Whale and Dolphin Mass Strandings
Spring 2008 Sees Unusual Number of Cetacean Deaths Worldwide
Jun 12, 2008
Mass strandings of whales and dolphins have been occurring for thousands of years. But the numbers of these strandings, the stranding sites and the types of whales and dolphins involved are changing.
The Link to Ocean Noise
In recent years there has been increasing evidence that links injuries resulting from ocean noise to whale and dolphin deaths. There are two types of post mortem finding that have been associated with the high intensity impulse noises that result from military exercises or oil and gas exploration.
The first is hemorrhaging in the inner ears and intracranial spaces, along with hemorrhaging and contusions in the mandible and/or jaw fat. The second is gas bubble formation consistent with Decompression Sickness (DCS), also called nitrogen narcosis. These gas emboli may be another result of the acoustic trauma or they may occur when whales or dolphins dive or surface under abnormal circumstances.
2008 Mass Strandings
In February 2008, 5 Cuvier’s beaked whales (Ziphius cavirostris) were found dead on beaches in the Hebrides. Beaked whale species have been the catalyst for much of the research into ocean noise’s effect on cetaceans, as they were rarely victims of mass strandings until recently.
This spring there were strandings of three Cuvier’s Beaked whale in the area around Hokkaido, Japan within three weeks. Unfortunately, all of the bodies washed ashore badly decomposed so cause of death remains unknown.
In late May, 81 whales, reported as either long finned pilot whales (Globicephala melas) or melon headed whales (Peponocephala electra), stranded on a beach in Dakar, Senegal. 38 died and the rest were refloated. This area has seen mass strandings in the past but this was the largest in thirty years.
At about the same time, Madagascar became the site of a mass stranding of melon headed whales. Somewhere between 150 and 200 animals were trapped in a small bay with a narrow opening. ExxonMobil had been carrying out oil and gas survey work in the area.
In early June, 26 common dolphins (Delphinus delphis)stranded at several sites in Cornwall, UK. An algal bloom was in progress and naval exercises were being carried out in the area.
Making Sense of Cetacean Mass Strandings
These incidents point to the problems in determining the causes of whale and dolphin mass strandings.
In Senegal, where pilot whales have stranded in the past, there has been no effort to perform postmortem examinations on any of the whales so there was no information gained regarding the cause of the strandings.
In Japan, the bodies of the whales were too badly decomposed to do any assessment
In Madagascar, where there is the possibility of an ocean noise aspect and no record of whale or dolphin mass strandings, experts have been brought in to do extensive examinations to determine cause of death.
In the UK, the Cuvier’s beaked whale strandings have resulted in efforts to determine whether there was any military activity at the time. And the Cornwall dolphin strandings will be looked at very closely as there are multiple possible triggering events.
Whale and dolphin mass strandings related to natural events or geography need to be separated from those with possible human causes. This means that thorough investigations must be done on as many mass strandings as possible. A clear picture of what is and isn’t human caused will make prevention of some whale or dolphin mass strandings possible.
 |
