On a research vessel in the Pelagos sanctuary, marine biologist Alice Barbarioli lifts her camera towards the horizon. Minutes earlier, a hydrophone under the hull picked up codas, the rhythmic click sequences that sperm whales use to talk to one another. Then the animal surfaces, breathes, arches its back and dives. As its tail rises, Barbarioli spots a precise, clean-edged wound on the left fluke, which she believes was probably caused by a ship.

Collisions have long been recognised as one of the main human causes of death among the Mediterranean's endangered sperm and fin whales. What a Guardian investigation now adds is a measure of how often the danger is created. Using data from the maritime intelligence firm Veson Nautical, reporters found that large passenger and cargo vessels run by four companies, the Grimaldi Group, Corsica Ferries, Baleària and Moby Lines, have frequently crossed a protected habitat including the sanctuary at 15 knots (17mph) or faster. At those speeds a strike almost always kills a cetacean, whereas below 10 knots it is unlikely to.

The stakes are high because the sea between Valencia and Genoa is home to about two-thirds of the Mediterranean's fin whales and more than a fifth of its sperm whales. In July 2023 the International Maritime Organization designated the area a particularly sensitive sea area and advised ships of 300 gross tonnes and above to sail at 10 to 13 knots. The advice is voluntary, however. Using additional data from Global Fishing Watch, the Guardian found that ships with IMO numbers spent more than 280,000 hours travelling at 15 knots or above through the zone.