Tuna is considered a real delicacy among sophisticated gourmets. Japanese fishermen 5000 years ago caught this strong and agile fish, the name of which is translated from ancient Greek as “throw or throw”. Now, tuna is not only a commercial fish, but also a trophy for many experienced, adventurous fishermen.
Origin of the species and description
Tuna is an ancient fish from the mackerel family of the genus Thunnus, which has survived to this day almost unchanged. Thunnus includes seven species, in 1999 the common and Pacific tuna were separated from among them as independent subspecies.
All tuna belong to the ray-finned fish, the most common class in the world’s oceans. They got this name due to the special structure of the fins. A large variety of ray-finned animals appeared in the process of long evolution, under the influence of adaptive radiation. The oldest find of fossil ray-finned fish corresponds to the end of the Silurian period – 420 million years. The remains of this predatory creature were found in Russia, Estonia, Sweden.
Species of tunas from the genus Thunnus:
- longfin tuna;
- bigeye tuna;
- yellowfin and longtail.
They all have different life spans, the maximum size and weight of the body, as well as the coloration characteristic of the species.
Interesting fact: Bluefin tuna is able to maintain its body temperature at 27 degrees even at a depth of more than a kilometer, where the water never warms up even to five degrees. They increase body temperature with the help of an additional countercurrent heat exchanger located between the gills and other tissues.
Appearance and features
All types of tuna have an elongated spindle-shaped body, sharply tapering towards the tail. The main dorsal fin is concave and elongated, the second has a crescent-shaped appearance, thin. From it towards the tail there are up to 9 small fins, and the tail has the shape of a crescent and it is he who makes it possible to achieve high speed in the water column, while the body of the tuna itself remains almost motionless during movement. These are incredibly powerful creatures, capable of moving at colossal speeds of up to 90 km per hour.
The head of the tuna is large in the form of a cone, the eyes are small, with the exception of one type of tuna – big-eyed. The mouth of the fish is wide, always ajar, the jaw has one row of small teeth. The scales on the front of the body and along the sides are larger and much thicker than on other parts of the body, due to this a kind of protective shell is formed.
The color of the tuna depends on its species, but most often they all have a light belly and a dark back with a gray or blue tint. Some species have characteristic stripes on the sides, there may be a different color or length of the fins. Some individuals are able to gain weight up to half a ton with a body length of 3 to 4.5 meters – these are real giants, they are also often called “kings of all fish”. Most often, blue or ordinary bluefin tuna can boast of such dimensions. Mackerel tuna have an average weight of no more than two kilograms with a length of up to half a meter.
Many ichthyologists agreed that these fish are almost the most perfect of all the inhabitants of the seas:
- they have an incredibly powerful tail fin;
- thanks to their wide gills, tuna are able to get up to 50 percent oxygen in the water, which is a third more than other fish;
- a special system of thermoregulation, when heat is transferred primarily to the brain, muscles and abdominal region;
- high hemoglobin level and fast gas exchange rate;
- perfect system of vessels and heart, physiology.
Where does tuna live?
Tuna settled almost throughout the world’s oceans, with the exception of polar waters. Blue or common tuna was previously found in the Atlantic Ocean from the Canary Islands to the North Sea, sometimes it swam to Norway, the Black Sea, the waters of Australia, Africa, felt like a master in the Mediterranean Sea. Today, its habitat has significantly narrowed. His relatives choose the tropical and subtropical waters of the Atlantic, Pacific and Indian Ocean. Tuna is able to live in cold waters, but goes there only occasionally, preferring warm ones.
All types of tuna, except for the Australian, very rarely come close to the coast and only during seasonal migration, more often they stay at a considerable distance from the coast. The Australian, on the contrary, is always in close proximity to the land, never goes into open water.
Tunas constantly migrate following the schools of fish they feed on. In the spring they approach the shores of the Caucasus, Crimea, enter the Sea of Japan, where they remain until October, and then return to the Mediterranean or Marmara. In winter, tuna stays mainly at depth and rises again with the advent of spring. During feeding migrations, it can come very close to the shores following the schools of fish that make up their diet.
What do tuna eat?
All tuna are predators, they feed on almost everything that comes across in the waters of the ocean or on its bottom, especially for large species. Tuna always hunts in a group, is able to follow a school of fish for a long time, covering vast distances, sometimes even entering cold waters. Bluefin tuna prefer to feed at medium depth on larger prey, which may even include small sharks, while smaller species remain close to the surface, content with everything that gets in their way.
The main diet of this predator :
- many species of schooling fish, including herring, hake, pollock;
- various sponges and crustaceans.
Tuna accumulates mercury in its meat more intensively than all other marine inhabitants, but the main reason for this phenomenon is not its diet, but human activity, as a result of which this dangerous element enters the water. Some of the mercury ends up in the ocean during volcanic eruptions, during the weathering of rocks.
Interesting fact: One of the sea travelers captured the moment when a particularly large tuna snatched from the surface of the water and swallowed a sea gull, but after a while spat it out, realizing her mistake.
