatch. More than half of the Tunas caught on Gulf longlines die before they reach the boats.
Such practices went on for the better part of the 20th century, rendering Atlantic Bluefins critically endangered. After years of unregulated exploitation, the first conservation initiatives finally started in the ‘60s. In 1966, fishing nations formed the International Commission for the Conservation of Atlantic Tunas (ICCAT). The ICCAT’s purpose was to establish a sustainable amount Bluefin catches for each participating country.
However, many experts contend that the ICCAT’s generous yearly quotas have not been taking the spawn timelines of Bluefins into account. More recently, the ICCAT did take steps to implement an electronic catch-tracking system to help stop illegally caught Atlantic Bluefin from making it to market. Still, the western Atlantic Bluefin Tuna stock numbers half of what it was in 1974.
Recognizing the threat, the NOAA implemented a series of rules to prevent overfishing Bluefins in the Gulf. In 2011, they made a requirement for longline fishermen to use weaker hooks, to allow for Bluefin survival post-release. In 2015, the NOAA prohibited the use of surface longlines during their spawning season in a vital 27,000-square-mile breeding area.
A recent study done by NOAA Fisheries and the University of Massachusetts Boston point to the existence of a second breeding ground of western Atlantic Bluefins. According to the study, Bluefins spawn in the area known as the Slope Sea, off the coast of New England. The implications of this study are that the western Bluefin stock is not as critically endangered as previously thought. According to the NOAA, Bluefins larger than 500 pounds migrate to the Gulf of Mexico to spawn, while smaller Bluefins make a shorter migration to the Slope Sea.
And while a number of experts are warning about the dangers of trusting a single study, one thing does seem to be certain: The eastern Atlantic Bluefin Tuna population is finally recovering. There are numbers showing that the western stock is starting to rebuild, too, but researchers are still skeptical as to how reliable this information is. The main reason for this is that the larger eastern stock often mingles with the western, which makes data processing difficult.
Pacific Bluefin Tuna
Like their Atlantic relative, Pacific Bluefin Tuna (Thunnus orientalis) are top-tier predators and fish of great commercial value. They inhabit the northern waters of the Pacific, from the East Asian coast to the western coast of North America. They usually prefer temperate waters, but sometimes venture into the southern tropics, too.
Where do Pacific Bluefin Tuna Spawn?
Pacific Bluefin Tuna spawn in the northwestern Philippine Sea and in the Sea of Japan. Their breeding season lasts from April to August, starting from the Philippines and moving north toward Japan. Bluefins reach sexual maturity between seven and nine years of age. When this happens, a single Bluefin Tuna can lay between 5 million and 25 million eggs in a spawning cycle. Unlike the Atlantic Bluefins, all the Pacific Bluefins are members of one single stock.
Before they reach maturity, the majority of Pacific Bluefins make a long voyage east to the North American continental shelf. To complete the impressive 5,000-mile journey, these juveniles make their way through the icy waters of the Arctic Sea. Bluefins pass through waters as cold as 16ºF. Not many fish can accomplish this feat. How are Bluefins able to do this?
All types of Bluefins boast a highly advanced circulation system, also known as a counter-current heat exchange system. In a nutshell, veins with warm blood leaving the muscles are aligned right next to the colder incoming arteries, so that the heat is passed in a continuous loop. The heat stays in the muscle and never gets the chance to be sapped away in the gills.
Once they reach the warm North American waters, Pacific Bluefins stay for two to five years, feeding and growing. While there, Bluefins traverse the waters from Mexico all the way to Washington State.
As superior predators, Bluefins are an important link in the ocean’s food chain. A link which can easily be broken by commercial fishers, especially if the Bluefins they catch are juvenile.
Pacific Bluefin Tuna Conservation
Since people first started fishing for Pacific Bluefin Tuna in the 50s, the population has declined by a frightening 96.4 percent. But that’s not the scariest part. Nine out of 10 PBTs caught by commercial fishermen are juveniles. This means that Bluefins are killed before they even get a chance to reproduce. On top of it all, Mexico, Japan, Korea, and the US have all exceeded their fishing quotas over the last few years. Why is this happening?
The Japanese Bluefin sashimi trade is one of the most lucrative food markets in the world. In 2013, a single 500 lb Bluefin Tuna was sold for a whopping US$1.76 million! With the vast majority of the global catch going to Japan, it appears to also be the driving force behind most commercial fishing expeditions.
