Why Do Whales And Dolphin Strand?
A. Single Strandings:
Live (or freshly-dead individuals) are probably found on the shore because they are old, sick, injured and/or disorientated. Dead individuals washing ashore could be the result of natural mortality or, perhaps, were drowned in nets. (Bodies sometime carry the characteristic marks of nets or even have pieces of rope or netting attached).
B. Multiple Strandings:
Live (or freshly-dead) animals of the same species coming ashore in a group typically belong to those species that have a “lead animal” and very tight social cohesion. Pilot whales are a good example. Usually when they strand it appears that either a lead animal has made a navigational mistake or one individual has become sick or wounded and led the rest of its pod onto the shore.
Such animals can also be deliberately driven ashore in a group, as in the case of the cruel pilot whale hunt in the Faroe Islands.
Disease can also cause animals of the same species to come ashore. There have been a series of recent mortality episodes in dolphins in Europe and the UK, for example, during which sick, dead and dying dolphins have stranded.
If groups of animals of different species strand together, this might indicate that they had been schooling together out at sea before they stranded. Mixed species strandings might also show that some major perturbation has occurred out to sea affecting a wide area and driving animals ahead of it to strand. This seemed to be the case in the Canary Islands a few years ago when a stranding of beaked whales of several species coincided with naval manoeuvres offshore.
Dead whale and dolphin bodies coming ashore in unusual numbers - either as one species or more - have often been found to be the result of interactions with fisheries (these animals are known as "bycatch").
If species other than whales and dolphins are also involved, for example fish or marine invertebrates, or if many different species come ashore together, an acute event such as a chemical spill or explosion may be to blame.
Some types of shore and some particular coast lines are more prone to strandings than others. Shallow, sloping shores made of soft sediments may be expected to confound the “echolocation” used by cetaceans to find their way around and strandings are indeed particularly common on such coastlines.
As indicated above, a combination of factors may cause whales and dolphins to strand and one theory to explain some strandings relates to the fact that they may be navigating using the earth’s magnetic field. Crystals of magnetite - which react to a weak magnetic field - have been detected in the brains and skulls of some whales and dolphins and a magnetic “sense” could be an important navigational aid, especially in the deep oceans. An analysis of strandings around the UK has found that live strandings occur more often on those unusual shores where lines of equal magnetic force meet the coastline perpendicularly. In other words, the dolphins or whales are disoriented by these abnormalities and follow them ashore.
Stranded animals, both dead and alive, can give important indications of the state of the population offshore. It is important that live animals are responded to appropriately. In many cases, they may be in distress and too badly injured or too ill for recovery. Rescuers need, therefore, to be prepared for the worst, as euthanasia is often required for such animals. It is also important that dead bodies, wherever possible, are subjected to a full post-mortem.
Between August 1990 and September 1996 necroscopies were conducted on 515 cetacean carcasses of 13 species stranded around the coasts of England and Wales. More than half of these were harbour porpoises, and bycatch was diagnosed (where a cause of death established) in 94 of these animals and in 80% of common dolphins. Live stranded harbour porpoises tended to be seriously diseased whereas live strandings of other species were often healthy. (Bycatch and other causes of mortality in cetaceans stranded on the coasts of England and Wales 1990-1996. Jepson, P.D. et al).
Mayer, S. 1996. A review of live strandings of cetaceans: implications for their veterinary care, rescue and rehabilitation in the UK. (download the PDF file) A report for the Whale and Dolphin Conservation Society April 1996: 57 pages.
Klinowska, M. 1988. Cetacean navigation and the geomagnetic field. J. Navigation 41(1): 52-71.
Simmonds, M.P. The Meaning of Cetacean Strandings. Bulletin de l’ Institut Royal Des Sciences Naturelles de Belgique. Biologie, 67-Suppl.: 29-34.