Conservationists have shared online a distressing photo of a whale with an almost completely severed tail likely caused by being hit by a boat propeller.

Despite the tragic image, the post on Instagram, which explains the whale had to be euthanized, has gone viral with most people horrified but applauding the marine conservationists for ending its suffering.

The photo was taken by underwater photographer Francis Perez off the coast of the Canary Islands, and shared online by Cristina Mittermeier, a marine biologist and photographer, on June 13.

In her post, Mittermeier explained that she, Perez, and a wildlife veterinarian were called to the scene of the young pilot whale and heard the “shrilling calls of pain and fear” as the whale struggled to swim. Realizing there was nothing they could do to help it, and it would not survive the injury, they made the decision to euthanize it.

Now Mittermeier is using the post to highlight the dangers of boat collisions with the huge mammals and how we can help reduce the risk of vessel strikes.

“Sparing more unnecessary suffering to an animal with no chance of recovery was what [we] had to do,” she says in the post. “What the rest of us need to do is to become more engaged it.” 

Boat strikes – when a whale and a boat collide or a whale gets injured by a boat’s spinning propellers – is one of the leading causes of injury and death for whales, according to the WWF. Many of the world's shipping and ferry lanes also coincide with areas where whales feed, breed, nurse, and migrate.

Often a ship or boat may not see a whale, or may be traveling too fast to avoid a collision if a whale rises for air unexpectedly. Sometimes whales are used to swimming in areas with heavy boat traffic and so don’t consider them a threat, getting too close. For larger ships, it’s likely the “bow null effect” – the bow of a ship can mask the sound of the engine from the stern, creating an acoustic shadow that sound waves can’t travel through –which means the whales simply can’t hear them.

“Was it hit by a ship, a ferry or a pleasure boat? We will never know, but I have certainly seen many boats traveling at high speeds through sensitive wildlife corridors,” Mittermeier wrote.

If a whale isn't killed instantly by a collision it can still die from its injuries. Sometimes a cut or gash means it will fall prey to predators, or stops it being able to feed or fend for itself. In this case, with no tail, it is highly unlikely – though not impossible – the whale would have survived. Whales' tails are vital for a number of functions, from propulsion to balance, regulating body heat, and communicating with other whales. If it was struggling to swim, as Mittermeier observed, it would likely not be able to propel itself to the surface to breathe and instead suffocate in the water.  

Mittermeier co-founded marine conservation non-profit SeaLegacy to help push for legislation regarding vessel speed limits in known whale habitats. The number of whale deaths each year from boat encounters is hard to determine as many researchers suspect many collisions go unreported. 

As John Calambokidis, a research biologist studying whale-boat collisions, told Hakai magazine in 2017, the responsibility of minimizing vessel collisions is down to us as whales have been the largest things in the oceans for around 50 million years, they are not prepared for anything bigger or more powerful than them.

“It’s not something they’re evolved to deal with,” he said. “It’s also something there’s very little opportunity to learn from. It’s not like you can get struck two or three times and then you know you should avoid them.”