The news is not all bad. Recognizing the danger, Bluefin-harvesting countries of the Pacific have come together to agree on a long term goal of achieving 20 percent of the Pacific Bluefin’s historic population by 2034.
Leading countries of the region have all established control groups to lead the conservation efforts. The groups have agreed to establish sliding catch quotas to reach the 20 percent goal, based on how well the stocks recover in coming years. They have also agreed to a harvest strategy timeline that includes partner meetings over the next two years. The agreement will include preventing illegally caught Bluefin Tuna from reaching markets, as well.
Given its resilience, if all goes as planned, Pacific Bluefin Tuna should be able to recover.
Southern Bluefin Tuna
A smaller cousin of the Atlantic and Pacific varieties, Southern Bluefin Tuna (Thunnus maccoyii) roam the oceans of the southern hemisphere, from the tropics to the sub-Antarctic. They may weigh “only” up to 570 pounds, but are fast swimmers, excellent predators, and distinctively intelligent among fish of their size. Like their larger relatives, they prefer warmer waters, but can easily withstand more temperate environments.
Southern Bluefins mature between eight and 20 years of age, and can live up to around 40.
Where do Southern Bluefin Tuna Spawn?
Thanks to large-scale electronic tagging efforts, scientists have been able to precisely measure the migratory behavior of Southern Bluefin Tuna. We now know that these Bluefins make yearly spawning trips to the warm waters southeast of Java. SBT spawning season is unusually long – it lasts from September all through April. During this time, Bluefins will move around to stay in waters around 75°F.
During austral winter (May to August), larvae and juvenile Bluefins move south with the Leeuwin Current down the coast of Western Australia. From WA, they proceed down to the Great Australian Bight, where they spend their first summer. As the GAB waters cool down, the young Bluefins escape to warmer waters, either in the southeast Indian Ocean or the Tasman Sea. The juveniles repeat this cycle every year until they are about five years of age.
When they reach the age of five, Southern Bluefins are big enough to withstand longer swims. They stop returning to the Bight for summer, replacing it with bountiful feeding grounds between New Zealand and South Africa. As they reach maturity, SBTs join the rest of the flock spawning in the tropics below Indonesia.
Southern Bluefin Tuna Conservation
Southern Bluefins have been declared as Critically Endangered by the International Union for Conservation of Nature. According to the Union, only 3−8% of supposed unfished SBT biomass (what it would be if not fished) still lives. For a species with so few natural predators, this is very worrisome. The primary market for this fish is, again, the Japanese sashimi industry. The landed market value of the SBT fishery is estimated to be around US$227 million.
Southern Bluefin Tuna commercial fisheries mostly land the fish by way of longlining. Only in Australia do they employ a different fishing tactic – the purse seine. However, instead of bringing the fish to the mainland, the fishers place the Bluefins into underwater cages for fattening. Once the Bluefins fatten up, they’re frozen, sold, and shipped to Japan.
The Commission for the Conservation of Southern Bluefin Tuna (CCSBT) is an intergovernmental organization responsible for the management of SBT throughout its distribution. The CCSBT imposed big quota cuts in 2006 and 2009, resulting in a positive population shift for the first time in years.
Based on their model, the Scientific Committee unanimously recommended an increase in the global SBT quota from 9,449 tonnes in 2011 to 14,647 tonnes in 2015. The Commission also created a sustainability model, with a goal of reaching 20% of its unfished levels by 2035. Better days seem to be coming for Southern Bluefin Tuna.
More than half a century of unregulated commercial fishing all but wiped one of the ocean’s precious creatures off the face of the earth. Seeing such a majestic predator on the verge of extinction poses a question we must all ask ourselves: If we are so reckless with a species of this importance, what chances do the ones lower in the food chain have?
There’s light at the end of the tunnel, though. The efforts put in by the leading international conservation organizations have finally started to yield positive results. As we celebrate the revival of the Bluefin Tuna population, we must applaud the years of research international marine scientists put into their conservation. We must also realize that rebuilding the Bluefin population will not take a year or two. It will take time. But we will save one of our planet’s most magnificent creatures because of it.
What are your thoughts on Bluefin Tuna conservation efforts? What’s the most impressive thing about Bluefins in your mind? Let us know in the comments below.