The Bluefin Tuna – Sagittarius

Seascope “Tuna” General facts.

When you think of Tuna, what comes to mind? Is it your favourite Sandwich at Subway? A mainstay at your local Sushi restaurant? Those small, cheap, and plentiful cans on the grocery shelf? Tuna is everywhere, and like most fish we consume, it is hard to imagine what it looked like before it was on our plates. It would surprise most people to know that Tuna are a large, impressive species to behold. Not insignificant, small, or lunch-bag friendly by any means!

As they are one of the most consumed fish on the market, it isn’t surprising that they are also over-fished. Most commonly this occurs near Japan and Australia. But due to their nomadic tendencies, they can be found all over the Atlantic Ocean. There are 15 species of Tuna, but the most common are: Bluefin, Yellowfin, Skipjack, and Albacore.


The Atlantic (Northern) Tuna are the largest Tunas in existence. From the family Scombridae, this family includes many of the world’s most familiar “food fishes” such as sardines and mackerels. These sea creatures are made for swimming long distances at great speed and can dive deeper than 900 meters. Aside from being a common food fish in our diets, they are also appreciated by scientists for their Apex predator traits.

Bluefin Tuna are made for speed. They’re built like torpedoes, have retractable fins, and their eyes are set flush to their body. Throughout recorded history, the Atlantic Bluefin tuna has been highly prized as a food fish. Besides their commercial value as food, they have great size, speed, and power as apex predators. These traits have attracted the admiration of many fishermen, writers, and scientists.

The Atlantic Bluefin Tuna: Out of the Blue!

image of tuna.

What is the size, weight & colour of the Bluefin?

Rivals to Swordfish and Marlin, Tuna one of the largest Perciformes. Fully mature Tuna are found to be 2-2.5m and weigh 225-250kgs. The largest Tuna ever found was caught in Nova Scotia, which weighed 679kgs and was 3.7m long.

Bluefin are differentiated from other Tuna varieties by their pectoral fins, which are much shorter. But the general deep blue black and gray stomach are another key indicators. When viewed up close, there are golden flecks covering the body and yellow caudal finlets.

Image of tuna.

What type of vision does the Bluefin have, and what sounds do they make?

Bluefins are equipped with green colour sensitivity, which allows them to spot prey in their deep blue surroundings. By this sight alone, they can spot food and hunt them down. Because of this sensitivity, they have the sharpest vision of any bony fish.

While the Bluefin does not make any sounds, they have acute hearing. They have a 100-800 hertz frequency range, which makes them attuned to high pitch sounds. Low sounds generated by boats are startling for this fish.

Are there any special or unique physical attributes?

Aside from its torpedo shape and size, this fish has a metallic colouring all over its body- light silver on the bottom and dark blue on the top. This helps them camouflage in open water from above and below.

How fast is the Bluefin?

The Bluefin’s “walking pace” of swimming is as fast as an Olympic sprinter at about 64km/h. But when they are in a hurry to swim away from a predator that speed can escalate to 88.5km/h. Tuna must be careful not to swim too fast, as the lack of nerve endings in their fins stops them from feeling pain. If a Tuna swims too quickly its fins can sustain serious damage in the form of Cavitation.

How do they hunt, and what do they eat?

Young Tuna feed on crustaceans, varied fish, and cephalopods. While adults feed on herring, anchovies, sardines, and horse mackerel. They are opportunistic towards food and will also eat jellyfish, octopi, crabs, and sponges if they come across them. This is due to the strong predator instincts they are born with.


Who are their natural enemies?

Apart from fisheries depleting 74% of Tuna in the last 50 years, Bluefin have natural predators in the ocean as well. Their primary enemies are Orca Whales, Killer Whales, and Sharks. But there have been theories that Giant Squids may also play a role in the hunt for Tuna as their populations grow.

Where do they live?

Atlantic Bluefins are largely found in the Mediterranean Sea, which is the largest bluefin tuna fishery in the world. They are also found in the western and eastern Atlantic oceans. Nova Scotia especially is known for the largest and most impressive Bluefin Tuna.

What are their migration habits and routes?

