RNR 2031 Chapter : Estes
Estes, J.A., M.T. Tinker, T.M. Williams, and D.F. Doak. 1998. Killer
whale predation on
sea otters linking oceanic and nearshore ecosystems. Science 282:473-
476.
This article takes place throughout the 1990’s around western Alaska
and deals with the decline in sea otter populations. In the middle to late
1800’s sea otter’s were over harvested to near extinction for their hides
and by the 1970’s recovered to maximum densities. Sea otters play an
important role in oceanic trophic systems. They eat herbivorous sea
urchins; an animal that left unkempt would destroy kelp forests. Surveys
were taken at various sites that were inhabited and uninhabited by sea
otters. The uninhabited locations had less biodiversity due to the loss of
kelp forests while the inhabited sites promoted a growth in trophic webs;
bring trophic systems from 2 level systems to a 3 level system.
Throughout the 1990’s sea otter populations began to decline by 25% a
year. Radio tagged otters were studied and reduced fertility and
redistribution were ruled out as factors causing the decline concluding
that increased mortality from killer whale predation was the main cause.
Areas where killer whales could not reach showed a higher survival rate
and larger populations of otters than did areas that were accessible to
killer whales. Disease, toxins and starvation were ruled out as factors
since there were very few beached carcasses found and plenty of sea
urchins were found in declining areas. The article estimated that one
killer whale eats up to 1825 otters in one year meaning that the decline
in sea otters could be caused by only 3.7 whales. Killer whales normally
eat sea lions and harbor seal and have coexisted with sea otters for much
of the past half century. They think that due to the decline in lion and
seal populations, killer whales have shifted their diet towards the sea
otter. The reason for this shift is speculated to be from reduced or altered
forage fish stocks in the oceanic environment. These fish stocks were
prey to the sea lions and seals and with their alteration/reduction sent
