At least 136 melon-headed whales are dead after a mass stranding event off the coast of western Africa last week, according to environmental conservation non-profit BIOS.CV.
Dozens of volunteers from local agencies assisted in relocating a total of 163 adult, juvenile, and calf whales back into the water after they were discovered on September 24 on the island of Boa Vista.
“Unfortunately, upon being re-introduced in the sea, most of the animals stranded again,” wrote the organization in a Facebook post.
Officials are working to bury the individuals to “prevent any environmental and public health hazards,” said BIOS.CV in an update posted on September 26. Samples were taken from 50 of the whales and another four individuals were frozen for future examination by veterinarians.
Though the International Union for Conservation of Nature considers melon-headed whales a species of least concern, the toothed cetaceans are threatened by a number of concerns including habitat changes from climate change, ocean noise, and fisheries bycatch. Closely related to pygmy sperm whales and false killer whales, Peponocephala electra are often found in deep tropical waters around the globe, according to the National Oceanic and Atmospheric Administration (NOAA). They typically live together in groups of hundreds to over 1,000 individuals.
Researchers are still unclear as to what caused the whales to beach themselves.
In recent years, a number of mass stranding events have occurred around the world. Last November saw several events, including two pods of pilot whales, totaling 145 individuals, dead after stranding on New Zealand shores. Just three days later, a humpback whale and 27 pilot whales were found beached in Australia. Earlier this year, at least 50 pilot whales were found dead on a remote beach in Iceland after possibly becoming caught in a strong tidal current that prevented them from reaching deeper waters. Since the beginning of 2019, at least 70 gray whales have washed up along the west coast of North America, from Alaska southward to Mexico – so many that NOAA has run out of space to bury decomposing carcasses.
Mass mortality events and whale strandings are becoming more common than before and the reason why is unclear. This could in part be due to the fact that protections in the last few decades have increased whale populations in waters around the world. However, it could be due to external factors such as disease or extreme weather. Cetaceans may also become stranded after being chased into shallower waters by predators or when chasing prey, increasing the likelihood that they become disoriented and caught by a retreating tide. Furthermore, studies have suggested that naval sonar could impact whales’ ability to navigate via echolocation, perhaps even giving them decompression sickness.
Even after human intervention, many whales die from dehydration and can drown if the tide rises over their blowholes.