Thunnus

The Thunnus group is a genus of bony marine fish with less than ten species included in it. The common name is tuna or tuna in the United States, Honduras, Panama, and Puerto Rico, although other fish belonging to different genera are informally known under this name.. In their first days of life, individuals of this genus are called cordilas.

They swim with cruising speeds of 3 to 7 km/h, but can reach 70 km/h and, exceptionally, are capable of exceeding 110 km/h in short distances. As they are pelagic oceanic animals, they travel long distances during their migrations (covering 14 to 50 km daily), which last up to 60 days. Certain species of Thunnus can dive to a depth of 400 m.

The meat of Thunnus is pink or red, and contains a higher amount of hemoglobin (up to 380 mg in 100 g of muscle) and myoglobin (up to more than 530 mg in 100 g of muscle). than that of other fish species.

Some of the larger species, such as the bluefin tuna, can raise their body temperatures above water temperatures with their muscular activity; this does not mean they are warm-blooded, but it allows them to live in deep waters. colder and survive in a wider variety of environments than other tuna species.

Features

A common characteristic of tunas is the presence of two dorsal fins, generally well separated, the first supported by spines and the second by soft stripes. Its body is hydrodynamic, entirely covered in scales, larger at the front than the rest. It has a dark blue back and a silver belly, without spots, which allows it to blend in with the aquatic environment. In young specimens there are vertical lines and light spots on the lower part of the body. The fins are bluish-grey. The size varies greatly, depending on the species, ranging from blackfin tuna (maximum size: 120cm, weight: 20.6kg) to bluefin tuna (maximum size: 4.6m, weight: 684kg). The largest individual recorded under the rules of the International Sport Fishing Association was a bluefin tuna caught in 1979 in Nova Scotia that weighed 679 kg and was 3.84 m long.

The tuna is a migratory and pelagic fish, which swims near the surface forming small schools. Look for water with temperatures above 10°C (17 to 33°C). It reaches sexual maturity at 4–5 years, when it measures 1–1.2 m (weighing 16–27 kg). Its average lifespan is estimated to be 15 years.

It has been documented that the speeds they can reach are the following:

Geographical distribution, management and marketing

Thunnus is a very popular fish in food in much of the world. The tuna market has been largely boosted by the development of the economies of East Asian countries, especially Japan, and by the global impact of certain dishes such as sushi.

The Department of Fisheries and Aquaculture of the FAO has a database with an atlas, which allows knowing the accumulated catches, according to the species and the years. It is estimated that the world catches of tuna, carried out in 2007, were of the order of 4 million tons. Of these catches, 69.0% were made in the Pacific Ocean, 21.7% in the Indian Ocean, and 9.5% in the Atlantic Ocean and the Mediterranean Sea. The tuna species that are exploited for commercial purposes are listed in the following table.

Proportions of tuna catches, carried out during 2007 by different countries.

The following graph shows the proportions of the catches made in 2007 by different countries. It is appreciated that Asian countries are the ones that make the most catches worldwide.

Other international organizations that provide information on the management of fishery resources, including those related to tuna, are:

• International Commission for the Conservation of Atlantic Tuna
• Indian Ocean Tuna Commission
• Secretariat of the Pacific Community of the Altitude Fisheries Program
• Inter-American Commission on Tropical Tuna

Globally, tunas are included in the UN Agreement for the management and conservation of straddling and highly migratory species.

Some varieties, such as bluefin tuna (or bluefin) and yellowfin tuna are the subject of different fishing projects throughout the world, likewise the fishing seasons and their areas are regulated by different organizations and governments. Something important to note is that when these organisms are captured, other animals are extracted along with them, whether it is the case of dolphins and sharks that feed on tuna and are dragged along with the school when they are captured.

An encouraging piece of information for the biological recovery of these fish is the success obtained in 2009 by a research project, developed by the Spanish Institute of Oceanography, through which bluefin tuna have been reproduced in captivity.

In Spain the most consumed and common varieties are bluefin tuna (Thunnus thynnus) and albacore tuna (Thunnus alalunga).

Gastronomic facts

Preparation of grilled roast tuna

Tunas are fish that admit a great diversity of culinary preparations, either fresh or canned (canned tuna). The slices or eggs that are extracted from them are varied: slices, ventresca (belly), flank, etc. To be consumed fresh, they are prepared roasted or cooked as part of popular dishes: sorropotún in the coastal towns of the community of Cantabria, marmitako in those of the Basque Country, or patacó in the kitchen of Tarragona (Catalonia). In some Latin American countries they are used to make cebiche. When they are subjected to conservation treatment through salting, another typical product is obtained: the mojama.

Data for human health

Tuna cut and salted

Aspects of nutritional interest

The average composition of tuna, referred to 100 g of fresh product, is shown in table 2. In relation to fat, it should be noted that its content depends on the area of the body in which it is deposited: it is minimum in the areas of the back and the caudal fin (6-10%) and is maximum in the ventral area (28%). Other factors that condition the proportions of body fat, until reaching values of 15-20%, they are seasonality (greater in October-November) and latitude (increases above 40ºN). It is because of such proportions of fat that tuna is considered a blue fish. This fat is made up of polyunsaturated (2.2% of the total weight), saturated (1.8% of the total weight) and monounsaturated (1.6% of the total weight) fatty acids. In polyunsaturated fatty acids, the so-called fatty acids of the Ω-3 series predominate: eicosa-5,8,11,14,17-pentaenoic acids (eicosapentaenoic acid or EPA) and docosa-4,7,10,13,16,19-hexaenoic (docosahexaenoic acid or DHA) register proportions of the order of 0.49-0.99% and 0.98-4.01%, respectively.^{,,< /sup>}

Aspects of toxicological interest

From a food safety point of view, it should be noted that tuna can contain high levels of histamine (≥ 50 mg/100 g of product) and accumulated mercury (in the form of methylmercury).

Histamine is produced because tuna have high contents of free histidine (more than 100 mg/100 g of fish), an amino acid that is degraded by the action of certain contaminating bacteria of the family Enterobacteriaceae (such as Enterobacter aerogenes, Klebsiella variicola, Klebsiella pneumoniae, Pantoea agglomerans, Proteus mirabilis or Serratia marcescens). Histamine formation, dependent on the amount of free histidine contained in the tuna and the initial bacterial contamination, is accelerated when the fish is exposed at room temperatures (20-25 °C). To prevent the formation of histamine to toxic levels, rapid refrigeration of tuna is recommended (at temperatures of 0-8 °C) and even packaging in a modified atmosphere (40 % CO₂ and 60% O₂). urticaria, inflammation, etc.), gastrointestinal (vomiting and diarrhea), hemodynamic (hypotension) and neurological (palpitations and headache).^,

Inorganic mercury, both anthropogenic (resulting from industrial activity) and naturally occurring, is transformed into organic mercury by aquatic microorganisms, which is more toxic (see mercury poisoning). This organic mercury accumulates through the food chain to predators such as tuna (see Presence of mercury in fish). In different tuna derivatives, marketed in Spain, mercury concentrations have been detected that vary between 0.17 and 0.40 μg/100 g of fresh product. It should be noted that these mercury contents are inversely proportional to the amount of that the animal possesses. For this reason, the mercury contents are higher in blue fin tuna than in yellow fin tuna. In addition, and as the fat content increases in the months of October-November, the content of mercury is reduced in this period.