Whales get a holiday break

Two wandering humpback whales remained in the Sacramento River on the eve of the busy Memorial Day weekend, but scientists announced plans to launch a new effort next week to herd them back to sea.

Authorities returned from the water Friday with a bit of optimism, declaring success in an experiment involving a Vallejo fireboat that sprayed water in the whales’ direction, prompting them to change course several times.

The plan now is to give the mother and calf a rest over the three-day weekend, the traditional start of the recreational boating season. Patrol boats will be on hand to keep boaters at least 500 yards from the pair.

Officials said that starting Tuesday, they expect to continue using their arsenal of tactics -- including banging pipes and playing recordings of whale sounds -- while also enlisting several fireboats to help keep the cetaceans on course.

Authorities kept the fireboat experiment short Friday so as not to “spoil the element of surprise” next week, said Trevor Spradlin, a wildlife biologist with the National Marine Fisheries Service.

The fireboat shot a stream of water a few hundred yards in front of the whales, who initially approached the spray before turning away and moving past the vessel, Spradlin said.

The whales turned away from the spray five times, and experts suspect a larger array of fireboats from various port cities might help keep the pair moving in the right direction next week.

The humpbacks still bear wounds probably inflicted by a ship propeller.

Veterinarians are intent on prodding the pair, first spotted in the Sacramento-San Joaquin River Delta on May 13, back into saltier waters where their wounds should heal better. In recent days their normally slick and glossy skin has become pitted and dull.

While the whales get a break from rescue attempts, veterinarians will visit them this weekend in hopes of giving each a dose of antibiotics.

Authorities believe it would be the first such attempt on whales in the wild.

At Grandpa’s Compounding Pharmacy in Placerville, Bill Wills was busy Friday preparing the first of the whale-size doses.

Wills, whose pharmacy mixes medicines not routinely offered by the drug industry, said the plan was for him to deliver the antibiotics to the docks in Rio Vista by 5 this morning.

An expert will try to administer the doses with a type of low-power gun normally used to fire tranquilizers.

Scientists also may attempt to capture air expelled by the whales.

They would analyze it for digestive byproducts that would indicate whether the calf was still nursing.

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eric.bailey@latimes.com