Atlantic bluefin tuna: the forgotten superpredator

We would expect that a huge, charismatic, and ecologically important predator would be celebrated in its own right. Especially if it is also endangered, has evolved some extreme physiological and behavioural adaptations to enable it to excel as one of the oceans’ top predators; this fish even hunts in coordinated packs much like a pride of lions. However, there is one such species that does not derive any such celebratory benefits, and this is the Atlantic bluefin tuna.

David Attenborough called the Atlantic bluefin tuna (Thunnus thynnus) the ultimate fish, but most people still perceive it is as food or a recreational toy rather than wildlife. This is a major injustice to such an amazing animal. It would be unthinkable to consider lions or wolves as not being wildlife, even if we ate them as food, but this is the exact plight facing bluefin tuna. It is this lack of awareness that has led to this animal, known mostly for sushi, becoming endangered.

Few people, even some that care about the fate of the bluefin, do not see the animal as anything other than a fish. As a result, people find it hard to relate to this animal – and that may well be the driving factor behind this species’ near extinction.

Bluefin tuna may have only a few decades, possibly just a few years, for us to stop its human-caused extinction. My hope is that this article doesn’t end up being an eulogy to a magnificent species, but realistically, it may well become one.

There is a good reason tuna are often referred to as superfish. Everything about them has evolved for peak athletic performance. Even among tunas, the bluefin stands out as extraordinary, having taken these adaptations to their ultimate perfection over thousands of years of evolution. All its organ systems have evolved to enable this fish to be a cut above the rest and that is how you create a swift-moving, high-seas predator.

The body shape matches a teardrop, with the widest part of the body occurring at around 2/5th of the distance from head to tail. This is the most hydrodynamic shape that is physically possible to achieve.

A tuna’s main engine is among the most specialized in the animal kingdom. The tail is tall, very stiff, narrow-bladed and swept-back, a form referred to as “lunate”. The horizontal keels on the caudal peduncle knife through water as the tail swings from side to side, reducing water resistance and thus the effort needed to beat the tail. The yellow finlets, extending like serrations on a saw blade, cut down on turbulence by altering water flow across their surface.

When swimming, tuna make only the minimum amount of side-to-side body movement, with nearly all of the forward thrust produced by their rapidly beating tail. It’s the most energy-efficient way of moving through water. So specialised is this method of swimming it is actually called thunniform swimming, after tuna.

To further streamline the body and make swimming more economical still, most of the fins on a tuna are fully retractable, like the wings of the F-111 military jet. The wing-like pectoral fins slot neatly into shallow, cowling-shaped depressions on the flanks, and the first dorsal fin and pelvic fins fold into grooves on the back and underside respectively. These fins are thus kept out of the way during high-speed attacks, but they unfold during tight turns or complex manoeuvres for better mobility. The pectorals can also function as a pair of swept-back hydrofoils, allowing the tail to focus on propulsion when the animal is just cruising at a moderate speed from one place to another. 

The remaining fins (the second dorsal and anal fins) are all slender and swept-back, producing as little water resistance as possible. They have also been modified in another, very crucial way: they are shape-shifting fins. At their bases lie special sinuses filled with fluid, an extension of the lymphatic system. Muscles around this sinus apply hydraulic pressure, which acts on the bones and on fin rays, finely changing the fin’s shape and angle as necessary for each particular situation.

The internal physiology of a bluefin tuna matters just as much as its hardware. As seen in the above diagram, red muscle makes up for a significant part of a tuna’s musculature, a much greater percentage of body mass than in any other fish. White muscle is used for bursts of intense, short-term activity, and red muscle is for sustained activity for long periods of time. With red muscle fibers running down the entire length of their body, tuna are built for endurance.

Bluefin need to swim constantly to breathe and they have massive gills to extract vital oxygen from the water, in fact they have the largest area for gas exchange found in any gill-breathing fish. Each gill plate is also fixed in place to reduce water resistance, another Darwinian evolution that maximises efficiency. Oxygen delivery from the gills to the body has also been improved. Tuna blood contains double or even triple the concentration of haemoglobin compared to other fish, and their hearts are several times as large – to pump blood at a high rate. All these adaptations mean the fish can swim further and faster, and dive deeper.

