Feature: Bluefin tuna - Oceana Europe

Feature: Bluefin tuna


In the last decade, fisheries bluefin tuna have become very profitable due to the high market demand for sushiand sashimi . According to the latest stock assessment of eastern Atlantic bluefin tuna and the Mediterranean, increased fishing pressure has caused a serious decline in the population and a drastic reduction in the spawning stock biomass. This has caused great concern for the survival of this species. In recent years, the rapid increase in the fleet has been fueled by the expansion of the cages of bluefin tuna in the Mediterranean and the increase in fleet capacity, particularly in the case of industrial fence, responsible for 60 -80% of the catch of bluefin tuna in this area.

Where do bluefin tuna live?

Bluefin tuna live mainly in the pelagic ecosystem of the North Atlantic and its adjacent seas, particularly the Mediterranean Sea.Geographically, its population throughout the North Atlantic Ocean is distributed over Ecuador to Norway and from the western Mediterranean to Mexico. Unlike other species of tuna, this is the only living permanently in temperate Atlantic waters.

There are two populations or stocks independent of bluefin tuna: a smaller population of the western Atlantic spawning in the Gulf of Mexico and a higher stock of the eastern Atlantic, distributed in the East Atlantic, the Mediterranean and in the past the Black Sea, the Mediterranean Sea as a spawning ground.

Moreover, it seems that bluefin tuna has a behavior return to the birthplace and fidelity to the play area in both the Mediterranean Sea and the Gulf of Mexico, which involves adults and young specimens of both populations feed together in particular off the coast of North America and in the Central Atlantic. 

Why do bluefin tuna migrate?

Electronic tagging studies have shown that bluefin tuna is a highly migratory species that performs two types of migration in his lifetime: a trophic migration to seek food and other reproductive migration to spawn.

When the breeding season begins, adults always return to the waters where they were born. Therefore, adults born in the Gulf of Mexico migrate to this area in spring to spawn in April-May, while adults born in the Mediterranean return there when they reach maturity, in late spring to spawn in June -July in the western and central areas.

How can they travel such long distances?

Bluefin tuna is particularly well developed circulatory system, which makes for a warm-blooded fish and fast swimmer (72.5 km / hr), with an enormous capacity for migration.

Like other species of tunas and sharks, Atlantic bluefin tuna is a pelagic predator that must swim continuously to vent and generate enough heat to keep the vital organs and raise body temperature above the water temperature. The complex structure of the circulatory system allows you to minimize heat loss and maintain the core body temperature above that of seawater.Thanks to its endothermic capacity, can withstand cold (3 ° C) and warm temperatures (up 30 ° C).

How old do bluefin tuna live for?

The Atlantic bluefin tuna is a long lived, slow-growing species. According to the data, the longevity of the eastern stock of bluefin tuna has been estimated at approximately 20 years, while the longevity of the specimens of the western population has been estimated at 32 years according to the radiocarbon traces.

What size can bluefin tuna be?

Adult specimens of bluefin tuna can exceed 3 m in length, making them the largest species of tuna and one of the largest teleost, and weigh 250 kg on average, although some can reach up to 900 kg.

How do bluefin tuna reproduce?

Several studies have shown that bluefin tuna reaches maturity at the age of 4-5 years in the Eastern Atlantic and the Mediterranean while specimens of the western Atlantic population reach sexual maturity at about age 8. The difference in the age at which they reach maturity the eastern and western populations in the North Atlantic may support the hypothesis of distinct populations

Like most fish, egg production appears to depend on the age (or size); therefore a 5 year old female can produce an average of five million eggs (of ~ 1 mm) per year, while females 15-20 years can carry up to 45 million eggs. Hatching happens without parental care after an incubation period of 2 days.

Bluefin tuna larvae (3-4 mm) are typically pelagic and can be found in surface waters throughout the Mediterranean Sea, with higher concentrations in areas where gyres and fronts, particularly in late summer. The larvae grow 1 mm per day until you reach a weight of 40-80 kg and separated into banks by size.

These larvae of bluefin tuna are mostly in surface waters with a temperature of 24 to 25, in mixing zones of the water masses of the Atlantic and the Mediterranean.

When do bluefin tuna spawn?

In the Mediterranean, bluefin tuna spawn during the months of May and August with spawning peaks, depending on weather and oceanographic conditions during the months of June and July. 

What do they eat?

Young specimens and adult bluefin tuna are opportunistic predators and large animals in their feed. Your diet can include several teleost species, invertebrates such as jellyfish and salps, and demersal and sessile species such as octopus, crabs and sponges, although there have been great differences in the study areas.

In general, young fish feed on crustaceans, fish and cephalopods, while adults feed on fish such as herring (Clupea arengus ), anchovy (Engraulis encrasicolus ), sandeel (Ammodytes spp.), sardine (Sardina pilchardus ), sprat (Sprattus sprattus ), bluefish (Pomatomus saltatrix ) and mackerel (Scomber scombrus ). Among the contents of the stomachs of bluefin tuna analyzed, we found three predominant species Atlantic herring and sand eels in the Atlantic Oeste26 or anchovy in the East Atlantic and Mediterranean.

Why are they endangered?

The condition of super predator of this species and thus slow growth make it a species vulnerable to overexploitation. Currently, the Atlantic stock has led to a situation of risk, mainly due to fishing pressure exerted industrial fence on this species, also linked to fattening cages. Fishing for individuals below their biological reproduction and size of the breeding areas is common in most Mediterranean fisheries and has a clear negative impact, has reduced the stock of breeding stock and caused a decrease in the size of the large tuna, which compromises the future of large bluefin tuna.

In addition, the few existing marine reserves leave the door open for direct destruction of essential habitats for reproduction and uncontrolled and too often illegal exploitation of the species.Currently, the situation of this stock is alarming, since it has been estimated that biomass is located below 15% of what was before it exploded. Consequently, in addition to strict fishing controls the declaration of marine reserves their breeding and protection of the species itself is necessary to drastically limit the abuses being committed on bluefin tuna fisheries.