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23 October 2012
Brooke Francisco
WFSC444 University of Arizona
Killer
Whale Predation on Sea Otters and its Effect on Marine Environments
BROOKE N.
FRANCISCO, Author, School of Natural Resources, University of Arizona, Tucson, AZ 85716, USA.
Abstract Sea otter, seal and sea lion
populations have collapsed across much of southwest Alaska over the past
several decades. Many theories have been proposed to explain why this is
happening such as nutritional limitation, and increased ocean temperature.
However, when compared to ecologically equivalent regions, the region
encompassing the western Gulf of Alaska, the Aleutian Islands and the eastern
Bering Sea has displayed high levels of biological productivity and strong
population levels of other species such as piscivourous birds and sea urchins.
It has been proposed that transient killer whales, either hunting individually
or in packs, are responsible for the abrupt decline in pinniped populations. Due
to post World War 2 whale harvesting, a large portion of available prey biomass
decreased, lowering the availability of food for killer whales in the area.
Forced to adapt to their changing environment, killer whales began
incorporating small cetaceans, pinnipeds and sea otters into their diet more
regularly, resulting in a trophic cascade that has ultimately led to kelp
deforestation in the area, as well as an observable decline in seal and otter
populations. This paper will show how anthropogenic influences and ecosystem
alterations have led to an ecosystem collapse in the Gulf of Alaska, and will
propose a management plan that will be useful in restoring the marine ecosystem
and all of the affected species that live there.
Keywords
killer whales, pinnipeds, sea otter, trophic cascade, whaling
History
and Anthropogenic Influence
It has been proposed that killer whales (Orcinus orca), which used to feed
extensively on great whales, recently have expanded their diet to include a
higher percentage of sea otters and pinnipeds (Estes et al. 2009). This was
caused by post- World War II whaling. Modern industrial whaling began in the
late 1940’s as Japan and the Soviet Union began to rely on maritime technology
that developed during the war, which helped foster postwar economic growth (Springer
et al. 2003). Extensive whaling removed more than half a million great whales
from the north pacific (Springer et al. 2003). This extensive industrial
whaling, which targeted great whales, but left killer whales unexploited,
reduced the per capita availability of prey biomass available to the killer
whales feeding in the area (Estes et al. 2009). The ultimate idea is that
anthropogenic impacts, which altered the ecosystem of the Southern Bering Sea and
the North Pacific Ocean, shifted a prey guild food web structure, which changed
the overall diet sources for a high level predator. In response to this change
in food web structure, killer whales expanded their diets to include more of
the smaller marine mammal species in the area, which effectively increased
seal, sea lion, and sea otter mortality to unsustainable levels (Estes et al.
2009).Pinniped populations began to decline in the late 1970’s and in addition
to extensive whaling following World War II, humans have also impacted the seal
and sea lion populations in conjunction with killer whale predation. Not only
are humans partially responsible for the decline in seal, sea lion, and sea
otter populations due to their extensive whaling, they are also guilty of
purposefully shooting seals and sea lions due to their competition with the
fishing industry in the area. Not only did humans shoot (and continue to shoot)
pinnipeds such as Harbor seals, incidental mortality has also occurred over the
past few decades due to incidental take, and accidental mortality due to
fishing gear and catch nets. Although the numbers of seals that have faced
mortality due to purposeful shooting and incidental take are low, they still
add to the ever growing mortality rate of seals in the area, which have proven
to be important to an already dwindling killer whale prey guild (Springer et
al. 2003). However, our anthropogenic influence goes beyond whaling and killing
of seals and sea lions (whether it be purposeful or accidental). After the
decline of the seal and sea lion populations in the area, killer whales began
relying on even smaller marine mammal species and turned to the sea otter for
nutrition. This nutritional limitation has far-reaching effects and ultimately
creates a trophic cascade, and has effectively reduced ecosystem production and
marine mammal populations to unsustainable levels. (Estes et al. 2009).
