The marine biologists said the mammals had finally reacted to spray from a fireboat. The new initiative was adopted after recordings of killer Orca's preying on whales, and loud metal banging failed to scare the two humpback whales from the Sacramento River back to the ocean, 11 days after the mother and calf were believed to have swam under San Francisco's Golden Gate Bridge.

Untill now they tried the luring calls of fellow humpback whales, the threatening sounds of killer whales, the banging of pipes and a flotilla of boats.

"We intentionally kept the experiment very short today because we don't want the whales to get used to it," Trevor Spradlin, a marine mammal biologist with the National Oceanic and Atmospheric Administration (NOAA,) was quoted as saying.

The biologists intended to return with the fireboat after the Memorial Day holiday weekend that ends on Monday. "We don't want to spoil the element of surprise, since this is definitely a novel stimulus that they would not be used to," he said.

Scientists also are working on a plan to inject the whales with antibiotics via dart gun to help heal wounds that are worsening the longer the marine mammals stay in fresh water.  Rescuers, puzzled by the situation, have even asked for ideas from the public, and encouraged suggestions to a special e-mail account, sacramento.whales@noaa.gov.