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In the Canary Islands, whales have become maritime roadkill.

In the waters off this northwestern African archipelago, an average of 2.3 sperm whales are killed each year by ship strikes. Worryingly, this death rate closely matches the estimated sperm whale birth rate in the area.

The implication, argues marine ecologist Natacha Aguilar de Soto in a new paper, is that the Canary Islands’ sperm whale population is facing an existential threat. That females and juveniles appear to be struck by ships most often is further evidence the whales are at risk of a gradual, long-term decline.

Aguilar de Soto and her colleagues’ estimate of whale deaths to ship strikes is based on observations of whale strandings. Yet, as Aguilar de Soto says, “everybody knows that strandings are only the tip of the iceberg.” Most dead whales sink rather than wash up on shore, meaning whales likely die due to ship strikes more often than people notice. Previous research showing that around 11 percent of the cetacean strandings detected in the Canary Islands from 1991 to 2007 were attributable to ship strikes supports this assumption.

Andrea Fais, a behavioral ecologist at the University of Veterinary Medicine in Hanover, Germany, worked with Aguilar de Soto to estimate the size of the Canary Island sperm whale population.

The team used hydrophones mounted on the bottom of a research vessel to eavesdrop on sperm whale vocalizations. After 225 hours and 2,668 kilometers traveled, they detected 85 individual whales. From that tally, they inferred a maximum sperm whale population of 224. A population of sperm whales that size can be expected to produce an average of 2.5 babies each year.

The data is further evidence of the threat posed to whales by ships.

Off the east coast of the United States, ship strikes have been identified as a main risk factor for the extinction of the North Atlantic right whale. In Greece, some 60 percent of stranded sperm whales show evidence of ship strikes. “Ship strikes are probably second only to entanglement in terms of the numbers that are killed” when it comes to direct human-caused mortalities, says International Fund for Animal Welfare whale researcher Russell Leaper.

Globally, shipping traffic is on the rise. To Aguilar de Soto, this means that, to spare the whales, shipping companies as well as inter-island ferries need to change their practices. “On land, we are used to knowing that vehicles can kill protected species, and we put measures [in place] to reduce that impact, like signs for reducing speed,” she says.

But when it comes to reducing the risks ships pose to whales, Leaper says the challenges at sea are different than they are on land. Reducing speed isn’t usually an option, he says, because that adds big costs for shipping companies. But, he adds, “a small change in route has little economic impact.”

Indeed, Leaper and his colleagues have proposed altering shipping lanes to spare blue whales from ship strikes off the coast of Sri Lanka. Aguilar de Soto says she is currently negotiating with shipping companies, as well as with the Spanish and Canary Island governments, to work out a solution—which may involve changing shipping lanes as well.

After a thousand years of intense pressure, during which humans hunted whales for their meat, oil, and bones, re-routing ships away from critical areas seems like an easy concession to these slowly recovering leviathans.

*Correction: Natacha Aguilar de Soto’s surname is Aguilar de Soto, not de Soto. This story has also been adjusted to more accurately reflect the preliminary status of the negotiations around changing the shipping lanes through the Canary Islands.