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Marine traffic is increasing worldwide, and where ships and whales overlap, collisions between the two are inevitable. The problem is particularly acute in the Canary Islands, off the northwest coast of Africa, an area that teems with both whales and large ships.
“There are many high-speed ferries in the Canary Islands, and we’ve seen an increase in stranded whales with signs of ship collision,” says Marina Arregui, a biologist at Spain’s University of Las Palmas de Gran Canaria. However, determining whether these collisions caused the deaths of the whales is not straightforward.
While some types of whales sink when they die, others float. Some particularly buoyant species, such as sperm whales, can take weeks to wash ashore, often arriving in advanced states of decomposition. Even when ship strike injuries are apparent—such as deep cuts made by the keel of a ship—it’s possible these wounds were inflicted postmortem, while the carcass drifted at sea.
To determine whether a ship strike killed a whale, Arregui and her team examined preserved lung tissue samples collected from 24 sperm whales that had died near the Canary Islands between 2000 and 2017. Sixteen of the whales had injuries consistent with ship strikes, while eight were examined to serve as a control group.
Arregui and her team examined the tissues, looking for small lumps of misplaced fat. When an animal suffers a major injury, fat cells from damaged tissue enter the bloodstream. The circulatory system carries these lumps around the body until they become tangled in fine blood vessels, such as those in the lungs. This is known as an embolism.
Because these clumps of fat cells can only be transported around the body while the whale’s heart is beating, their presence in the lungs indicates the animal was still alive when the trauma occurred. In the crime lab, these fat emboli indicate someone was a victim of trauma before they died. Arregui says this is the first time the technique has been performed on whales, although it’s been used in animal pathology before.
Of the 16 whales that had injuries associated with ship strikes, 13 had fat emboli in their lungs. The scientists also examined the eight whales in the control group and found that two had small numbers of fat emboli, likely the consequence of small, non-fatal injuries. This data, Arregui says, is enough to establish a protocol for diagnosing fatal ship strikes.
Accurately recording ship strike fatalities is an important advance for whale conservation, says Arregui’s colleague Yara Bernaldo de Quirós. “It’s all part of an effort to make a diagnosis. If you have more pieces of the puzzle, you get a better, stronger case.”
Armed with better diagnostics, whale conservation campaigners will be able to more convincingly argue that mitigation measures are needed to protect whales.
“There is little understanding of how frequently ship strikes happen,” says James Robbins, a doctoral candidate at the University of Portsmouth in England who wasn’t involved in the study. “Being able to say this animal was killed by a ship could prove very useful for getting closer to the true number of animals affected.”