Large fish in the open sea have declined by at least 90 percent over the past century due to overexploitation.

To pull fish like tuna, swordfish, and marlin back from the brink, scientists argue we need to protect their migration superhighways known as 'blue corridors'.

A recent study on the Pacific Ocean has mapped the busiest of these underwater traffic lanes using a fish's tendency to return to its birthplace.

This behavior is known as philopatry, or natal homing, and it's not just an impulse for salmon.

Other fish species also return to their birth location to reproduce, and experts want to use that information to reveal where we need to limit or ban fishing.

Tracking large fish as they swim across vast swathes of ocean is incredibly difficult, which means scientists don't know much about migratory routes in the high seas.

If some fish are assumed to return to their spawning grounds, however, then their travels should create an annual loop through certain parts of the ocean.

Comparing data on where fish are caught most and where fish spawn, researchers at the University of British Columbia have inferred the migration loops of 11 fish species in the Pacific Ocean.

The 11 species considered were skipjack, yellowfin, bigeye tuna, albacore, pacific bluefin tuna, swordfish, common dolphinfish, striped marlin, black marlin, wahoo, and Indo-Pacific sailfish.

The results are only tentative and are based on several assumptions, but they provide important clues about where fish might be swimming at certain times of the year.

When all the migration pathways are superimposed on a map, the overlap reveals several "high priority" and "very high priority" areas for conservation.

Below is the final map, showing which areas of the Pacific should receive protection first. The red and orange spots represent ocean regions traversed by all or nearly all of the fish species considered in the study.

sustainabilitybluecorrirodors(Relano and Pauly, Sustainability, 2022)

Above: Habitat use maps for large pelagic species in the Pacific, generated by superposing the habitat use maps of the different stocks.

In the busiest blue corridors, the authors recommend banning or reducing industrial fishing of large pelagic species, like skipjack tuna, yellowfin, striped marlin, and swordfish.

"Those high-traffic areas, two of which are in northeastern and central sections of the Pacific Ocean and two in the southwestern and central sections, should become parts of blue corridors, which are routes where strict fisheries management measures or partial bans of industrial fishing ought to be enforced to allow for increased connectivity of habitats and thus allow populations of marine species to maintain themselves," says Daniel Pauly, the principal investigator at the UBC's research institute, the Sea Around Us.

Today, very few marine reserves exist in the open ocean. Blue corridors could help extend the protection bestowed on coasts right out into the high seas, ensuring both large fish and whale migration routes remain relatively undisturbed.

For large pelagic species that roam far and wide, blue corridors are especially important. And the bigger, the better.

"[T]he best-case scenario for conserving and rebuilding stocks," the authors write, "would be an even larger and continuous blue corridor extending from 30° N to 40° S and from 160° E to 110° W of the Pacific."

A blue belt of that size could help rebuild fish stocks and boost fisheries throughout the Pacific.

The study was published in Sustainability.