Mass epidemics of viruses and parasites have resulted from a warmer climate, pollution and human activities such as sea farming, the scientists warn in a report which is published today. They believe there are dire consequences for the future diversity of marine life if the outbreaks of disease persist and so continue to cause mass deaths within the oceans.
Just 30 years ago, scientists believed that the oceans were so vast that they would remain relatively immune from human influence, but recent evidence shows that the seas have become as vulnerable as the terrestrial environment.
Some of the world's most distinguished marine researchers have highlighted a series of disturbing reports over the past two decades that indicate an ever-growing threat of infections. These have resulted in the mass mortality of fish, sea-mammals, coral reefs and sea water plants. In a major review article in the journal Science, they point to a continuing increase in the number of reported outbreaks of disease, the worrying shift in infections from one host species to another and the emergence of new diseases due to the human exploitation of the sea.
They cite 34 instances of mass mortality since 1938 where 10 per cent or more of a population had been wiped out. All but seven occurred in the past 20 years. "In the past few decades, there has been a world-wide increase in reports of diseases affecting marine organisms," the authors of the Science article say.
Many other outbreaks have probably gone unnoticed or unreported, warn the 13 biologists whose expertise ranges from microbiology and ecology to immunology, oceanography and epidemiology. Drew Harvell, associate professor of ecology and evolutionary biology at Cornell University in Ithaca, New York and a lead author of the report, warned of the potentially catastrophic dangers posed by marine pathogens - viruses, bacteria, fungi and other parasites.
"The combined effects of rising temperatures, human activity and pollution are producing a volatile mix that may threaten tropical corals and temperate species alike," said Professor Harvell, an authority on coral reefs.
Across the globe, dramatic examples have emerged to illustrate the threats posed by the breakdown of natural barriers that controlled the spread of marine pests. The waters of the Caribbean are now highlighted by the scientists as a toxic "hot spot", where mass mortalities have resulted in a dramatic breakdown in the delicate web of marine life.
For instance, an outbreak in the Eighties of a mysterious pathogen virtually wiped out the dominant sea urchin of the Caribbean, which the authors describe as a "keystone herbivore". The sea urchin's near extinction resulted in the replacement of coral reefs in the area with algal growths.
At the same time other, unknown, infectious agents wiped out some of the most common species of marine organisms in the region, notably 4,000 hectares of turtle grass seaweed in Florida Bay. "Of the dozen or so coral diseases currently described for the Caribbean region, the identity of the causative agent is known only for three; nonetheless, the severity and novelty of many of the disease symptoms suggest that the diseases are indeed new," the scientists say.
They highlight their concerns about the cross infection of marine species directly due to human influence. The rare freshwater seals of Lake Baikal in Siberia are known to be infected with the distemper virus of domestic dogs, for example.
Other seal populations - the harbour seals and grey seals of the seas around North-western Europe - also died in their hundreds as a result of becoming infected with the viruses of dolphins and porpoises. The viruses were transmitted to the seals by infected harp seals which had migrated to Europe from their normal hunting grounds off Greenland, which had themselves been decimated by overfishing.
Further examples of pathogens jumping from one species to another are cited by the scientists, such as influenza in birds causing mass deaths in seals and whales, and the infection of sea fan corals with a soil fungus - indicating the breakdown of the land-sea barrier, the scientists say. Warmer seas and chemical pollution are lowering the natural resistance of marine organisms to infections. For instance, higher temperatures caused European harbour seals to group together on land in bigger colonies, making them more vulnerable to infections.
The El Nino event - a warm water current in the Pacific Ocean - has become more frequent in recent years, causing coral reefs to become bleached as a result of the death of vital algae on which the coral depends. "The coral bleaching of 1998 was the most geographically extensive and severe in recorded history, causing significant mortality world-wide ...
"The demise of some corals is likely to have been accelerated by opportunistic infections," the scientists say.
Pollutants, such as organochlorides, are thought to be implicated in the mass mortality of Mediterranean monk seals off the coast of Mauritania, which died after becoming infected with a distemper virus of dolphins. Direct human activity is also taking its toll. The scientists cite the example of a virus brought to Anxious Bay in South Australia. The virus infected frozen pilchards which were used to feed the blue-fin tuna farms of the area.
The inability of science to even identify many of the diseases in the oceans means there is an "urgent need" to study the problem across many academic disciplines, the scientists say.