If you’ve ever seen tuna in action, then you know just how powerful these massive fish are. Much sought after as an edible commodity, and captivating to dive with, these enormous pelagic fish deserve respect. There are around ninety fish species commonly referred to as “Tuna” and another eight which are considered to be “True Tuna.” Found in oceans and seas throughout the world, they are severely overfished in many places and are not as commonly encountered as they once were.
Tuna Basics
A group of pelagic fish belonging to the Scombridae family, and part of the genus Thunnus, Tuna are closely related to mackerel, bonitos, and skipjacks. With streamlined, torpedo-like bodies, the fish are easy to identify. Although size and coloration varies from one species to another, most have several characteristics in common. First, they have a pair of closely spaced dorsal fins. The forward dorsal fin can be laid down in a groove along the fish’s spine, while the rear dorsal fin is long, stiff, and pointed, with a distinct sickle-like shape. Between this fin and the tuna’s tail, which is curved just like a crescent moon, are a group of smaller finlets. Most tuna have a dark blue, metallic looking dorsal side; their ventral side is usually white or light silver. Their anal fins are similar to their rear dorsal fins, and their pectorals are strong and stout. Thanks to their shape and musculature, they are swift swimmers, with some species being capable of reaching speeds of over 40 miles per hour.
Tuna have a unique circulatory and respiratory system which supports their athleticism and enables them to maintain a body temperature which is slightly higher than cool marine environments. In order to breathe properly and oxygenate their bodies though, they must swim at a rate equivalent to one body length per second. Besides being speedy, they are agile, feeding on schooling fish that are ordinarily nimble enough to escape predation. Their favorite prey include herring, whiting, bluefish, hake, and sardines; in addition, they enjoy feeding on squid and crustaceans, including shrimp.
Normally traveling in schools that number from six to forty individuals of a similar size, and not particular about whether schools include more than one species, tuna are often encountered along walls at deep dive sites where cool upwellings and swift currents combine to create an ideal feeding environment. Very large tuna weighing more than 500 pounds are occasionally encountered; they tend to be solitary rather than swimming with a school, probably because few tuna of that size exist. The largest tuna recorded to date was caught off the coast of Massachusetts in November of 2011; it weighed in at 881 pounds.
Due to their large size, tuna have very few predators other than man; these include sharks and billfish, along with toothed whales such as orcas. Once abundant due to their ability to thrive, they reproduce at a high rate, with spawning females releasing as many as 100,000 eggs per kilogram of body weight, meaning that a 100 kilogram (220 pound) female would produce up to ten million eggs per spawning season. The eggs, when fertilized, hatch after just 30 hours, however very few of the fry survive to reach sexual maturity and reproduce, even though they grow rapidly. When they do survive, however, they can live for years; Albacore and Bigeye tuna are believed to live for almost ten years, and Bluefin are believed to be capable of achieving an age of at least 30 years.
Most tuna species are considered to be epipelagic to mid-water fish. The largest are capable of diving to depths of five hundred meters or so, and they make long migratory journeys throughout all the world’s oceans with the exceptions of the polar seas. Scientists and conservation groups tag tuna to track their migration patterns, which are extensive. Tuna tagged in the Bahamas, for example, have been recovered in places such as Uruguay and Norway; those tagged in the Northeastern portion of the United States have been found off the West coast of various European nations. As testament to their speed, some of these migrations have been found to have occurred quickly; one of the Bluefin which was tracked took just 119 days to travel a stunning 7,700 kilometers across the Atlantic.
Migration occurs annually; for example, northern Bluefin can be found off Newfoundland during the summer months and in the Caribbean during the northern hemisphere’s winter.
Diving With Tuna
As many tuna are enormous, and as even the smaller fish are fascinating to observe, they are always amazing to encounter. There are several dive sites that feature tuna as a main attraction; while you’re never guaranteed tuna encounters at most of them, dive operators that frequent these areas do a good job of letting travelers know when to plan vacations in order to increase the odds of swimming with these giant fish. Some of the world’s best tuna dives include:
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Donsol’sTuna Alley – Located in the Philippines, this site is one most divers can enjoy as maximum depth is just 25 meters. Tuna come here to feed on sardines, and sharks can be seen hunting the tuna and other large fish that visit the reef.
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Tuna Alley in Bimini – Situated off North Cat Cay, this site features walls and is almost constantly washed by current from the Gulf Stream, which brings the tuna in to hunt. This is a slightly more challenging site than the one in Donsol, however maximum depth is an easy 30 meters.
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Back Faru – located just off Male, this site features shallow coral heads and a wall that drops into a channel where tuna and other large pelagics can be seen passing on a regular basis. Maximum depth is 30 meters, and the site is often great for a drift.
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Blue Corner – Just one of Palau’s many fantastic sites, this one is exciting not only for its schooling tuna and soaring walls, but for its gray reef sharks and other pelagics.
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Gordo Banks – Off Cabo San Lucas, this seamount rises up from the depths, with its top situated at about 32 meters beneath the surface. Not only will you encounter huge schools of tuna here, you’ll also see mantas, whale sharks, and even schooling hammerheads. This is a very challenging site suitable for experts only.
Tuna are known to associate closely with dolphins, and while fishing regulations are enforced in some areas, both the tuna and dolphins suffer the results of overfishing and irresponsible fisheries practices in other areas. If you eat tuna, be sure to check the source before deciding to purchase it, and be aware that these fish tend to carry heavy loads of mercury and other toxins in their bodies, thanks to pollution. Be sure to spread the word, too – with better education and careful conservation, we can help bring these magnificent fish back from the edge.