Why do whales and dolphins strand?
With combined forces, the helpers guide the stranded orcas back into deeper water. But after a short time they run onto the beach near Cape Town again. Why whales and dolphins die like this is still an unsolved mystery.
Summer is coming to an end - time to start the hike to the equator. Their special tracking system shows the male sperm whales the way back from the Arctic Ocean to warmer waters. Again and again, however, some of the giants deviate from their course and swim into the flat North Sea instead of Scotland and Ireland. Shallow tidal creeks and flat beaches can be fatal to them. If the mammals run into sand, they can dry out, overheat and suffer internal injuries from their own weight.
There are numerous theories that could explain why whales and dolphins strand. According to the Kiel marine biologist Boris Culik, in most cases it is a matter of navigation errors by the animals. Toothed whales orient themselves with the help of echolocation, which means they produce tones or clicks, the echo of which tells the mammals exactly where an object is in front of them.
Crescent-shaped beaches that taper off slowly and shallowly, or semi-circular bays that lie in the migration direction of the whales pose a great threat to marine mammals. Because these shores do not reflect back from any direction and deceive the whale, he would be in the infinite expanse of the ocean. "Their natural navigation system no longer works in shallow water," emphasizes Culik.
The fact that marine mammals lose their orientation during their migrations could also be due to the sun: solar storms, during which our central star shoots particularly large amounts of ionized matter into space and which occur regularly every eleven years, can occur shift the lines of the Earth's magnetic field that certain whales use to orient themselves. Klaus Heinrich Vanselow and Klaus Ricklefs from the Research and Technology Center West Coast (FTZ) clarified this relationship for sperm whales, which have been stranding in the North Sea at regular intervals since the 18th century. Mammals have been stranded again and again over the centuries, so it's not a new phenomenon.
Moreover, many whale species are extremely socially organized, so that the group, also known as a pod of whales, stays with an injured or stranded animal to help and protect it. "If a beached whale screams for help, it can even happen that the other animals in the group react and follow the whale onto the beach," explains Culik. These models could also explain the behavior of the killer whales stranded in South Africa at the end of May 2009, which returned to shore after being guided back to deeper water by rescuers.
It is also possible that the leader loses orientation and all other members of the school swim in the wrong direction. From killer whales, for example, it is known that the mother or grandmother leads the clan and the other animals follow. However, when parasites invade the marine mammal's ear, it can cause them to miss the echo of the clicks they emit and become disoriented.
Underwater noise generated by humans, such as oil rigs, ships, military sonars, or strong noise from seismic surveys, can affect whale orientation and communication, or even frighten the animals so much that they flee "blindly" from the noise. "Behavioural changes, physical damage and even the death of the whales are conceivable consequences. After NATO maneuvers, rare beaked whales washed up on the coasts of Cyprus, the Canary Islands and the Bahamas," reports Culik. "They had been injured by the strong military sonar and were suffering from divers' sickness, so to speak." The acoustic signals triggered the formation of gas bubbles in the blood vessels and organs of the marine mammals. These cut off the blood supply and resulted in her death.
Animals that wash up dead already have been wounded or sick before. They can die as a result of infection, parasite infestation and injuries from ship collisions, fishing nets or shark attacks, as well as poisonous or contaminated food. When the mammals are driven from their feeding and breeding grounds by underwater noise, or become disoriented, they can starve to death.
The most common mass strandings are of pilot and sperm whales. Baleen whales, on the other hand, very rarely beach. With the help of their baleen, horny plates in the upper jaw that they have instead of teeth, they filter krill, animal plankton and small fish out of the water. This group includes all large whales except for the sperm whale. Unlike toothed whales, their ability to echolocate has not yet been proven.