The Bluefin Tuna migrates for 2 reasons: a trophic migration for food, and to spawn in the waters where they were born. In addition to these, the Bluefin cannot breathe if it is not swimming, therefore it must always be on the move. Their Journey begins at their most common spawning ground: The Gulf of Mexico. Then it can go as far south as Brazil or as Far North as Scotland. If their spawning ground is in the Mediterranean, they tend to not cover such long distances, as they need to travel less to seek warm waters.

How can they travel such long distances?

These fish have a complex circulatory system. They are warm-blooded and need to keep swimming in order to generate this heat. Their vital organs rely heavily on their temperature being above the water around them. This structure allows it to withstand cold temperatures (3°C) and warm waters (30°C). However, you will find them in warmer water year-round, as it takes less energy to raise their body temperature. This especially is why their mating grounds are in temperate climates.

Image of tuna.

All about Reproduction.

From mid-April to June, the Gulf of Mexico houses a large proportion of Bluefins for the mating season. These eggs are lightweight and are carried by surface currents away from their original spawning site. Female Bluefins have been known to lay between 5 and 45 million eggs a year in this season depending on their age. The incubation of eggs lasts for 2 days without parental supervision. The Larvae, 3mm long, then separate into schools and it becomes the survival of the fittest.

Life Span.

Bluefin are slow growers, reaching sexual maturity at age 4-5. This combined with their position on the food chain allows these creatures to live up to 40 years. Sadly, many Bluefins are caught by fishing nets before they have reached maturity. Most Tuna do not get to live to the maximum lifespan.


A common misconception about Tuna is that it is naturally high in Mercury. However, all fish are exposed to mercury through their environment. Tuna accumulates mercury more than other fish because of its size and density. This is a consequence of metallurgical oceanic spills polluting the water, and eventually into the food we consume.

What threats does the Tuna face?

The largest threat to Tuna is Global appetite. As it stands currently, there are about 500,000 Atlantic Bluefin tunas left. Overfishing is a main contributor to the problem, but also the lack of reliability in Bluefin Aquaculture. If these fish farmers found a more sustainable method of breeding their Tuna, this would greatly help the overall numbers in the wild.

Why are they endangered?

As previously stated, the Bluefin takes 4-5 years to develop before it can reproduce. This makes their populations vulnerable, as fisheries will often catch Tuna that have yet to reproduce once in their lifetime. The pressures on their populations are further enhanced by fisheries hunting in breeding grounds. These territories themselves need to be protected as much as the sea life within. It’s like cutting down a forest without planting new trees. When mating season is interrupted by hunting- we risk a world without Bluefins. The best example of this is the Black Sea: once populated by Tuna, but they have not been sighted there since the 1980s.

Endangered Ranking – CITES.

In 2010, the Convention on International Trade in Endangered Species (CITES) rejected acknowledging Bluefin Tuna as an endangered species. Since then, Bluefin Tuna have been ranked “Endangered” in North America and “Near Threat” in Europe by the International Union for Conservation of Nature (IUCN). The discussion itself is widely disagreed upon, but small steps to protect mating grounds in the Gulf of Mexico have been put in place. Foundations such as PEW have developed a “Transatlantic Insurance Policy” to allow Tuna to reproduce and protect their populations.

Why should you care?

The extinction of Tuna would destabilize the food chain in the Ocean dramatically. These apex predators control the populations of many species. Without that population control, there would be an ecological collapse.

On top of this, food supply chains around the world would be greatly affected, along with people’s livelihoods. The Tuna industry is worth 42 billion dollars, and over-fishing jeopardizes thousands of jobs across industries.

What can you do to help?

  1. Ethical Diet: Decrease the amount of Tuna you consume, or make sure it is coming from more responsible sources. There are a lot of resources online that list the most ethical seafood you can opt for. Tuna Ranching requires more support and research in order to truly become the best viable source.
  1. Spread the word! Talk to family members and friends about what they’re doing to help the cause. Inform family on more sustainable seafood they can substitute instead of Tuna.
  1. Get Involved: Lastly, spend time signing petitions and getting involved in online spaces. If you can, donate whatever you can to organizations that focus on Tuna conservation.

If there is a public demand for change, then change will come. With everyone’s help, we can create new standards for the ocean and all of its inhabitants! With cleaner oceans we will continue to see the Bluefin Tuna populations stabilize.

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Bethany MacDonald

Bethany MacDonald

Bethany MacDonald has contributed articles since 2020. As their Blog Lead, she specialises in informative pieces on culture, education, and language learning

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