A growing number of shipping companies are slowing down as they approach San Francisco and other California ports so they are less likely to injure or kill whales.

Marine experts say four of the 10 gray whales found dead near San Francisco this year were killed by ships, the San Francisco Chronicle reported Friday .

When the campaign started about six years ago, only 17% of incoming ships were slowing down, said Greater Farallones National Marine Sanctuary superintendent Maria Brown. Now 45% are throttling down by half, to 10 knots.

There are no speed limits at sea, so vessels can zip along as fast as they want, usually 20 knots, or about 23 mph (37 kph). The large vessels often travel through national marine sanctuaries to get to their destination ports.

“Endangered blue whales come to the sanctuary every year. We’re the bread basket, the restaurant for blue whales,” Brown said. “And the National Marine Fisheries Service has determined that the No. 1 human-caused death of blue whales is ship strikes.”

The campaign began in 2013, when NOAA extended shipping lanes several miles beyond the continental shelf, where blue and humpback whales feed. A year later, the Greater Farallones and Cordell Bank sanctuaries began voluntary vessel speed reductions from May to November, peak time for blue and humpback whale visits to the area.

Twenty-two shipping firms, which represent 45% of the 8,000 inbound vessel trips through the Golden Gate every year, were honored Thursday for cutting their speed in 2018 to 10 knots (11.5 mph) or less in areas populated by whales. Those include the Farallones, Channel Islands, Monterey Bay and Cordell Bank marine sanctuaries.

“Just as anyone driving down the highway doesn’t want to hit an animal, nobody in a ship wants to hit a whale,” said John Berge, vice president of the Pacific Merchant Shipping Association, which deals with maritime trade issues.

While there is a growing awareness by shippers to reduce speed for the whales’ sake, it’s difficult to track how much the 5-year-old campaign has affected them, said Mary Jane Schramm, a spokeswoman for the sanctuary.

“It’s hard to say, these whales live and die offshore,” Schramm said.

Also, large vessel traffic has increased over the last few decades, and it is widely recognized that shipping lanes adjacent to San Francisco Bay ports are especially susceptible. That’s because record numbers of humpback and blue whales have been feeding in recent years off the coast and even in San Francisco Bay.

The injuries have extended this year to gray whales, which normally do not enter the bay or linger in the area during their winter migration, which usually ends in May. But those whales have been hanging out for long periods and feeding on bay mud, a highly abnormal practice for the species, said Pádraig Duignan, the chief research pathologist at the Marine Mammal Center in the Marin Headlands.

Of the 10 gray whale carcasses found in the Bay Area this year, four died of malnutrition, four after being hit by ships, and two were too decomposed to tell what happened.

Tired, malnourished whales are more susceptible to ship strikes, attacks by orcas or entanglement in fishing gear, Duignan said. In a typical year, only one or two gray whales wash ashore in the Bay Area, Duignan said.