Yet all of the above is nothing compared to the most extreme physiological adaptation the bluefin has up its sleeve. While not truly endothermic, all true tuna are able to raise and maintain their body temperature, and it is the bluefin that takes this feature to its zenith. This predator is capable of keeping its body temperature between 77–91°F (25-33°C) in the frigid waters of the North Atlantic, which is its main hunting grounds. Even the Arctic Circle isn’t beyond the reach of the bluefin. In fact, the bluefin is hot-blooded to the point that if it is forced to exert itself for hours at a time, such as being hooked on the end of a fishing line, it can cook itself to death (a dreaded phenomenon called “burn” by fishermen, which makes the fish worthless).

The secret to this ‘warm-blooded’ trick is a specialized countercurrent heat-exchange system called the rete mirabile, that prevents metabolic heat from escaping into the surrounding water. Heat lost from blood leaving the musculature is re-absorbed by chilled, oxygenated blood coming from the gills. This enables the bluefin to keep its brain, eyes, spinal cord, muscles and heart running at high temperatures constantly, giving quick reaction times and sustained speed in cold waters. It also allows for faster digestion, enabling the bluefin to more efficiently process and store the calories gained by eating, and to be able to eat again after a short period of time.

Bluefin are fast swimmers, with a maximum speed of 25 mph. This burst of speed is reserved only for short periods of time when it’s really needed, however, and most of the time is spent at a much slower pace. Normal cruising speeds are only around 5 mph.

While tuna (and other fish also known for high-performance adaptations, such as billfish) have been stated as achieving high speeds of over 50 mph, these are based entirely on non-scientific sources or outdated studies using poor methods. Swimming at over 30 mph would be physically impossible for any animal: cavitation affects body tissues at such speeds, cutting down speed and causing injuries.

In addition to not tiring while travelling long distances at their cruising speed, tuna also recover quickly from a short, fast hunt after chasing their prey. Like many other animals, the bluefin resorts to anaerobic metabolism for quick bursts of activity (like the final rush at the climax of an attack), leaving an oxygen deficit. But due to its adaptations, it can recoup this deficit easily and without ceasing to move, enabling it to repeat the process within a short period of time (this is very different to the fatal ‘burn’ from long periods of stressful activity as described above). Because their entire lifestyle relies on being in perpetual motion, taking a rest as most fish do, would be fatal.

In short, the bluefin is a superb animal, superbly adapted for out-swimming, out-lasting, out-diving, out-travelling, out-eating, and out-growing other fish. All are skills critical in the pelagic environment.

There are two breeding populations of Atlantic bluefin: one breeds in the Gulf of Mexico, and the other in the Mediterranean. The species is widespread throughout both these areas, and throughout the North Atlantic. There used to be a third population in the Black Sea, and a fourth off the Brazilian coast, but fishing has wiped out these two populations.

Until recently it was thought that the North Pacific population of bluefin was the same species, but they have now been reclassified as a separate species, the Pacific bluefin tuna (Thunnus orientalis). It’s just as much, if not even more, endangered than the Atlantic bluefin, and its migrations are even more spectacular.

The hunting ground of most tunas is the epipelagic zone (the surface to 200m). While bluefin tuna can dive down to over half a mile deep to track down squid or lanternfish, they don’t stay there for long, unlike, for example, bigeye tuna (which specialize in rapid, vertical movements and deep diving).

Wherever the tuna hunts however, is a harsh environment where there are often few sources of prey and even less cover. The pelagic waters of the world’s oceans are called “marine deserts”, and this is especially true in tropical waters. The lack of sustenance in clear tropical seas is part of why tuna became capable of controlling their body temperature, in order to penetrate the colder, richer waters in temperate latitudes.

But every desert has its oases, and the open ocean is no exception. The edges of the epipelagic zone, where the high seas meet more fertile coastal waters, are far richer in life. Oceanic islands, as well as submerged sea mounts, influence currents and are a magnet for large pelagic predators. Upwellings also bring nutrients to the surface and allow for plankton blooms, which attract the plankton-feeding fish the bluefin preys on. Perhaps that is why that, unusually among tunas, the bluefin will often venture inshore to hunt bringing it into close contact with humans.

The cold, nutrient-rich waters of the North Atlantic are the bluefin tuna’s main summer haunts and after an absence of nearly 40 years they have returned to UK waters. Intense pressure is mounting in the UK to catch these amazing fish for sport and profit. We are urging George Eustice, Secretary of State for Environment, to make the UK a safe place for bluefin tuna and a world leader in protecting these magnificent animals. Sign our petition here and support our campaign here.

Byungkyu Jeong studies ecology and evolutionary biology, is a nature enthusiast, animal geek, and ardent follower of new paleontological discoveries. Follow Byungkyu on Imgur.

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