Cetaceans stranding: humans are partly to blame

Cetacean stranding, more commonly referred to as beaching, refers to the phenomenon of dolphins and whales stranding themselves on beaches. Its recently reported that worldwide, about 2,000 whales and dolphins die from stranding every year, with most resulting in the death of the animal. Some whale and dolphin species are more prone to mass beaching. Toothed whales (include dolphins, porpoises, and all whales with teeth) are the most commonly affected. When they are stranded, they will experience severe dehydration, overheating, depression and suffocation. Their very heavy body makes it difficult for them to return to the sea.

New Zealand and neighbouring Australia are hotspots for mass whale stranding, thanks to large colonies of pilot whales living in the deep oceans surrounding both island nations. In Macquarie Harbour, Tasmania, the most recent mass stranding involved approximately 470 pilot whales In September 2020. At least 380 had died, and only 20 remained well enough to rescue. But the largest mass stranding in modern recorded history was 1,000 whales on the shores of the Chatham Islands, a New Zealand territory in the Pacific Ocean in 1918. There have also been cases of cetacean single stranded on shore after being previously injured in a collision with a boat, fishing net, or shark attack. The wound then became infected and made the animal sick. Another factor that might cause this could also be because they took shelter from other larger predators up to shallow water. Or as they ventured too far into shallow areas when hunting for prey. However, the stranding of these mammals that resulted in mass deaths is a complex story related to the disruption of the navigation system when they swim, both by nature and human activities.

Like migratory birds, some species of cetacean also travel large distances each year. In winter, they migrate from the cold northern seas to the warmer waters of the south. Vice versa, whales and dolphins in southern waters will move north in the same season. It was only a few months later that they would begin their return journey. How do they navigate their long migratory journey? Small-toothed whales such as dolphins have powerful underwater sonar. On the way, they will emit sound waves in the form of clicking sounds. When these sound waves hit an object, they are reflected back as echoes to their ears. The faster the sound returns, the closer prey, obstacles, or shore are. This sonar also keeps the whales from stranded on the beach. However, in the case of large baleen whales, underwater sonar is not as sophisticated as that of small-toothed whales. Under certain circumstances, underwater sonar may not function properly, especially if there are shallow or semi-circular bays, sandy underwater embankments, or mud banks. This type of beach and obstacles makes the echo received by the whales unclear, so the warning system on the whale fails. Other types of whales, such as pilot whales, do not just use underwater sonar to navigate −small fluctuations in the Earth’s magnetic field function like map. They depend on the Earth’s magnetic field lines, as their migration routes are often parallel to those lines. Significant changes to the Earth’s magnetic field causing by solar storms and the increasing activity of sunspot is believed to be the caused of their navigational errors resulting them to get lost and stranded. However, all these reasons have not been studied in depth. For example, in relation to the social behaviour of many species of whales, which roam in groups and are guided by a leader. The strong social bonding of some species of whales can cause mass stranding. Whales that strand in groups are usually deep-water species with highly evolved social structures. When the leader loses orientation, due to confusion or parasite attacks his ears, it is unable to hear the echo of the clicking sound that is sent. And unfortunately, the party behind it will follow in the wrong direction. If the leader of the group were stranded in shallow water, the members of its group would follow, even when it meant death.

In addition to natural factors, underwater noise resulting from human activities (such as: sounds from ships, icebreakers, drilling activity, or military sonar equipment) also interferes with the navigation of whales and dolphins, causing them to be disoriented and eventually stranded. In the ocean the speed of sound propagation can reach 1,500 meters per second, about four times faster than the propagation of sound in air. For some marine biota such as whales, dolphins, and even small invertebrates such as shrimp, crabs, and sea urchins, they use sound to get food, to communicate, and to regulate themselves to maintain group relationships, all of which are basically done to survive in the ocean. According to research by the National Oceanic and Atmospheric Administration (NOAA), the biggest source of noise pollution in the oceans comes from the activities of submarine sonar test systems, oil company ships at sea, and traffic from cargo ships. Another study was also conducted by NOAA by sending a microphone to the deepest ocean floor, namely the Mariana Trench with a depth of 10,984 meters below sea level which shows that the seabed is not silent. Loud noise in the ocean has become a major conservation concern as humans introduce sounds of varying intensities and frequencies into the marine environment from different technologies and even explosions. Seaquakes are another source of intense underwater sound and those might also lead to physical damage or behaviour resulting in stranding, although no one has yet produced a statistical link between the two. Although completely inaudible to the human ear, ocean noise pollution causes serious problems that will continue to grow. Imagine being a sea animal that must communicate amid noise but cannot do anything to stop it. Disruption to the way whales and dolphins communicate is a hindrance to their efforts to survive, so it is time to take steps to resolve this global crisis.

It was reported in 1995 that the New Zealand’s Department of Conservation (DoC) was looking at two lines of research for directing pilot whales away from potential stranding areas. In January, a bubble net or “air curtain” was used for the first time in Golden Bay. This comprised air compressors and a perforated hose which was draped two or three metres below the surface. The bubble “wall” acted as a barrier and reflected the whale’s sonar back to them. It had reasonable success in turning whales at sea, but once the whales discovered the wall was an illusion then its effectiveness was reduced. It was a mixed success, possibly useful as a short-term measure. In future the bubble net is likely to be most effective in herding refloated whales out to sea after a stranding. The second avenue that DoC was investigating was to use the same mechanism that draws so many whales onto the shore in the first place – the animal’s own distress signals. DoC was hoping to record pilot whale distress calls and use the recordings to attract refloated whales away from danger areas.

And what to do if you find a live stranded or injured whale, dolphin, or porpoise on the beach or in the shallows? Stranded cetacean usually does not have much time. They must be helped immediately by cooling their bodies and keeping them moist by constantly dousing them with water. At the same time, push them as quickly and gently as possible into deeper waters so they can swim again. Some whale rescuers suggest that if the leader of a pod is put to sea, then the other whales will follow. The problem here is that it is usually impossible to read the social dynamics of any pod. As pilot whales are socially matriarchal, the leader will usually be female, but it is difficult to identify the leader of a pod. There may be more than one, the pod may have several subgroups within it, and the leader will not necessarily be at the front.