A dog named Tucker with a thumping tail and a mysterious past as a stray on the streets of Seattle has become an unexpected star in the realm of canine-assisted science. He is the world's only working dog, marine biologists say, able to find and track the scent of orca scat, or feces, in open ocean water -- up to a mile away.

Through dint of hard work and obsession with an orange ball on a rope, which he gets to play with as a reward after a successful search, Tucker is an ace in finding something that most people, and perhaps most dogs, would just as soon avoid.

And it is not easy. Scat can sink or disperse in 30 minutes or less. But it is crucial in monitoring the health of the orcas here, an endangered group that is probably among the most studied animal populations in the world. Most of the 85 or so orcas, or killer whales, that frequent the San Juans, about two hours northwest of Seattle, have been genotyped and tracked for decades. And none of this could happen as easily as it does without Tucker and his nose -- or the new tricks that he taught the scientists.

"Sometimes he'd just turn around and sit down and stare at me, waiting for me to figure it out," said Deborah A. Giles, who is completing a Ph.D. on how orcas here are affected by the thousands of whale watchers and scores of commercial whale-watch vessels that cluster around the animals. "He's very subtle," said Giles as Tucker, an 8-year-old black Lab mix, paced at the prow.

One thing to get out of the way quickly: Orca scat really does not smell that bad. Unlike, say, a narcotics-sniffing dog that can lead its human around by a leash, the research boat itself is, in effect, Tucker's legs. He cannot physically go where the sample is to be found, but must somehow signal where he wants the boat to go. Tucker might lean to one side, then another, then suddenly sink back onto his green mat with his head between his paws, the scent lost -- and all must follow.

"The slightest twitch of his ear is important," said Elizabeth Seely, a trainer who has worked with Tucker for four years at a nonprofit group called the Conservation Canines, which specializes in dog-assisted research on behalf of endangered species.

Out on these waters, though, it seems that every creature is learning new tricks. Salmon have taken to hiding under commercial whale-watch boats when they are being hunted by the orcas. The boats, in turn, are filled with people -- upward of 500,000 during the peak season from May to October -- who have paid to see whales.

The orcas are becoming, in a strange way, more in sync with the rhythms of their human watchers -- resting less during daylight and more at night than they used to in the 1980s or '90s.

For Tucker, though, it mostly comes down to his ball toy, which he plays with in exuberant, wild abandon, tossing it into the air and staging bouts of tug of war. Another dog in training, a retriever named Sadie, was donated to the program by an owner who could not deal with her ball fixation. In frustration, the owner put Sadie's ball on top of the fridge. Eight hours or so later, she returned and found Sadie still sitting there, staring up at the object of her desires.

"When the owner told me that story, my immediate response was, 'We'll take her,'" said Samuel Wasser, the director of the Center for Conservation Biology at the University of Washington and the director of the orca scat research project.

The research is raising new questions about how to protect the orcas. Wasser said that when he started the project four years ago, he thought boat activity would be a crucial element of whale stress, reflected through stress hormones in their scat. But it turned out, he said, that food supply was more important. Knowing to focus on fish supply, he said, means knowing where to focus public policy efforts.

For all his hundreds of hours on boats, Tucker will not get wet. He hates to swim, Seely said.