Science shows a path to recover Pacific bluefin tuna

The ruby-red slice of maguro presented on a piece of nigiri sushi does nothing to convey the sheer power of Pacific bluefin tuna. These top ocean predators can grow to be twice the size of lions; at top swimming speed, they’re faster than gazelles. But it’s been a huge challenge to halt the decline of these incredible fish.

Pacific bluefin tuna at the Monterey Bay Aquarium. Photo by Monterey Bay Aquarium/Randy Wilder

The Pacific bluefin population is down to just 2.6 percent of its unfished level—yet it continues to face intense fishing pressure. The fish are prized commercially, command staggering market prices, and are difficult to manage because they cross through national and international waters on trans-Pacific migrations.

Monterey Bay Aquarium has long advocated for use of the best available science to inform management decisions that can bring the Pacific bluefin population back to a healthy level. Now researchers at the Aquarium, together with colleagues from Harvard University and the National Museum of National History, have identified new evidence of migration trends that underscore the need for comprehensive fishing restrictions and enforcement across the Pacific—especially in the Western Pacific, where all Pacific bluefin spawn, and where most of the fish are caught.

The source of spawning-age fish

The analysis, published in Science magazine, concludes that—in many years—the majority of spawning-age bluefin tuna in the Western Pacific are migrants who left the waters off Japan when they were just one to two years old, and spent the next four to six years on rich feeding grounds off the coasts of California and Mexico, before returning to the Western Pacific.

New research supports the need to limit fishing for Pacific bluefin tuna — and to enforce the limits — in order to recover the species. Photo courtesy NOAA

If too many of the young fish are caught in the Western Pacific before they can make the migration east, there won’t be enough returning fish years later to maintain or recover the already-depleted population.

And if fishing pressure is too great in the Eastern Pacific, the fish won’t survive to make the migration back to their spawning grounds near Japan.

“These fish were passing through two gauntlets, in the west and in the east, before they had a chance to spawn,” said Dr. Andre Boustany, the Nereus Principal Fisheries Investigator for the Aquarium. “Many fish have to pass through both the Western and Eastern Pacific Ocean. So by taking too many of them out in both locations, we end up with a severely depleted population.

“We need much better management of the fishery in the west, and to continue to at least maintain current management in the east,” he added.

Draws on the latest science

The research is grounded in recent work by Japanese and U.S. scientists, who used stable-isotope analysis to pinpoint the recent geographic origin of Pacific bluefin tuna based on the chemical signatures of the food they’d been eating.

Fish that eat prey from different parts of the ocean each have a different stable-isotope profile. This lets scientists distinguish recent migrants to the Western Pacific from long-term resident fish.

It’s one of the new research approaches shared by scientists when they convened in Monterey in 2016 for the Bluefin Futures Symposium. Experts from around the world shared breakthroughs in bluefin tuna science, best practices for bluefin management, and new understandings of bluefin markets—all in order to chart a course for recovery and sustainable management of bluefin tunas around the world.

The new paper in Science highlights exactly the kind of rigorous research that should inform management approaches for Pacific bluefin tuna.

“This paper reinforces the urgent need for coordinated, science-based conservation and management across the Pacific Ocean to recover this iconic species to a healthy level,” said Margaret Spring, chief conservation officer for the Monterey Bay Aquarium. “We cannot afford further delay. It is time for all Pacific nations to make a serious commitment to recover Pacific bluefin tuna during international negotiations this summer.”

Learn more about what we’re doing to recover Pacific bluefin tuna.


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