Marine Mammal Mass Mortality Events

The Issue

• Over the past two decades, there has been an unprecedented number of marine mammal mass mortality events involving pinnipeds, dolphins, whales, manatees and dugongs. Some of the species impacted are endangered -- the Florida manatee, the humpback whale, and the Mediterranean monk seal -- and can ill afford such dramatic population losses.
• In U.S. waters there has been a number of bottlenose dolphin mortality events, including the death of over half of the inshore population along the east coast during 1987-88, four such occurrences in the Gulf of Mexico between 1990 and 1994 (each involving the deaths of at least 200 animals), and 107 animals in an event along the Florida Panhandle in 2004. Sixteen humpback whales were found dead along the Maine coast in an event in 2003. Over 150 manatees died in a major die-off during the early part of 1996 while another die-off in 2003 claimed 96 animals. Two hundred and eighty-three and 368 gray whale strandings were reported along the North American Pacific coast in 1999 and 2000 respectively, exceeding by far the number of strandings in previous years. Over 400 California sea lions died in an algal toxin-related event in 1998.
• Globally, recent marine mammal mass mortality events include, for example, the death of over 18,000 seals in Europe during 1988/89 and a further 22,000 in 2002; some 6,000 striped dolphins in the Mediterranean Sea from 1990 to 1992; three large mortality events between 1995 and 2004 involving sea lions and dolphins in the Gulf of California; well over 100 dugongs off eastern Australia in 1992; and over 150 highly endangered monk seals off the northwest African coast during the earlier part of 1997. In addition, an unprecedented number of sperm whales stranded along European coasts from 1994 through 1996.

The Causes

• Disease caused by morbilliviruses -- from the same family of viruses that causes distemper in dogs or measles in humans -- has been the main factor in the mass mortalities of European harbor seals, Mediterranean striped dolphins and monk seals, and in at least three of the U.S. bottlenose dolphin mortality events. Their sudden appearance as causative agents in marine mammal mass mortalities is surprising. However, changes in ecological conditions or the impact of other stress agents may also be implicated.
• Chemical contaminants have been found in high levels in some of the afflicted populations-notably the European seals and the Mediterranean and US. dolphins-suggesting that pollution may have compromised the animals' immune system making them more vulnerable to infection or reducing their ability to recover. Studies have shown that some marine mammal species, including bottlenose dolphins in U.S. waters, do indeed have decreased immune system function as a result of exposure to such chemicals as PCBs and DDT.
• Algal toxins have also been implicated in, among others, a mortality event of Hawaiian monk seals in 1980, in Florida manatee die-offs during 1982, 1996 and 2003, in humpback whale deaths during 1987/88 and 2003 and may have been involved in the U.S. east coast dolphin mortality of 1987/88 as well as in recent events in the Gulf of California. Though clearcut evidence of biotoxin involvement is lacking in some cases, there is rising concern that the recent increase in the frequency, duration, intensity, and geographic distribution of harmful algal blooms, or "red tides," along various coastal environments will increase the risk of marine mammal poisoning.
• Climatic conditions such as El Nino have resulted in the starvation of U.S. west coast pinnipeds and birds because of changes in prey availability; the mass mortality of dugongs in Australia was a result of starvation due to the loss of seagrass beds from storms and floods. Similarly, dolphins involved in the Mediterranean die-off may have been weakened, and thus left more vulnerable to disease agents, because of reduced food availability due to warmer climatic conditions.

The Context

• The spate of major mortality events in marine mammal populations over the past 20 years has coincided with an increase in reported events involving diseases and die-off occurrences among a variety of other marine species.
• As seemingly minor ecological changes can lead to the rapid proliferation of disease, there is obvious concern over the role of human activities-such as fishing, nutrient and chemical pollution, or impacts on the ozone layer and climate. The documented involvement of "red tide" toxins and potent toxic chemicals in some marine mammal mortality events are areas of increasing concern.
• Though superbly adapted to their environment, marine mammals are still susceptible to natural environmental stresses such as pathogens, El Nino occurrences and other climatic conditions, and fluctuations in prey availability. Robust populations are a buffer to environmental changes and disease events; however, many marine mammal populations are not only much reduced from historical abundances but now face a compounding range of stresses associated with human activity.

Further Reading

Gulland, F.M.D. and Hall, A.J.  2007.  Is marine mammal health deteriorating? Trends in the global reporting of marine mammal disease.  EcoHealth 4(2): 135-150.

Harvell, C.D. et al.  1999.   Emerging marine diseases - Climate links and anthropogenic factors.  Science 285(5433): 1505-1510.

Härkönen, T. et al.  2006.  A review of the 1988 and 2002 phocine distemper virus epidemics in European harbour seals.  Diseases of Aquatic Organisms 68(2): 115-130.

Jepson, P.D. et al.  2003.  Gas-bubble lesions in stranded cetaceans - Was sonar responsible for a spate of whale deaths after an Atlantic military exercise?  Nature 425(6958): 575-576.

Le Boeuf, B.J. et al.  2000.  High gray whale mortality and low recruitment in 1999: Potential causes and implications.  Journal of Cetacean Research and Management 2(2): 85-99.

Scholin, C. A. et al.  2000.  Mortality of sea lions along the central California coast linked to a toxic diatom bloom.  Nature 403(6765): 80-84.