Energetic
Requirements of Killer Whales and its Effect on the Ecosystem
It has been shown that anthropogenic
influences and practices have effectively reduced great whale populations in
the Gulf of Alaska and surrounding areas. This in turn has led to a collapse of
populations of sea otters, seals, and sea lions across much of southwest Alaska
over the past several decades (Estes et al. 2009). It has been discovered that
recent killer whale predation (due to lack of great whale biomass) is to blame
for this sharp decline. A dramatic and sudden change in the food web of the Alaskan
ecosystem has begun to define and illustrate trophic linkages among the species
that live there. At current population levels neither great whales alone nor
smaller marine mammals can effectively sustain a large population of killer
whales. Under these implications, it is even more unrealistic for a small prey
population of seals, sea lions and sea otters to sustain the transient killer
whales unless the cause of death is greater than 50% (predation) for all of the
aforementioned species (Estes et al. 2009). Research done over the years by the
NOAA (National Oceanic and Atmospheric Administration) has estimated that there
are about 400 predators in the area. These killer whales have been identified
and monitored over time using tail fluke identification. Even if 50% of all
small marine mammals are killer whale food, per the requirement mentioned
above, 20-40% of great whale deaths (including those achieved during the post
WW2 industrial whaling era) would be needed to sustain the 400 predators in the
area (Estes et al. 2009). Coupled with our anthropogenic history of whaling and
seal and otter population declines, it is clear that we are reaching an
unattainable level of ecosystem balance. After the decline of seals and sea
lions began in 1970, killer whales turned to another form a prey in order to
meet their nutritional requirements, the sea otter. Killer whale predation on
this species has been absolutely detrimental to their population and
reproduction rates. Studies were conducted on radio-tagged sea otters at
Amchitka Island in 1992-94 and Adak Island from 1995-96. The impacts of killer
whales on sea otter populations was evaluated in its entirety at Adak Island.
The experiment contrasted otter population trends and survival rates between
two regions: Clam Lagoon and Kuluk Bay. Clam Lagoon is an area that is uniquely
inaccessible to killer whales, however Kuluk Bay is an open coastal environment
and is home to several killer whales. It was documented that sea otter numbers
were stable from 1993-1997 in Clam Lagoon, whereas in Kuluk Bay, they declined
by 76% (Estes, Springer et al, 2009). The first attack on a sea otter by a
killer whale was observed in 1991, which is very recent in the realm of
scientific research and discovery. Over a span of 4 years, killer whales were
able to almost entirely wipeout sea otter populations in the area. This shows
that sea otters are not a sustainable food source for killer whales, and
without a stable seal, sea lion, and great whale populations in the area, their
nutritional requirements will continue to be pushed. Prior to the onset of the
seal and sea lion decline, an estimated 52,656 sea otters inhabited this area.
These life table statistics were estimated and formulated from data that was
collected during earlier field studies and natural observations. Using the life
stable statistics documented, scientists were able to construct a Leslie matrix
for a stationary population size of otters (Estes et al. 2009). In addition to
achieving a stationary population size, researchers were also able to construct
and formulate an age constant death rate, which was then added to killer whale
predation rates. The results were staggering, and showed that killer whale
predation was sufficient enough to reduce the population by 78 percent of the
six years the magnitude was observed to decline at the Adak station.
Essentially, two data sets have demonstrated that killer whales are entirely
capable of pushing the sea otters to the brink of extinction, something they
already had to face in light of the 17th and 18th century
fur and trap industry (Springer et al. 2003). To sum up this section, and to
provide more insight into the severity of the situation, from the energetic
requirements of free-ranging killer whales, and the caloric value of sea
otters, Estes and Springer estimated that a single killer whale could consume
1825 otters per year, therefore, the sea otter population decline could have
been caused by as few as 3.7 killer whales (Springer et al. 2003).
Trophic
Cascade and Further Implications
It has been demonstrated that killer whale
predation has detrimental effects on sea otter populations. However, the
decline in population rates and growth of sea otters is not all that has
scientists and researchers worried. Sea otters are associated with kelp forest
ecosystems, which in light of the sea otter’s recent absence, have been
transformed into deforested sea urchin barrens (Estes et al. 2009). Not only
have kelp forests disappeared, biological productivity and species associated with
kelp forests have disappeared or have been lowered with it. Sea otters rely on
sea urchins as a food source. By preying on sea urchins, sea otters initiate a
trophic cascade in which kelp is enhanced and favored. Thus, ecosystems that
lack otters are deforested by sea urchin grazing, which means that sea otter
populations check sea urchin populations (Estes et al. 2009). Although kelp may
not seem too important to an ecosystem, it actually has far reaching effects
when it comes to increasing production. The presence of kelp increases the
habitat structure from a two-fold system to a three dimensional habitat, and it
also attenuates water motion from waves and currents (Estes et al. 2009). In
the research done by Springer and Estes, many other species interactions were
halted, prevented, or altered in the absence of sea otters. Data and
observations showed that when sea otters are lost from a system, the growth
rates of filter feeding barnacles and mussels decline two-fold, rock greenling
population rates (a common kelp forest fish) decline ten-fold, and even the
diet of bald eagles is altered from approximately equal parts of fishes, birds
and mammals to domination by marine birds (Spring et al. 2003). The changes
such as those observed in the bald eagle’s diet demonstrates that the absence
of sea otters initiates a cascade in which kelp, mussels, piscivourous birds,
barnacles, fish, and small marine mammals are all affected. It must be noted
and re-enforced that all of the aforementioned ecosystem and species
alterations are the results of the killer whales shift in diet from a
combination of great whales and seals to a diet that focuses on seals and small
marine mammals such as sea otters. Before we go into management implications
and conservations efforts, it must also be noted that the initial momentum
given to this trophic cascade began with humans hunting and killing great
whales in the post-World War 2 industrial whaling era. In order to correct and
redirect our somewhat controversial past anthropogenic practices, we must come
up with anthropogenic solutions that will increase and restore productivity in
the waters surrounding Alaska.
