Some orcas won't tolerate being tagged, but a few, Candice Emmons says, are willing to play ball -- like K33, who on a gray September day is swimming high and slow in Puget Sound. Emmons, a National Oceanic and Atmospheric Administration biologist, angles her boat at the big male, who almost seems to like the tags, having worn one twice already. This particular tag, a $20,000 piece of technical wizardry worn by the whale for up to six hours before being retrieved, shows how fast he swims, how deep he dives, how he pitches and rolls, what sounds he makes, and what sounds he might hear -- the most thorough accounting yet of what it is like to be an orca, short of actually being an orca.

The problem is sticking the darn thing on him. Jeff Hogan, another biologist, stands on a platform, the tag attached to the end of an 18-foot pole. Twice today he has tried to affix it to K33's back; twice, its suction cups have failed to hold. Once more, Emmons guns the engine and the boat surges forward. Hogan leans out, the pole poised like a harpoon, and strikes. "Got 'im! Got 'im!" he shouts. A few seconds later, the tag bobs back to the surface. "Dammit," Hogan mutters.

"Let's call it a day," says Brad Hanson, the leader of the crew. "We're becoming too much the center of attention." He gestures to the dozen or so whale-watch boats that have encircled them. Obliged by federal law to stay at least 200 yards from the Puget Sound orcas, known as "the southern residents," the whale watchers are a potentially nettlesome peanut gallery. It's their effect on orcas that the NOAA biologists are here to study.

Every year, half a million people come to see the three southern resident pods --called J, K and L. Individual whales can be identified by unique markings, and for almost 40 years scientists have sketched family trees, and tallied births and deaths. It is an intimacy rare for people and (wild) whales.

Relations weren't always so rosy. In the early 20th century, fishermen shot or threw seal bombs at orcas to eliminate potential competitors for salmon. The Canadian Air Force used them as target practice to test new machine guns.

Then, in 1964, the Vancouver Aquarium commissioned a sculpture of an orca for a new exhibit. The sculptor, Samuel Burich, wanted a life-size model, so he fired a harpoon gun at an orca he spotted off Vancouver Island. He wounded his target, but rather than finish it off, he hauled it to the shallows. Instead of a sculpture, the aquarium got one of the world's first live orca displays.

Wranglers would eventually herd one-third of the southern residents off to aquariums and water parks. Those that survived became the Shamus and Namus of international fame, nurturing the esteem in which the public now holds their descendants. For them, this meant a more benign but still keen interest in their habits. Closeness may have costs, though. The southern residents declined from a high of 98 in 1995 to 81 animals in 2001. They were listed as endangered in 2005, and as of last July, there were 85.

Biologists have focused on two competing but not necessarily exclusive reasons for their decline: The orcas either aren't getting enough salmon, their primary food, or they are being loved to their detriment, if not their death, by whale watchers, whose presence can cause behavioral changes and may be a source of stress.

Finding out which issue is more pressing has become a question of knowing the orcas both inside and out. Where Hanson and his crew focus on the orcas' outer lives, University of Washington biologist Sam Wasser analyzes orca feces for hormone levels. He looks for glucocorticoids and fecal thyroid hormone. Glucocorticoids are released during acute stress events -- when an animal faces a food shortage or is chased by a predator (or a boat). Thyroid hormone levels in feces indicate the abundance or shortage of food. High glucocorticoid levels when both salmon and boats are abundant would indicate that whale-watch boats are a major stressor.

Wasser has found that a lack of food is the more powerful stressor by far. Glucocorticoids were lowest during periods of peak vessel traffic, when as many as 25 boats might shadow the pods, since these coincided with the largest salmon runs. But, he says, that doesn't mean whale watchers are off the hook. In years of low food, boat stress can exacerbate the impacts of that shortage. The effect isn't significant statistically -- at least, not yet. "Boat impacts may be hard to detect because there were already a hell of a lot of boats when we started the study in 2006," Wasser says.

What he needs is more data, and the NOAA biologists' tags may help. If the hormone signature is too coarse to pick up an effect from whale watch boats, perhaps an orca's movements will show one. Researchers working on resident pods in Canada have shown that as few as three boats can lead to behavioral changes. The orcas, in effect, often react to a boat the way a salmon does to an orca -- that is, as prey does to its predator.