Marine Mammal Mass Mortality
- 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.
- 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
- 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
- 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
- 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.
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