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Goal of Research
In the last century, the average global temperature has increased by 0.6ºC and the
warming is even greater in some areas such as the Polar Regions. The average atmospheric
temperature in Antarctica has increased 0.65ºC just in the last 50 years, with some regions, such
as the Antarctic Peninsula, increasing 3.5ºC during that period (IPCC 2001). The warming of the
Southern Ocean is showing a smaller, but similar trend with a mid-depth temperature increase of
0.17ºC between the 1950’s and 1980’s (Gille 2002). The continued increase in the global
temperature will have an unknown impact on the Southern Ocean ecosystem. Marine mammals
are a prominent apex predator in the Southern Ocean and play a major role in the ecosystem.
The foraging behavior of marine mammals must be able to accommodate seasonal
environmental fluctuations that cause significant variation in prey abundance and location. To
understand the foraging strategies utilized by marine mammals it is necessary to have
information on the spatial and temporal variation of their environment, in addition to information
on how organisms respond to these changes. Such information will allow us to determine how
the seals are using oceanographic features to find prey. Furthermore, variations in
oceanographic features are coupled with climate driven processes. I will investigate, in southern
elephant seals (Mirounga leonina) off the Antarctic Peninsula, the relationship of specific diving
behavior and animal movements to oceanographic and bathymetric features, develop and test
models of these features’ role in defining habitat use, and predict how these animals might
respond to oceanographic variations that are coupled with climate change. Although a three year
study may not document changes in foraging behavior associated with global climate change, the
data collected can be used to make predictions about how they might respond based on their
response to annual variation.
Rationale
Marine apex predators forage in areas where oceanographic features such as currents,
frontal systems, thermal layers, sea mounts and continental shelf breaks help to increase prey
availability (Boyd et al. 2001, Field et al. 2001). In the Southern Ocean additional features, such
as icebergs, local eddies, and characteristics of the marginal ice zone contribute to predator
abundance (Veit et al. 1993, Ichii et al. 1998, Bornemann et al. 2000). All of these features are
thought to increase foraging efficiency by physically forcing prey to aggregate (van Franecker
1992, Veit et al. 1993). Our understanding of these associations is limited to population level
studies that have found correlations between animal abundance and oceanographic features. The
current advances in technology of satellite telemetry, dive recorders, and remote sensing
methods, allow us to identify specific diving behavior and relate it to different environmental
conditions. This study will use location and diving behavior data, animal derived water column
temperature and salinity data, and remotely sensed oceanographic data to expand our
understanding of how seals are using oceanographic and bathymetric features.
Understanding the link between oceanographic features and foraging behavior is essential
if we are going to be able to predict the response of pelagic organisms to global climate change.
For example, the warming of the Antarctic Peninsula appears to have resulted in a shift in
penguin habitat. Adelie penguins in the northern Antarctic Peninsula are moving south and
decreasing in numbers while the Chinstrap penguins are increasing in numbers as they are
moving south (Smith et al. 1999). The mechanism of this shift remained a mystery until winter
foraging behavior data was collected for the two species. Although there is no difference in
foraging behavior between the two species during the summer, during the winter Adelie
penguins primarily forage in the winter pack ice while Chinstrap penguins forage in open water
(Fraser et al. 1992). The decrease in seasonal pack ice associated with the warming of the
peninsula would mean less wintering habitat for the Adelie penguins and may be responsible for
the decrease in numbers. In addition to changes in winter pack ice, the climate change has more
far-reaching effects such as the increase in summer wind stress in the region of South Shetland
and Elephant Islands (US Antarctic Marine Living Resources program - AMLR). Changes in
wind stress modifies local currents, eddies, and frontal features that in turn affect the foraging
behavior of marine predators that rely on these features to concentrate their prey.
Southern elephant seals may be experiencing a shift in habitat use similar to that found in
the penguins. In 1983 four elephant seals gave birth at Palmer Station (Anvers Island, 64º46’S
64º03’W), much further south than their usual sub-Antarctic breeding grounds (Heimark and
Heimark 1986). This was the first time elephant seal births were recorded at Palmer Station. It
was originally attributed to an unusually warm summer but now a small number of elephant seal
regularly give birth at Palmer Station (Fraser, pers comm). Additionally, although some
populations of elephant seals are stable or increasing, others, like the Macquarie Island
population, have been declining since the 1950’s (Slip and Burton 1999). The influence of
oceanographic variation on prey abundance and distribution may be important in these regional
differences.
Southern elephant seals are one of the top predators in the southern ocean. They spend
most of their lives at sea but are tied to land for reproduction and for the annual molt. Elephant
seals demonstrate high site fidelity with most seals returning to the same rookery every year and
sometimes even the same beach. At Cape Shirreff adult females come ashore to give birth
between October and November and depart after approximately one month on the beach. In
January and February, after approximately 2 months at sea, females return for the annual molt.
Once again, after a month on shore they return to sea for their long foraging trip and will not
return until the following breeding season.
Previous studies on southern elephant seal females breeding at South Georgia and
Macquarie Islands suggest wide coverage of the Southern Ocean, with foraging areas associated
with the Antarctic Polar Front, the continental shelf margin or the ice margin (Hindell et al.
