Returning home to breed

In May, adults from the eastern population of Atlantic bluefin tuna return from the all around the North Atlantic Ocean to spawn in the Mediterranean Sea. No one knows exactly why they spawn here, except that they need warm water to breed.

Not long ago, these tuna formed giant schools as they moved through the Straight of Gibraltar, several kilometres wide and made up of millions of individuals. Even for the depleted population today, this annual migration is an impressive event, easily comparable to the better-known wildebeest and caribou migrations on land.

Spawning and early life
For those tuna that avoid the fishing fleets, spawning takes place from late-May to July. Spawning sites appear to be concentrated around the Balearic Islands, the Tyrrhenian Sea, and the central and eastern Mediterranean.

Spawning occurs at night when a female tuna releases up to 30 million eggs, depending on her size. The eggs float just below the water surface, and are widely distributed in surface currents. Many end up in the bellies of various fish and invertebrate species.

Around 3 days after fertilization, the surviving eggs hatch. The tiny tuna larvae, just 3mm long, have huge heads and jaws. They are predators from the start, feeding on smaller organisms such as brine shrimp and other fish larvae. Many are in turn eaten by larger animals.

The larvae grow incredibly fast, at a rate of 1-2mm per day. A month after hatching, they have increased in length 12-fold, to 3.6cm. By three months, young bluefin tuna can weigh over half a kilogram.

Leaving home
Juvenile bluefin tuna stay in the Mediterranean for around a year, by which time they have reached a weight of around 4kg. Then, between September and November, many leave their Mediterranean nursery through the Strait of Gibraltar, and move into the Atlantic Ocean. Not all leave the Mediterranean however - some stay in these waters until they are adults.

The juveniles form large schools to help reduce the chance of predation by other fish, marine mammals, and seabirds. The schools can contain thousands of individuals, and are based on age and size. Similar-sized fish from other tuna species are also found in bluefin tuna schools.

The juveniles that migrated out into the Atlantic Ocean stay there until they reach maturity at 4-5 years and a size of around 120cm and 30kg. As they get older and larger, the number of fish in each school decreases to just a few tens.

Of all the larvae that hatched in the Mediterranean, only 1 in 40 million makes it to adulthood.

Back to the ocean
It’s not just the juveniles who leave the Mediterranean - so do adults once spawning season is over. In contrast to juveniles, adults appear to migrate north to feed in cooler, rich subpolar waters. The largest, which by now are mostly solitary, travel the furthest.