A stranded group of long finned pilot whales.
Although, there are several theories to why whales strand,
this occurance remains a mystery to scientists.
Photo: Iain Kerr
June 25, 2001
The Mystery of Whale Strandings
In many parts of the world, the only first hand experience local people have with whales are when they strand and thousands do so every year along the coastlines of the world.
During our last four months in Papua New Guinea we have heard several stories about stranded sperm whales from the local people. In some villages whale strandings are considered a windfall, a huge quantity of food, fuel and building materials delivered to the front door. In other areas, the people of the villages come together to try and help the animal or animals back out to deeper water. It seems that whenever a whale strands anywhere in the world, it is an event, which never goes by unnoticed.
The sight of a sick or distressed animal struggling in the shallows or stranded high and dry has for centuries evoked much concern and considerable interest. The question that invariably arises, as a result of these incidents is always the same, why does this happen?
Strandings and beachings occur worldwide, the terms often being used interchangeably. However, 'beached' refers to any dead marine mammal that washes up on shore. 'Stranded' refers more restrictively to live cetaceans or sirenians that intentionally swim into, or are unintentionally trapped on shore by waves or a receding tide. (Hofman 1991)
Stranded individuals often appear to be old, sick or injured animals, mass strandings on the other hand, which can include a handful, or at times several hundred animals may be cloaked by more complex causes.
Baleen whales are rare standers, usually doing so alone. It is the odontocetes, or toothed whales who tend to strand 'on mass'. The victims are most commonly pilot whales, false killer whales, dwarf and pygmy sperm whales and the largest of the odontocetes, the mighty sperm whale.
We do not know for certain why these animals strand, but several possible causes have been put forward. Most mass strandings consist of the deep water, odontocete species that are usually found far from shore. It has been suggested that maybe these animals become disorientated when they venture by mistake into shallower waters. Acoustically they are not designed to function in coastal waters and could be confused by echolocation waves received when swimming near gently sloping, sandy shores. Other theories range from fear reactions when being pursued by predators, to poor weather conditions, to herdwide disease. Another possibility lies behind the naturally strong social cohesion and interdependence observed amongst odontocetes. Perhaps the particularly strong social structure finds the group reluctant to leave a sick or injured companion who has ventured to close to shore. When individual animals from a mass stranding are removed from the shore and taken to deeper waters and presumed safety, they invariably turn and head strait back, apparently to rejoin their stricken companions.
Odyssey Scientific Manager, Rebecca Clark, and Science Intern, Ariel Bass, examine the
vertebrae of a sperm whale that stranded in Madang approximately five years
Photo: Chris Johnson
Stranding events have been recorded and observed throughout history and have always been considered to be a natural phenomenon. In recent years, strandings resulting in a number of mortalities have affected a number of marine mammal populations. Studies are showing that perhaps not all strandings are as natural as we first thought and that human impact on the marine environment, such as pollution may be playing a significant role. Toxic metals in the blood of stranded sperm whales and pilot whales have shown concentrations that dramatically exceed levels associated with severe toxicity in other mammal species. It has been suggested that such toxic loads may have an indirect effect on the health status or behavior of cetaceans.
One theory that ties in directly to the research that is being carried out by the Ocean Alliance, is that toxicants are linked with health problems in marine mammals. Scientists from the Queensland Institute of Medical Research theorize that environmental pollution may weaken immune systems, making marine mammals vulnerable to viruses they previously resisted such as encephalitis, an arboviruses that causes a swelling in the brain and may effect their navigation and communication skills. Such viruses were previously thought not to effect marine mammals.
There is also compelling evidence to suggest that man made noise may 'mask' or cover the active or passive acoustic cues used by an animal to navigate.
Whatever the cause for marine mammal strandings, we hope to help solve the mystery with data collected during the Voyage.
This is Genevieve Johnson speaking to you from the Odyssey in Madang, Papua New Guinea.
- Cetacean Societies: Field Studies of Dolphins and Whales
Mann, Connor, Tyack and Whitehead.
- Sperm Whales
- "Virus may explain Whale Beaching" - The Australian, June 18, 2001
Log by Genevieve Johnson