That's the verdict from scientists who acknowledged the humpbacks' quiet departure was bittersweet for the whale rescue team.

The last confirmed sighting of the mother and calf was Tuesday night near Tiburon, about four miles from the Golden Gate Bridge.

The lack of other sightings Wednesday -- despite a concerted effort -- convinced rescuers that the pair made it to the open sea.

"I don't think we could reasonably expect them to wave as they left," said Brian Gorman, speaking for the team. "There is obviously an enormous amount of excitement, both emotionally and professionally, but at some point, we have to bring this to closure."

Gorman, a spokesman for the National Marine Fisheries Service, said emotions ran high during the whales' odyssey through the Sacramento River Delta, eventually reaching the Port of Sacramento 90 miles upstream.

After dallying in fresh water for more than two weeks, the whales made a final dash for the ocean Tuesday, moving through San Pablo Bay at a brisk 5 miles per hour.

One factor making it difficult to track their departure, he said, is the unusually high number of whale sightings off the San Francisco coast on Wednesday -- mostly of California grays. Delta and Dawn may just have gotten lost in the crowd.

The whale multitudes may have been drawn by a huge "bloom" of krill near the Farallon Islands, 26 miles west of the Golden Gate. Krill is sort of a plankton soup -- the primary food of humpbacks and other baleen whales.

It is anybody's guess whether the krill played a role in attracting Delta and Dawn back to the open sea, marine mammal biologist Trevor Spradlin said. But the pair were observed to be feeding Tuesday in the bay, probably their first meal in weeks.

Researchers regret they were unable to attach a tracking device to either the mother or the calf. Such a locator could have minimized the frustration Wednesday over locating the whales.

Gorman said an early attempt to attach a monitor was called off because of concern about the health of the animals. Then, on Tuesday in the Carquinez Strait, a malfunction in the tracking device itself stopped the tagging.

Researchers will rely instead on a dedicated network of trained observers who monitor whale movements up and down the coast.

The distinctive fins and flukes of Delta and Dawn were well documented during their Sacramento sojourn. Their pictures will be posted in the official whale identification catalog.

Since it began May 13, the multiagency whale rescue effort involved nearly 100 people. Total costs are still being tallied, but the California Department of Fish and Game estimates it spent about $300,000, including salaries.

Whatever the final expenditure, scientists say the wealth of first-hand information they gleaned is unparalleled in the study of humpbacks.

They now know, for instance, that the whales can last about two weeks in fresh water before their skin starts a rapid deterioration. They also learned how to administer antibiotics to whales in the wild.

And they discovered that some whale-herding tactics -- playing recordings of feeding humpbacks, for example -- just don't work.