OFF THE COAST OF SAN JUAN ISLAND, Wash. — 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, in the smallest of specks.

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 on the water, 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 whales 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, down to their birth years and number of offspring.