Chapter V 



MARINE MAMMAL STRANDINGS AND DIE-OFFS 



There appears to have been an increase in the 

 incidence of unusual marine mammal mortalities in 

 the past 15 to 20 years. There also appears to have 

 been an increase in unexplained marine mammal 

 population declines, such as those involving sea otters 

 off Adak Island, Alaska, where numbers dropped 

 from approximately 1,800 animals in 1994 to about 

 400 in 1996, and along the California coast, where the 

 resumption of sea otter population growth that fol- 

 lowed the ban on coastal gillnet fisheries in the mid- 

 1980s has now stopped. (See the discussions in 

 Chapter II on the continuing declines of sea otters, 

 Hawaiian monk seals, and Steller sea lions.) Further, 

 there appears to have been a general increase in the 

 number of marine mammal strandings in some coastal 

 areas. For example, the number of dead marine 

 mammals found on beaches in the southeastern United 

 States has doubled since the mid-1980s although this 

 may merely reflect better reporting (see Figure 14). 



Unusual marine mammal mortality events in the 

 United States over the past two decades have involved 

 a broad range of species in widely separated geo- 

 graphic areas. They include monk seals in the North- 

 western Hawaiian Islands, harbor seals and humpback 

 whales in New England, sea lions in California, 

 bottlenose dolphins along the east and Gulf coasts of 

 the United States, and manatees in Florida. World- 

 wide, the largest and most publicized events in the 

 past decade 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, more than 1,000 striped 

 dolphins in the Mediterranean Sea in 1990-1991, 

 about 150 manatees along the southwestern coast of 

 Florida in 1996, perhaps as many as 200 Mediterra- 

 nean monk seals off the northwestern coast of Africa 

 in 1997, and more than 1,600 New Zealand (Hook- 

 er's) sea lion pups on Auckland Island rookeries in 

 1998 (see below). The Florida manatee, Mediterra- 

 nean monk seal, and New Zealand sea lion die-offs 



demonstrate the devastating impact that unusual 

 mortality events can have on marine mammal species 

 threatened with extinction. 



Several of these mass mortality events appear to 

 have been caused by a morbillivirus, congeners of 

 which cause distemper in dogs and measles in hu- 

 mans. It is not known whether cetaceans and pinni- 

 peds have only recently been exposed to the virus, 

 and thus have no acquired immunity to it, or whether 

 more virulent forms of the virus have evolved. 



Furthermore, it is not known whether animals in 

 affected populations had been stressed in ways that 

 could compromise their immune systems or whether 

 there are simply better means now for detecting both 

 viruses and unusual mortality events than in the past. 

 Morbillivirus infections are now recognized to be 

 common in a wide range of marine mammal species 

 and populations in many areas although there is little 

 or no evidence of associated serious illness in most 

 instances. 



High levels of certain environmental contaminants 

 were found in the blubber, livers, and other tissues of 

 some of the bottlenose dolphins and striped dolphins 

 that died during the unusual mortality events noted 

 above. 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, at what 

 levels, or in what combinations, environmental con- 

 taminants may compromise the immune systems or 

 otherwise affect marine mammals. As described in 

 Chapter VI, the Commission, the National Marine 

 Fisheries Service, the Environmental Protection 

 Agency, and the National Fish and Wildlife Founda- 

 tion jointly sponsored a workshop on contaminants in 

 marine mammals in October 1998 to better document 

 and determine how to resolve the most critical uncer- 

 tainties. 



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