Tracking the giants

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!

Tagging fish

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.

Working with fishers, 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% of the population may cross the Atlantic Ocean. Previously it was thought that only 2-4% of the population made the trip, with the rest divided into separate western and eastern populations.

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.

Management consequences

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

Since the eastern and western populations have been considered almost completely separate, they have been managed separately, with different quotas set for each. However, this doesn’t make sense if the two stocks mix extensively.

The studies have also identified a possible third spawning area for the tuna in the Sargasso Sea. If this is the case, then spawning tuna in this sea need to be protected from fishing.