for National Geographic News
Spotted and spinner dolphin populations in the eastern tropical Pacific Ocean have yet to recover, despite nearly 20 years of friendlier tuna fishing practices, researchers say.
But no one is quite sure why the dolphins are having a hard time bouncing back. So a team of researchers regularly probes an 8.1-million-square-mile (21-million-square-kilometer) swath of the remote ocean for clues.
As part of the Stenella Abundance Research Project, the team counts dolphins as well as seabirds, fish, turtles, and squid. It also measures water temperature and salinity and maps the ocean currents.
"You can't count dolphins in a vacuum," said Lisa Ballance, a marine ecologist with the National Oceanic and Atmospheric Administration (NOAA) and chief scientist for the research voyages.
"If you see a change in abundance over time, you need to know why that change is occurring," she added.
The surveys, which started in the 1970s, currently go out every three years to survey the ecosystem. The most recent outing returned last December.
Tuna fisheries in the eastern tropical Pacific killed millions of dolphins in the 1950s, '60s, and '70s, Ballance said.
"It's a relatively unique and prevalent phenomenon that in the eastern tropical Pacific, large schools of big-bodied yellowfin tuna are found in the same aggregation with spotted and spinner dolphins," she said.
The tuna and dolphins both chase smaller fish to the ocean's surface, which in turn attracts seabirds.
"This whole assemblage gets together and essentially serves as a very highly visual signal that indicates the location of a school of yellowfin tuna," she said (see a photo of yellowfin tuna).
Tuna fishers learned to use this visual signal as a cue to launch speedboats to round up the dolphins—and the tuna around them—in purse seine nets.
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