California clean air regulations intended to protect people from pollution spewed by ships are having an unintended but happy effect – saving whales.

The rules, implemented in 2009 and expanded in 2011, require vessels plying California waters to use cleaner but more expensive fuels and to limit emissions in specified areas. To conserve fuel, vessels have slowed down, researchers found. As a result, fewer whales have been killed in collisions with ships, according to a new study published in the journal Ocean & Coastal Management.

The Santa Barbara Channel is a main shipping route for vessels en route to the ports of Los Angeles and Long Beach and also a prime feeding ground for humpback whales and endangered blue whales. Researchers found vessel speeds in the channel fell from 19.7 knots to 13.4 knots (22.7mph to 15.4mph) between 2008 and 2015. Based on previous research, that would result in a 20 percent reduction in the probability that a whale struck by a ship dies. The researchers also noted that strandings of dead whales on California beaches suspected to be caused by ship strikes declined from a four-year average of 7.75 before 2011 to a four-year average of 2.75 in 2012–15.

The association of higher vessel speeds with deadly strikes is well known, said T.J. Moore, a research analyst at the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in La Jolla, California, and lead author of the study. He said he had suspected there would be evidence of ships slowing down off California in response to the new rules. “However, the widespread extent of the slow-steaming trend over time was a bit of a surprise,” he said.

The findings could have global implications as other countries and the International Maritime Organization move to impose similar rules.

But despite the reduction in ship strikes, there are still many whales being killed by collisions.

Douglas McCauley was reading the new study while glancing out at the ships traversing the Santa Barbara Channel. He tracked the speeds of a couple of them.

“There was one that was coming in at just over 19 knots, above a blue whale foraging ground, and another at 16 knots,” said McCauley, a marine biologist at the University of California, Santa Barbara. He noted that about 25 percent of the ships traveling through the area in March had been traveling at speeds of at least at 16 knots. “That’s a lot of ships that are blasting through the channel and right over the top of these blue whales.”

McCauley, who was not involved in the new study, said he thought much of what the researchers concluded was correct. He called the clean air rules a quadruple win – lower emissions, reduced risk of respiratory disease in coastal communities, less noise that can disturb marine life and fewer fatal ship strikes of whales.

But he cautioned that the phenomenon could be short-lived if cleaner fuels become cheaper and ships no longer need to slow down to save money.

“It’s a wonderful unintended consequence,” McCauley noted, but emphasized that solutions that directly address whale-ship strikes are needed. Strandings on beaches are just the tip of the iceberg, he said, as most whales sink when they die, and ship strikes are the biggest cause of whale fatalities. Ships kill an estimated 80 whales annually off the coasts of California, Oregon and Washington.

“Let’s say Walmart truckers were running over an endangered species the size of a whale. We wouldn’t be satisfied with a 20 percent cut,” McCauley said. “This is great progress in terms of notching down these mortalities, but we still need to make a lot of progress in closing this gap.”

He offered another analogy. If a law had an unintended consequence of slowing down cars in front of an elementary school, that wouldn’t be good enough to address the problem of child safety. “We’d need a direct solution, one that gets people to slow down in exactly the right places at exactly the right times,” he said.

But McCauley does want to see clean air rules like California’s implemented more widely around the world. He said it’s a “pretty good moment for that, as shipping companies are coming to the table to clean up the industry.”

Sean Hastings, resource protection coordinator for the Channel Islands National Marine Sanctuary, said he recently received a call from a corporate sustainability officer at a major user of container ships who was concerned about whale strikes. “You work on this for 10 years and then there’s finally this moment of breakthrough in awareness,” Hastings said.

“We need ships to slow down,” he said he had told the corporate executive, saying that ships can either be funneled into lanes where whales are not present or vessels can reduce speeds when moving through whale feeding grounds.

Moore noted that slower speeds might increase the number of ships on a company’s route needed to meet delivery requirements.

It’s a complicated calculation. “More ship transits may increase the risk of a strike, but slower speeds may decrease the risk that a strike is fatal to a whale,” he said.

Hastings sees the study’s finding of a 20 percent reduction in likely whale fatalities from ship collisions as a “pretty low bar.”

“We’re trying to minimize, reduce ship strikes overall,” he said. “And once we’ve done those things, we can try to reduce the likelihood of a fatal ship strike.”

Even with the fall in fatalities, ship strikes are still hindering the recovery of endangered whales, particularly blue whales. Hastings said scientists have estimated that only three of the world’s largest animals can be removed from a population in a year without hurting recovery prospects.

Ship strikes of other whales, he noted, are more easily, if gruesomely, identified. “For some reason, the fin whales get wrapped around the bow of the ship and don’t come off,” Hastings said. “They come into port.”

The Channel Islands sanctuary has implemented a voluntary slower-speed program that pays companies for complying. But the budget is limited and the program affects less than 10 percent of the ship traffic wildlife officials would like to slow down.

Another initiative called WhaleWatch gauges when whales are likely to be in a certain areas of the channel and alerts vessels so they can voluntarily lower their speeds. And a new program funded by the Benioff Ocean Initiative, where McCauley serves as a director, aims to deploy a more high-tech solution by using hydrophones to detect blue whale songs and then alert ship captains to the presence of the massive marine mammals.

Hastings said people care about where products come from, how they were produced, whether they are fair-trade or organic or conserve water. He thinks asking whether they were shipped sustainably is the next step.