Tuna

Why tag and track bluefin tuna?

Click here to view recently collected data that helps us understand where tuna travel in the Mediterranean Sea.

The population of reproductive adults of the eastern Atlantic population of bluefin tuna is currently less than 40 percent of the level it was in the early 1970s.

In this crisis situation, fisheries managers require solid tools – such as accurate and up-to-date information on the biology and ecology of bluefin tuna – in order to develop a successful and effective recovery plan.

The results of the WWF tagging project in the Mediterranean are expected to greatly assist in this feat, by providing valuable scientific information on the migratory routes, daily behaviour, reproduction, and even swimming depth of bluefin tuna individuals.

Tracking Atlantic bluefin tuna in the Mediterranean Sea

Join WWF's Dr. Pablo Cermeno as he sends his notes from the Mediterranean Sea where he is tagging bluefin tuna.

Just how do you study Atlantic bluefin tuna? They travel huge distances and spend a lot of their adult life in the remote and wild North Atlantic Ocean - not the easiest place to visit.

One way to track the migrations of bluefin tuna is to attach tags and then hope that these will be retrieved by fishermen at some point.

Early tagging studies revealed just how far Atlantic bluefin tuna travel. For example, tuna tagged in the Bahamas were caught off the coast of Brazil, as well as off the coast of Norway.

Enter satellites and electronic tags

New technology has allowed more sophisticated studies to be carried out.

Different scientific teams have braved the seas over many years to tag hundreds of bluefin tuna with sophisticated electronic devices. These devices recorded around-the-clock data on the tuna's location, diving depth, body temperature, and surrounding water temperature.

Some devices were “pop-up” satellite tags. At a pre-programmed date, they detached from the tuna and floated to the ocean surface, where they transmitted the data via satellite back to the lab.  Others were permanent “archival” tags, which recorded data over many years. Like early tags, these devices were retrieved by fishers who happened to catch a tagged tuna, and then returned to the scientists.

Rewriting the biology books

Tagging studies have given new insights into Atlantic bluefin tuna biology. A key finding was that up to 30 percent of the population may cross the Atlantic Ocean. Previously it was thought that only 2 to 4 percent of the population made the trip.

Another key finding was that the eastern and western populations are actually quite mixed. For example, juveniles from both populations appear to feed together off the coast of North America, while adults feed together in the Central Atlantic.

Despite their trans-Altantic travels bluefin tunas always swim back home to spawn. So the spawning grounds of the Gulf of Mexico and the Mediterranean Sea are exclusively composed of tuna of purely western and eastern origin respectively.

Management consequences

These new findings have major implications for the way Atlantic bluefin tuna are managed.

Until now the eastern and western populations have been considered almost completely separate and have been managed separately, with different quotas set for each. The problem needs to be addressed of how the fishery should be managed if the two stocks mix without a clear geographical separation.