Concerns about pollution and endangered whales in southern California have prompted six global shipping companies to try slower speeds in exchange for cash incentives, environmental groups said.

The companies agreed to slow down -- from typical speeds of 14-16 knots down to 12 knots or less -- in exchange for $2,500 each time they pass through the Santa Barbara Channel.

The program was announced this month by the Santa Barbara County Air Pollution Control District, the Environmental Defense Center and the National Oceanic and Atmospheric Administration's Channel Islands National Marine Sanctuary.

"Reducing ship speeds to 12 knots or less reduces emissions of smog-forming air pollutants that harm our health," said Dave Van Mullem, Director, Santa Barbara County Air Pollution Control District.

Marine experts said the slower speeds could also save the lives of whales by cutting down on collisions that can be deadly.

Ship strikes play a role in the deaths of one to three whales per year on average, though environmentalists say more are likely killed by ships but never found.

"Slowing ships down reduces the likelihood that a ship strike on a whale will be fatal," said Chris Mobley, Superintendent for the Channel Islands National Marine Sanctuary.

About 2,500 container ships pass through the area each year.

The trial, announced August 4, began on July 1 and runs through October 31 "to coincide with the busiest whale season and the prime period for high levels of air pollution," the groups' statement said.

Companies involved so far include COSCO, Hapag Lloyd, K Line, Maersk Line, Matson, and United Arab Shipping Company.

It is modeled on another program under way at the Ports of Long Beach and Los Angeles, where 90 percent of shipping lines participate.

A study last month found that blue whales off the US west coast habitually return to the same areas to feed on krill each year, and these areas tend to be heavily used as shipping lanes.

This habit could be the reason why their population is not rebounding as experts had hoped, and researchers suggested in the journal PLOS ONE that changing shipping routes could help.

The blue whale population was depleted by commercial whaling early last century but gained protection from the International Whaling Commission in 1966.

Between 10,000 and 25,000 of these endangered whales are believed to swim in the world's oceans today. Their numbers have not increased much in recent decades.

Blue whales (Balaenoptera musculus) can live 80 to 90 years on average, and their 100-foot (30-meter) long bodies can weigh around 20 tons.