The technique yielded tons of tuna, Ballance said, but proved lethal to the dolphins.
By the late 1970s, scientists estimate, the region's spinner and spotted populations had declined to approximately a quarter of what they had been before fishers started using this technique.
Since then the tuna industry has adopted new fishing methods that have reduced incidental dolphin mortality to nearly zero. But the dolphin populations have yet to make a comeback.
"We're pursuing a number of hypotheses that might explain why the dolphin populations are not recovering, but right now we don't know the answer," Ballance said.
Carl Safina is a marine scientist who is co-founder and president of the conservation advocacy group Blue Ocean Institute based in New York State.
He joined NOAA's 2006 research trip during its leg off the coast of Central America.
He said ongoing tuna fishing, despite today's low dolphin mortality, is still the cause of the marine mammals' low numbers.
The new fishing techniques still involve chasing the dolphins in speedboats, he said. The fishers just use nets that allow the dolphins to escape once the tuna are caught.
"Before you can set a net around dolphins, you have to chase them until basically they are too exhausted to keep running, and they run very hard when the boat starts chasing them," he said.
"And so the leading hypothesis is that [females] lose their babies during those chases," he said.
Researchers report that they have seen lactating mothers in the nets but so far no babies, indicating that the youngsters are left behind during the chase. If they can't reunite with their mothers, they most likely die (related video: "How Baby Dolphins Learn the Secrets of Survival").
But more and stronger evidence is required to confirm this hypothesis, Safina noted.
According to Ballance, other hypotheses include insufficient time for the dolphins to recover and a biological effect stemming from ongoing changes in the ocean environment.
"What we really need to do is to tease out fisheries' effects from other effects, and that is the reason why we conduct ecosystem studies," she said.
The spin-off from this "ecosystem approach" to studying dolphin abundance is an opportunity to learn more about a remote ocean region, Ballance noted.
For example, she has learned that the Parkinson's petrel, which breeds on New Zealand, flies to the eastern tropical Pacific to feed on chunks of fish left in the wake of two rare porpoise species.
"That's a pretty academic piece of information about a rare species of seabird, but it's very exciting and interesting that you get insight into what these birds are doing when they are off their breeding colonies," she said.
Safina, of the Blue Ocean Institute, noted that most seabirds in the tropics rely on tuna and dolphins to chase prey to the water's surface. Overfished tuna stocks, he said, could result in less food for seabirds.
But it could also result in more food, because there are fewer tuna eating the smaller fish, he added.
"There are relationships between and among the different organisms that are out there," Safina continued.
Studying those relationships is important, whether they are "of commercial interest or moral and aesthetic interest or scientific or intellectual interest."
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