The Davis Strait - DCE - Nationalt Center for Miljø og Energi

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eastern edge of the Davis Strait pack ice. Marine mammals common in the
area during summer include harp seal, hooded seal, ringed seal, harbour
seal, fin whale, humpback whale, minke whale, sei whale, harbour porpoise,
white beaked dolphin, bottlenose whale, sperm whale, and pilot whale. Blue
whale occurs only rarely in the assessment area, but is vulnerable due to a
very small population and the survival of single individuals is important for
the recovery of the population.
Assessing oil-related mortality of marine mammals is difficult as carcasses
are rarely found in conditions suitable for necropsies. Nevertheless, increased
mortality of killer whales, sea otters and harbour seals exposed to
the Exxon Valdes event in Prince William Sound has been well documented
(e.g., Spraker et al. 1994, Matkin et al. 2008). In the Gulf of Mexico, the rate of
stranded cetaceans increased after the Deepwater Horizon event in 2010,
from a 2003-2007 mean observed rate of 17 standings per year to 101 in 2010.
Both numbers are expected to represent only a small fraction (approx. 2%) of
the true death toll (Williams et al. 2011).
The banks on the shelf of the assessment area are important feeding grounds
for seals and baleen whales. If the prey species are contaminated with toxic
substances after an oil spill this may affect the top-predators relying on this
feeding area.
11.2.9 Long-term effects
A synthesis of 14 years of oil spill studies in Prince William Sound since the
Exxon Valdez spill has been published in the journal ‘Science’ (Peterson et
al. 2003), and here it is documented that delayed, chronic and indirect effects
of marine oil pollution occur. Oil persisted in certain coastal habitats beyond
a decade in surprisingly high amounts and in highly toxic forms. The oil was
sufficiently bio-available to induce chronic biological exposure and had
long-term impacts at the population level. Heavily oiled coarse sediments
formed subsurface reservoirs of oil, where they were protected from loss
and weathering in intertidal habitats. In these habitats e.g. harlequin ducks,
preying on intertidal benthic invertebrates, showed clear differences between
oiled and unoiled coasts. On oiled coasts they displayed the detoxification
enzyme CYP1A nine years after the spill. Harlequin ducks on oiled
coasts displayed lower survival, their mortality rate being 22% instead of
16%; body mass was smaller; and they showed a decline in population density
as compared with stable numbers on unoiled shores (Peterson et al.
2003). The oil still lingers in the environment and both the harlequin duck
and other populations of coastal birds are still assessed as ‘recovering’
(NOAA 2010).
Long-term chronic effects of oil on marine mammals can include decreased
survival and lowered reproductive success (NOAA 2011b). In the first year
after the 1989 Exxon Valdez spill, a well-known group of local killer whales
experienced a 41% loss; there has been no reproduction since the spill
(Matkin et al. 2008). The cause of the apparent sterility is unknown, but this
case shows that immediate death is not the only factor that can lead to longterm
loss of population viability.
Many coasts in the assessment area in West Greenland have the same morphology
as the coasts of Prince William Sound, where oil was trapped. This