Management
Implications, Ecosystem Concerns and a Look to the Future
Many scientists
and researchers deny that increased predation by marine-mammal eating killer
whales is the cause of the sea otter declines. Some have attributed the decline
to reduced fertility or redistribution by which the otters simply relocate to
another place. However, research eliminates reduced both of these ideas.
Studies of radio-tagged otters at Amchitka Island in 1992-94 and Adak Island in
1995-96 show that birth rates of adult females and pup survival rates from
birth to weaning were similar to those of stable populations. Furthermore,
redistribution can also be eliminated on the bases that the declines were
synchronous over large area, meaning that there have been no populations
buildups on other islands to account for the losses on others. In addition, the
radio-tagged otters provided no indication of redistribution during the peak of
the declines (Estes et al. 1996). The only logical thing causing the population
decline of sea otters is increased mortality. This observed increased mortality
can be directly attributed to killer whale predation in many areas surrounding
the Gulf of Alaska. This recent prey adaptation should concern and worry
wildlife managers across the globe. Killer whales and sea otters have
co-inhabited the west central Aleutian archipelago for most of the past half
century, and probably for millennia before that. Therefore, it is necessary to
explain why the behavior of killer whales towards sea otters has recently
changed, and what we as managers can do to restore the original linkage of this
particular ecosystem (Estes et al. 1996). It has been effectively shown that
the original cause of the killer whale’s diet shift can be attributed to
anthropogenic influences including the killing of whales post-World War 2, as
well as incidental take and purposeful shooting of seals and sea lions. In
order to ensure long-term survival of all the species involved in this
ecosystem interaction (seals, sea lions, sea otters and killer whales), habitat
restoration must be considered, especially when it comes to the prey base of
the killer whale, which is a high level predator that relies on a particular
habitat that can support great whales, pinnipeds and sea otters. Although we
cannot undo our past and reverse our anthropogenic effects of whaling, we can
set in motion an effective conservation plan under which all species can
flourish. First, we must reduce the mortality rates of seals, sea lions and sea
otters. To do this, purposeful shooting due to competition issues, as well as
incidental take should be monitored, regulated, and fined. Small marine
mammal-safe nets can be used to ensure that only the targeted species of fish
are removed from the ecosystem. Furthermore, we must encourage a sustainable
population of great whales. Whether it be an open-ocean captive breeding area,
or simply protecting the great whales that are left in the area, great whales
must remain a viable food choice for killer whales in the Gulf. It was shown
earlier that neither great whales nor small marine mammals alone can sustain a
killer whale’s energetic and caloric requirements. With that being said, it is
obvious that both species must be protected and monitored if killer whales are
to persist. The declines of these populations are of substantial concern to
conservation management. The Alaskan marine system is unique and can be used to
address questions of causality and can also test the strength of possible
ecosystem linkages in the face of human alterations (Estes et al. 1996). By
using the Alaskan system as a model, we can gain information that can help
understand linkages between ecosystem quality and food-web dynamics.
Literature
Cited
Estes,
J.A., Doak, D.F., Springer, A.M., Williams, T.M. 2009. Causes and Consequences
of Marine Population Declines in
Southwest Alaska; a Food-Web Perspective. Physical Translations of the Royal
Society. 364:1647:1658.
Estes,
J.A., Doak, D.F., Tinker, M.T., Williams. 1996. Killer Whale Predation on Sea
Otters Linking Oceanic and Nearshore Ecosystems. Science. 282:5388.