Character and lifestyle features
Tuna is a schooling fish that needs constant movement, because it is during movement that it receives a powerful supply of oxygen through its gills. These are very dexterous and fast swimmers, they are able to develop tremendous speeds under water, maneuver, move over long distances. Despite constant migration, tuna always return to the same waters again and again.
Tuna rarely take food from the bottom or the surface of the water, preferring to look for prey in its thickness. During the daytime, they hunt at depth, and when night falls, they rise. These fish are able to move not only horizontally, but also vertically. The temperature of the water determines the nature of the movement. Tuna always tends to layers of water warmed up to 20-25 degrees – this is the most comfortable indicator for it.
During a schooling hunt, the tuna bypasses the school of fish in a semicircle and then swiftly attacks. In a short period of time, a large flock of fish is destroyed, and it is for this reason that in the last century, fishermen considered tuna to be their competitor and purposefully destroyed it so as not to be completely without a catch.
Interesting fact: Before In the mid-20th century, meat was more commonly used as a raw material for animal feed.
Social Structure and Reproduction
Tunas reach puberty only by the age of three, but they start spawning no earlier than 10-12 years, in warm waters a little earlier. Their life expectancy averages 35 years, and can reach half a century. For spawning, fish migrate to the warm waters of the Gulf of Mexico and the Mediterranean Sea, while each zone has its own spawning period, when the water temperature reaches 23-27 degrees.
All tuna are fertile – at one time the female produces up to 10 million eggs about 1 millimeter in size and all of them are immediately fertilized by the male. After a few days, fry appear from them, which gather in large quantities near the surface of the water. Some of them will be eaten by small fish, while the rest will quickly increase in size, feeding on plankton and small crustaceans. Juveniles pass to the usual diet as they grow, gradually joining adults during their pack hunting.
Tuna is always in a pack of its relatives, single individuals are rare, unless it is a scout in search of suitable prey. All members of the flock are equal, there is no hierarchy, but there is always contact between them, their actions during joint hunting are clear and coordinated.
Natural enemies of tuna
Tuna has few natural enemies thanks to its incredible agility and the ability to quickly accelerate to great speed. There have been cases of attacks by some species of large sharks, swordfish, as a result of which tuna died, but this happens more often with subspecies of small sizes.
The main damage to the population is caused by humans, since tuna is a commercial fish, the bright red meat of which is highly valued due to the high content of protein and iron, excellent taste, and resistance to infection by parasites. Since the eighties of the 20th century, a complete re-equipment of the fishing fleet has taken place, and the commercial catch of this fish has reached incredible proportions.
An interesting fact: Tuna meat is especially valued by the Japanese; price records are regularly set at food auctions in Japan — the cost of one kilogram of fresh tuna can reach $1,000.
The attitude towards tuna as a commercial fish has changed dramatically. If for several thousand years this powerful fish was held in high esteem by fishermen, its image was even engraved on Greek and Celtic coins, then in the 20th century tuna meat was no longer appreciated & # 8212; it was caught for sporting interest to obtain a spectacular trophy, used as a raw material in the production of feed mixtures.
Population and species status
Despite the almost complete absence of natural enemies, high fertility, the tuna population is steadily declining due to the huge scale of the fishery. The common or bluefin tuna has already been declared endangered. The Australian species is on the verge of extinction. Only a number of medium-sized subspecies do not cause concern among scientists and their status is stable.
Since tuna takes a long time to reach sexual maturity, there is a ban on catching young fish. In the event of an accidental hit on a fishing vessel, they are not allowed to go under the knife, but are released or transported to special farms for rearing. Since the eighties of the last century, tuna have been purposefully grown in artificial conditions using special pens. Japan has been particularly successful in this. A large number of fish farms are located in Greece, Croatia, Cyprus, Italy.
In Turkey, from mid-May to June, special ships track down flocks of tuna and, surrounding them with nets, move them to a fish farm in Karaburun Bay. All activities for catching, growing and processing this fish are under strict state control. Divers monitor the state of the tuna, fatten the fish for 1-2 years and then poison it for processing or freeze it for further export.
Common tuna, which is distinguished by its impressive size, is on the verge of extinction and is listed in the Red Book in the category of endangered species. The main reason is the high popularity of this fish meat in gastronomy and uncontrolled fishing for several decades. According to statistics over the past 50 years, the population of some species of tuna has declined by 40-60 percent, and the number of ordinary tuna in natural conditions is not enough to maintain the population.
Since 2015, an agreement has been in place among 26 countries to halve the catch of the Pacific tuna species. Additionally, work is underway on the artificial cultivation of individuals. At the same time, a number of countries outside the list of countries that supported the agreement to reduce catches are significantly increasing their fishing volumes.
Interesting fact: Tuna meat was not always as highly valued as it is now, in some At that time, it was not even perceived as a fish, and consumers were frightened off by the unusual bright red color of the meat, which it acquired due to the high content of myoglobin. This substance is produced in the muscles of the tuna so that it can withstand high loads. Since this fish moves very actively, myoglobin is produced in huge quantities.
Tuna is a perfect inhabitant of the seas and oceans, having practically no natural enemies, protected by nature itself from extinction. fertility and longevity, still found itself on the verge of extinction due to the immoderate appetites of man. Will it be possible to protect rare species of tuna from complete extinction — time will tell.