An Oasis in Motion
The basic movements of water impact fish — and people, too.
Swirling bodies of water, 10 to 100 kilometers wide and full of life, travel around an otherwise barren region spanning most of the North Pacific Ocean. Fishermen too travel around this region. And in their search for tuna, billfish, and sharks, they often find themselves in these same bodies of water.
The region is the North Pacific Subtropical Gyre, and the bodies of water are eddies. A study recently published in Nature explores the relationship between eddies and fish by comparing how many fish fishermen caught in and out of eddies. Their exploration had an urgency. The gyre is expanding and fish populations in it are declining, so eddies may be becoming more and more important for sustaining these populations, which Pacific Island communities, including the very fishermen who helped collect the data, depend on for food and work.
“There are certain things that once I start thinking about them, I can’t stop,” says Martin Arostegui, Woods Hole Oceanographic Institution postdoctoral scholar and coauthor of the study. This time that was the relationship between eddies and fish. Arostegui’s research group began by using sea temperature and level data to find out where different types of eddies were. Eddies that rotate counter-clockwise presented as cold depressions in the data, and eddies that rotate clockwise presented as warm bumps.
Arostegui’s research group then turned to the people who know the gyre best — the fishermen. Specifically, the fishermen operating boats in the Hawaiian deep-set longline fishery, who have long been recording what they catch and where they catch it. The research group looked at where the fishery had caught fish in the last 20+ years and compared how many fish they caught in counter-clockwise eddies, clockwise eddies, and outside of eddies. Arostegui says using this data, which was not collected with scientific research in mind, to study ocean currents “challenged my abilities more than anything that I’ve had to do.”
In the end, the challenge was well worth it and blazed the trail for future research using similar methods. Alice Della Penna, University of Auckland lecturer who was not involved in the study, said Arostegui’s research made her excited about using data collected by fisheries: “That’s something that I’ve been curious about, but I think I was always very much intimidated by the complexity.”
Arostegui’s research group found that the fishery caught many more fish in clockwise eddies. These eddies are “a mobile oasis, if the ocean is the desert,” says Arostegui. The relationship between clockwise eddies and fish exemplifies how basic movements of water can have effects that cascade up the entire food chain, how an oasis is no simple creation.
Other researchers have studied the relationships between eddies and specific fish species, but no one had studied this many species at once. “The other aspect of it that’s really exciting is the fact that none of this kind of work had been done before in the subtropical gyres, which constitute the single largest marine biome and the biggest ecosystem on the planet,” says Arostegui. By studying more species in a larger area, Arostegui’s research group discovered broader trends that will help fishery managers sustain the ecosystem as a whole.
Why those broader trends exist, however, is still not fully understood. “It looks like [clockwise] eddies are really important, and they really seem to host more life. But I think the big question is still why,” says Della Penna.
Despite the questions that remain, this improved understanding of the relationship between fish and eddies will help fishery managers maintain that relationship and stabilize the ecosystem, supporting both the fish and the people who depend on them.
