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No Pacific Dolphin Recovery, Despite Protection

Jennifer Hile
National Geographic Channel
June 25, 2004
 
Marine ecologists from the U.S. National Oceanic and Atmospheric Administration (NOAA) are combing the eastern Pacific for clues to why the dolphin population there is not growing, despite more than a decade of conservation efforts.

One mystery is wrapped in another. Spotted and spinner dolphins inhabit tropical seas around the world along with yellowfin tuna. But only in the eastern Pacific do the tuna and dolphin regularly stay close—or school—together. Nobody knows why.



That tuna-dolphin connection has led to fisheries practices that devastated the dolphin population. Dolphins are a marker for the tuna. In helicopters and speedboats, fishers prowl for dolphins, then herd them toward a mother ship and encircling nets.

"When they catch the dolphins, they catch the tuna," said marine ecologist Lisa Ballance, chief scientist in the NOAA mission, based in La Jolla, California. "The dolphins are just incidental catch."

Dolphin-Safe Tuna

In 1990 NOAA mandated that all tuna-fishing boats must have an observer on board if companies want their products to qualify for a "dolphin safe" label. The label means that no dolphins were captured or killed for the catch.

The label was a response to the public outcry after estimates that populations of spotted and spinner dolphins had plummeted by 75 percent since the tuna fishery started targeting dolphins in the 1950s.

Congress authorized NOAA to track the dolphin populations to make sure that they rebounded. "We've discovered something we didn't expect," Ballance said. "There's no recovery at all [from ten years ago]. Now we're trying to find out why."

The research area is a vast triangle—7.7 million square miles (19.9 million square kilometers)—from southern California to Hawaii to Peru. On their recent mission, the NOAA research vessels David Starr Jordan and McArthur plied these seas for four months.

For the scientists aboard, the workday began at 4 a.m. At sunrise, six marine observers stationed themselves at high-powered binoculars mounted on the bridge, where they remained until sundown.

"We're out there to look for trends in the dolphin population, but we also try to get in as much other science as we can," said Bob Pitman, another NOAA marine ecologist. "We take notes on whales, sea turtles, and whatever else we find."

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 interbreed—there 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 ecosystem—perhaps in ocean temperature or abundance of prey—altered 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.

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