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Traveling tuna

Young Atlantic bluefin tuna swim the sea from coast to coast

Apparently, even fish want to break the rules sometimes.

After spending weeks and months swimming in the open sea, mature Atlantic bluefin tuna return to the area where they first hatched. They do so in order to spawn, or reproduce. These bluefin tuna are divided into two groups: western Atlantic bluefins and eastern Atlantic bluefins. When it’s time to mate, the mature westerners head toward the Gulf of Mexico, while the mature easterners return to the Mediterranean.

But a new study shows that when bluefins are young, they don’t always stick to their side of the ocean. Scientists found that juvenile bluefins like to socialize on both sides of the Atlantic. To these young fish, the ocean is just one gigantic aquarium.

While this mingling may be good for the fish, it can complicate strategies designed to manage how many of the fish can be caught. Scientists say the new findings may help in developing better management strategies, and help save the fisheries from collapse.

Atlantic bluefin tuna are one of the most sought after fish. Sushi lovers relish the fish for their savory taste, and fishermen prize the tunas for the high price they fetch at the fish market.

Over the past few decades, as the number of bluefin tuna has declined, scientists have sought ways to better manage these fish. A critical part of setting fishing quotas, or limits on how many fish can be caught, is estimating the number of fish located in a geographic area.

Though scientists have known for some time that the western and eastern bluefin populations mix, management agencies that set fishing quotas have treated the westerners and easterners as two separate populations.

To get more specific information on where the tuna spend time, Jay Rooker of Texas A&M University at Galveston set out to identify the origins of fish by examining the chemicals of their otoliths, or “ear stones.” Nearly 200 Atlantic bluefins, from both sides of the ocean, were tracked over a six-year period.

Otoliths are found inside the ear and receive sound. As the fish grows, the otoliths also grow in layers that contain carbon and oxygen. These layers grow like tree rings throughout the fish’s life. The amounts of oxygen and carbon that are laid down can differ, depending on the water that the fish spends time in.

By examining the material deposited in the otolith, Rooker and his team were able to track the tunas’ travels.

The researchers found that more than 90 percent of the older, larger bluefins looked at in the study and living in waters off Maine and Canada tend to be westerners, born in the Gulf of Mexico.

Many of the smaller, and younger, bluefins caught off the eastern United States, however, originated in the Mediterranean.

Molly Lutcavage, director of the Large Pelagics Research Center at the University of New Hampshire in Durham, who tracks bluefins off Nova Scotia, Canada, says patterns like these need to be considered when setting fishing quotas and developing plans to rebuild the tuna populations.

If not, Lutcavage says, “We could be missing the boat.”

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