A pair of humpback whales, believed to be a mother and her calf, swim ahead of a Coast Guard cutter Monday in the Cache Slough near Rio Vista, Calif. The pair had spent the past few days in the Port of Sacramento before suddenly swimming south Sunday to Rio Vista. Authorities are trying to herd the pair back to the ocean.

Sacramento, Calif. ? In an attempt to stop the mother and calf humpbacks from continuing north again up the Deep Water Ship Channel, rescue teams are banging on metal pipes to create unpleasant underwater noises that, it’s hoped, will send them south toward the San Francisco Bay.

The noise therapy follows a successful attempt by researchers to obtain a tissue sample from the mother whale.

Using a cross-bow dart shot from a boat, animal rescuers collected the sample, which will enable scientists to determine the fatty acid content of the animal’s flesh, provide genetic information and help them make an overall health assessment of the animal, said Laurie Gage, a veterinarian with the National Marine Fisheries Service.

The sample was collected by experts with Cascadia Research Collective, a nonprofit scientific firm in Olympia, Wash., that has been involved in the rescue efforts of the mother whale and her calf. The tissue sample came from the larger whale’s right side, just below the dorsal fin. The rescuers collected it at 5:20 p.m. CDT about six miles north of Rio Vista, where the animals were swimming and milling about.

Rescuers were attempting to use a flotilla of boats to herd the two animals back downstream after the sample was collected.

On Sunday, the whales abruptly began swimming south in the channel, after several days of swimming in the Port of Sacramento and entertaining tens of thousands of spectators.

Researchers moved with the whales, taking up positions at the mouths of sloughs and rivers they wanted to keep off limits to the whales, dubbed Delta and Dawn.

But the whales’ sudden northward turn Monday, near the Ryer Island Ferry Bridge, also hastened plans to tag the mother with a small satellite tracking device – the size of a chicken egg – that will allow scientists to track the pair in the Delta and, later, in open waters.

While harmless to the whale, the tag must be spot-on, hitting flesh just behind the dorsal fin. If it hits blubber, it won’t stay.

The tag is designed to fall off in a couple of months.

The Coast Guard also continues to enforce 500-yard boating restrictions around the whales, as well as a 1,000-foot flight ceiling for aircraft.

The wayward whales who were headed south Sunday were playing a cat and mouse game Monday with a flotilla of boats attempting to guide them safely to the ocean.

Shortly after daybreak, State Fish and Game officials reported that the whales were spotted heading south just north of Rio Vista. A few minutes later, the whales were spotted heading back north.

“They were circling the boats, which was kind of cool,” said Jim Milbury, a spokesman for the government whale-rescue effort.