Like many open ocean bony fishes, Atlantic bluefin tuna start out as extremely tiny larvae, no more than a few millimeters long and weighing only a few hundredths of a gram. Within three to five years, individuals reach lengths of three feet (1 m) and are sexually mature. Because Atlantic bluefin tunas undergo such an amazing transformation in size (from being nearly microscopic to being one of the largest open ocean predators), they eat a wide variety of prey throughout their lifetimes. At a young age, they eat tiny zooplankton, and their prey increases in size as they do. As adults, they eat fairly large bony fishes and invertebrates. Similarly, Atlantic bluefin tuna are eaten by a wide variety of predators. When they are newly hatched, they are eaten by other fishes that specialize on eating plankton. At that life stage, their numbers are reduced dramatically. Those that survive face a steady increase in the size of their predators. Adult Atlantic Bluefin are not eaten by anything other than the very largest billfishes, toothed whales, and some open ocean shark species.

Bluefin Tuna are known to be highly migratory, with individuals making long migrations every year. These migrations correspond with their spawning behavior and with their food needs. This species reproduces via broadcast spawning, where several females and several males release millions of eggs and sperm into the water column at the same time. This method increases the likelihood that the eggs will be fertilized and decreases the chances that they will be eaten by egg predators. There are at least two known populations of Atlantic bluefin tuna, one that reproduces in the Gulf of Mexico and one that reproduces in the Mediterranean Sea. Some researchers believe that the Mediterranean population actually represents two populations (one in the west and one in the east plus the Black Sea). This further division has some implications for fisheries management, because the fish in the eastern Mediterranean are generally considered more of a conservation concern than those in the western Mediterranean.

Though almost all fishes are cold blooded, Atlantic Bluefin have a specialized blood vessel structure – called a countercurrent exchanger – that allows them to maintain a body temperature that is higher than the surrounding water. This adaptation provides them with a major advantage when hunting in cold water, by allowing them to move more quickly and intelligently. The Atlantic bluefin tuna is one of the fastest swimmers in the ocean. Like some shark species, Atlantic Bluefin must constantly swim. In order to obtain oxygen from the water, fishes pass water over their gills. The tunas lack the ability to do so while stopped, so they must continuously swim forward with their mouths open to keep their blood oxygenated.

The Atlantic bluefin tuna is a highly prized food fish and is fished heavily throughout its range. Its value in the high-grade sushi market in Japan and elsewhere is high enough to cause fishers to target this species with almost unmatched effort. Scientists believe the species to be endangered (highly vulnerable to extinction), but fisheries managers continue to allow commercial fishing to target it. While the Atlantic Bluefin was formerly common in the Black Sea and off the coast of Brazil, it has not been observed in significant numbers in either of those places for several decades. As a result of these and other losses, the Atlantic bluefin tuna has experienced the largest range contraction of any open ocean species. Early indications imply that recent strong management regulations may be allowing populations to rebound, but continued conservation and management efforts are required to ensure that more subpopulations are not lost.

Note on closely related species: For many years, bluefin tunas around the northern hemisphere were assumed to all be one species. Scientists have recognized Southern Bluefin Tuna (Thunnus maccoyii) as a distinct species for quite some time, but the northern species was divided only in the last decade. Now, the Atlantic bluefin tuna and the Pacific Bluefin Tuna (Thunnus orientalis) are generally considered to be distinct species.

Fun Facts About Atlantic Bluefin Tuna

1. Atlantic bluefin tuna are the largest tuna species. The western Atlantic stock reaches a maximum length of 13 (4 m) feet and weight of 2,000 pounds (907 kg),1 while the eastern Atlantic stock reaches a maximum length of 15 feet (4.6 m).2

2. Atlantic bluefin tuna can live for 35 years, possibly longer.2

3. Female Atlantic bluefin tuna produce up to 10 million eggs a year. Eggs hatch 2 days after being fertilized.

4. Atlantic bluefin tuna are top predators, feeding on fish such as herring and mackerel as adults.

5. Atlantic bluefin tuna are characteristically torpedo-shaped with short pectoral fins and a dark red dorsal fin.1

Engage Youth with Sailors for the Sea

Oceana joined forces with Sailors for the Sea, an ocean conservation organization dedicated to educating and engaging the world’s boating community. Sailors for the Sea developed the KELP (Kids Environmental Lesson Plans) program to create the next generation of ocean stewards. Click here or below to download hands-on marine science activities for kids.

References:

1 NOAA Fisheries

2 IUCN Red List