Chapter VII 



MARINE MAMMAL STRANDINGS AND DIE-OFFS 



Since the late 1970s there has been an apparent 

 increase in the incidence of unusual marine mammal 

 mortalities throughout the world. These incidents 

 have involved a broad range of species in widely 

 separated geographic areas, including monk seals in 

 the Northwestern Hawaiian Islands, harbor seals and 

 humpback whales in New England, sea lions in 

 California, manatees in Florida, and bottlenose 

 dolphins along the east and Gulf coasts of the United 

 States. The largest and most publicized events were 

 the deaths of more than 700 bottlenose dolphins along 

 the U.S. mid-Atlantic coast in 1987-1988, more than 

 17,000 harbor seals in the North Sea late in 1988, and 

 more than 1 ,000 striped dolphins in the Mediterranean 

 Sea in 1990-1991. 



As noted in previous Marine Mammal Commission 

 reports, a number of these mass mortalities appear to 

 have been caused by a morbillivirus, congeners of 

 which cause distemper in dogs and measles in hu- 

 mans. The etiology of the disease is uncertain; i.e., 

 it is not known whether cetaceans and pinnipeds have 

 been exposed to the virus only recently and thus have 

 no acquired immunity to it; whether more virulent 

 forms of the virus have evolved; whether animals in 

 the affected populations have been stressed in ways 

 that compromise their immune systems; or whether 

 there simply is increased awareness and better means 

 for detecting such viruses. 



High levels of a number of environmental contami- 

 nants were found in the blubber, liver, and other 

 tissues of some, but not all of the bottlenose dolphins 

 and striped dolphins that died during the unusual 

 mortality events. These contaminants may have 

 affected the animals' immune systems and made them 

 more vulnerable to the virus. Available information 

 is insufficient, however, to determine how, or at what 

 levels and in what combinations, environmental 

 contaminants may compromise the immune systems or 

 otherwise affect marine mammals. 



Unusual Mortality Events in 1995 



There were two reported and one possible unusual 

 marine mammal mortality events in 1995. In addi- 

 tion, there were indications that some populations of 

 cetaceans and pinnipeds in the Pacific have come into 

 contact with morbillivirus and other disease-causing 

 organisms, perhaps for the first time. These matters 

 are described below. 



Common Dolphins 



Early in February 1995 more than 200 common 

 dolphins (Delphinus delphis) were found dead on 

 beaches and floating offshore along the northwest 

 coast of the Gulf of California (Sea of Cortez), 

 Mexico. Many dead seabirds also were found. 

 Cyanide compounds were found in liver and lung 

 samples taken from the dead dolphins for toxicological 

 analysis, suggesting that the mortalities may have been 

 caused by cyanide poisoning. A possible source of 

 the poison was not identified. 



Sea Otters 



Between 16 and 22 July 1995 ten dead sea otters 

 were found in Monterey Harbor and the Del Monte 

 Beach area of California. A decomposed carcass of 

 another otter, thought to have died the preceding 

 week, was found on 25 July. The cause of this 

 unusually high sea otter mortality could not be deter- 

 mined. Gross necropsies and analyses of tissue 

 samples collected from the dead otters found no 

 evidence of consistent gross lesions, naturally occur- 

 ring biotoxins, or unusually high levels of environ- 

 mental contaminants (e.g., chlorinated hydrocarbons 

 and heavy metals). 



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