When the crew spotted dolphins, the scientists noted the number, then closed in. With a crossbow, they took skin samples about the size of a pencil eraser from the animals for genetic analysis.
"The population structure is much more complex than we originally thought," Ballance said. "Dolphins in the eastern Pacific don't just freely interbreedthere are discrete units below the level of species for these animals, comparable to races in people. If you wipe out a race, you've lost something irreplaceable for the human species. The same is true in dolphin and whale populations."
Two leading theories explain the population mystery. "It could be that changes to the ecosystemperhaps in ocean temperature or abundance of preyaltered or dismantled the niche these dolphins once filled," said Andrew Read, chair of marine conservation biology at the Duke University Marine Laboratory in Beaufort, South Carolina.
But Read and the NOAA scientists have not found evidence to support that scenario. "It's much more likely that dolphins aren't recovering because of the long-term effects of chase and capture by international tuna fishermen."
Catching Tuna by Tracking Dolphins
Mexico, Venezuela, and other Latin American countries target dolphins to catch tuna using a "back down" procedure designed to prevent dolphins from drowning. A net full of dolphins and tuna is dragged backward, pushing the far edge underwater and allowing the dolphins to escape out the top of the net. Tuna tend to dive to the bottom of the net.
"Fishermen work very hard to get dolphins out of their nets," Read pointed out. "The annual mortality of dolphins has fallen from hundreds of thousands per year to less than 2,000."
No tuna caught by this method can be sold under the "dolphin safe" label in the United States. But international fisheries, pointing to the low dolphin mortality, are lobbying the U.S. secretary of commerce to change the label to include the catch.
The chase continues. Helicopters and speedboats typically pursue dolphins for 40 to 60 minutes, scattering the schools and causing panic among the animals.
"It must be incredibly stressful, because dolphins are highly social animals," Read said. "Mothers and calves often get separated. Studies have shown that the number of lactating females in tuna nets don't match the number of calves." With their mothers caught, young dolphins then are vulnerable to starvation or predators.
"Many scientists suspect that the continued chase and capture by tuna fishermen, and the cumulative stress on dolphins, may be the reason they aren't rebounding," Pitman said. "That's part of what's standing in the way of loosening the legislation."
Meanwhile the NOAA scientists scan the horizon for evidence that can make the case beyond a reasonable doubt.
For more news on marnine life, scroll down.
SOURCES AND RELATED WEB SITES