2023-09-19 10:06:48
By María López, Anna Aguiló, Patricia Reglero y Daniel Ottmann *
Every year, starting in April, large banks of Atlantic bluefin tuna (Tunnus tunnus) they cross the Strait of Gibraltar heading towards one of his main breeding areashe Western Mediterranean. This large marine predator travels thousands of kilometers from the North Atlantic in search of the perfect conditions to complete the reproductive cycle, one of the most important moments in its life.
The Western Mediterranean: good place to start life’s adventure
Adult specimens take this issue very seriously and they leave nothing to chance. The goal is for fertilization to be successful and for as many fish as possible to survive. The tuna groups of reproductive age participating in this migration are so numerous that can be seen from the air.
You may wonder why such a long trip takes place. There is no single answer to that question. The most obvious is who seek optimal conditions for reproduction. Bluefin tuna are very demanding and need, first of all, that the water reaches a temperature of 20 degrees Celsius. When this happens, they release millions of eggs and large clouds of sperm into the water. The laying takes place at a depth of about five meters between two and four in the morning. It seems that the reason they stay up late is to avoid predators.
Eggs of Atlantic red tuna, Thunnus thynnus / Ferdinand de la Gandara
However, the warmth of the water is not enough for success. It is also necessary that their young do not lack food. But against all logic, bluefin tunas They choose very nutrient-poor waters to breed. CSIC researchers are trying to decipher the reason. It is possible that the advantage is that, since the waters are poor, fewer predators come to the hatchlings. And yes it is known that the babiesto compensate, They practice cannibalism and often eat each other.. When they reach a certain size they stop doing so and from then on they always swim in groups.
In addition, there are other factors that may be important when choosing this place. The proximity to the islands may influence it, or the currents that are created as a result of the denser waters of the Mediterranean coming into contact with the less saline waters of the Atlantic.
Only two out of every 30 million fertilized eggs reach adulthood.
The bluefin tuna is an impressive marine predator and is an excellent and fast swimmer that can reach 400 kilos in weight. When you reach adulthood there are few predators you need to fear…. But first, in its first phases of life, the colossus is also prey.
Curiously, it will be a small invertebrate that will considerably decimate its population. It is about the ephyras: baby jellyfish between 4 and 12 millimeters that are They eat tuna eggs and larvae up to 5 mm. Ephyras, which coincide with tuna larvae in the first 20 meters of the water column, hunt their prey by filtering the water. Its chemical sensors tell it where the food is and they go after it.
The reproductive strategy of tunas seems to have adapted to avoid one of their large predators: it basically consists of dodge the ephyras of jellyfish looking for areas where they are not usually found. While the tunas stay in the front waters coming from the Atlantic, they are located in resident waters of the Mediterranean. It also happens that the jellyfish reproduces before the tuna. Their breeding season begins in April and reaches its peak in May, while tuna begins to spawn in mid-June.
Until It was now believed that the prized Atlantic bluefin tuna reproduced only in the Mediterranean and the Gulf of Mexico.. However, new studies have shown the existence of a third area on the North Atlantic coast of the United States, and it seems that there could even be more areas still undiscovered. Every year, the research staff of the Spanish Institute of Oceanography of the CSIC carry out an oceanographic campaign to study the state of tuna populations in the Western Mediterranean and learn more about this large and fast migrator.
* Maria Lopez He is a journalist; Anna Aguiló, disseminator; and Patricia Regero y Daniel Ottmann, are researchers at the CSIC Oceanographic Institute in the Balearic Islands. All of them, together with the illustrator Flavia Gargiuloparticipate in the dissemination project Planet Tuna.
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