1991, Field et al. 2001). Elephant seals travel thousands of kilometers from the rookery and
regularly dive to depths over 1000 meters (Slip et al. 1994). For this reason, southern elephant
seals are the ideal species for remote sampling of the Southern Ocean. Furthermore, southern
elephant seals may be a key species for indicating ecosystem change. During the 1997 - 1998 El
Niño northern elephant seals showed low rates of mass gain, long foraging trips, and
demonstrated dive behavior that suggested reduced patch residence time and more transit time
compared to non-El Niño years. Crocker et al. (in press) suggest that the difficulty in foraging
might have been due to alterations in the oceanographic features the seals rely on to concentrate
prey or as cues to locate prey.
This study will determine what features elephant seals use to find prey and how they deal
with changes in these features. I will investigate this by testing the following hypotheses: 1)
Elephant seal foraging behavior will be associated with oceanographic features and 2) elephant
seals will exhibit variation in diving behavior depending on the water mass. Elephant seals
travel long distances between breeding rookeries and feeding sites, and will therefore likely
return to oceanic regions where prey are in high abundance due to predictable oceanic processes.
I predict that seal foraging behavior will be associated with strong frontal zones that are thought
to aggregate prey. This will result in the seals exhibiting variable dive behavior in the different
water masses. 3) Variation in the oceanographic features seals use will lead to changes in
habitat use and/or decreased foraging success. During the 1997-1998 El Niño, when there was
major changes in the structure and function of the North Pacific sub-tropical gyre, northern
elephant seals (Mirounga angustirostris) spent significantly less time in foraging patches and
more time in transit between the patches and exhibited significantly lower rate of mass gain
compared to non-El Niño years (Crocker et al. in press).
Approach
This study will be conducted at the US AMLR summer field camp at Cape Shirreff,
Livingston Island during the 2004-05, 2005-06, 2006-07 seasons. Cape Shirreff is an ideal site
for this research because of its location on the Antarctic Peninsula. The Antarctic Peninsula is a
long and narrow extension of the Antarctic continent with a strong climatic gradient, making the
climate highly variable and particularly sensitive to climate change. The region has also been the
site of US AMLR’s long term oceanographic monitoring focusing on krill and apex predator
interactions since 1986.
I will deploy satellite tags and time-depth recorders on 10 adult female elephant seals at
the end of the breeding season. The tags will be recovered approximately 2 months later when
seals return for the annual molt and then redeployed on 10 different adult females at the end of
the molt. I will recover the tags during the following season when the seals return to breed. The
seals will be instrumented with Satellite Relay Data Loggers (SRDLs – Sea Mammal Research
Unit) and archival dive recorders (Wildlife Computers). The SRDLs collect, summarize, and
transmit data on the animal’s behavior (location, dive depth, and dive duration) and environment
(temperature, salinity, and light) via the ARGOS satellite system. This allows for near real-time
tracking and accurate geopositioning. The archival dive recorders must be recovered to obtain
the data. These tags measure dive behavior (dive depth, dive duration), environmental
temperature, light, and pressure at high resolution.
For tag deployment and recovery, seals will be immobilized with an intramuscular
injection of Telezol and anesthesia will be maintained with intravenous injections of Ketaset.
The female will be weighed using a hand winch and scale suspended from a metal tripod. Body
composition will be determined by blubber depth measurements with a portable ultrasound
scanner and morphometrics. The SRDLs will be attached to the top of the head with 5 minute
marine epoxy with the antenna angled forward so that it will be out of the water when the seal
surfaces. The archival dive recorders will similarly be attached to the pelage on the dorsal
midline between the shoulders. The tags will be retrieved when the seal returns and the epoxy
mount will be left to fall off during the molt.
The tag data will be integrated with remotely sensed, in situ, and model derived
oceanographic data using a live access server hosted by the Pacific Fisheries Environmental
Laboratory. This system allows animal tracks to be visualized along with bathymetry, weekly
sea surface temperatures, chlorophyll observations, sea surface height and surface wind vectors.
The live access server allows us to extract data in a variety of formats that are easily imported
into data analysis programs.
I will use the animal derived temperature and salinity data to place behavior in the
context of water masses by defining habitat types using hierarchical agglomerative cluster
analysis on the temperature and salinity profiles. Each dive's combined temperature and salinity
profile can then be assigned to a cluster that will be examined for a relationship to known water
types in the Southern Ocean. I will then examine the diving behavior in the different clusters.
Elephant seals exhibit a number of distinct recurring dive profiles that have been associated with
diving, foraging, and food processing (Crocker et al. 1994, 1997, in press; Le Boeuf et al. 1992,
1993, 2000; Slip et al. 1994; Hindell et al. 1991). Using logistic regression I will look for
features associated with different dive profiles. This analysis was used successfully on
temperature-depth data from 22 female northern elephant seals (Crocker et al. 2003). They were
able to assign 82 ± 9% of dives from each individual into clusters representing thermally defined
water types. Additionally there were highly significant changes in diving behavior parameters
between water types. This approach should be much improved by the inclusion of salinity data.
Habitat usage will be modeled based on individual seal utilization. Animals show
preference for a habitat when it uses an area more than would be expected on the basis of relative
availability of the habitat. Because seals are central place foragers this approach is complicated
by the relative accessibility of habitat from haulout sites. I will model the relative accessibility
of habitat mechanistically based on distance from the haulout, speed of movement, and the
observed distribution of trip durations (Aarts et al. 2003). I will then use a Generalized
Additive model approach to relate the spatial utilization of the seals to the environmental
variables defining the habitats.
These data will be used to investigate foraging behavior and see how the seals change
their behavior to deal with seasonal and annual variation.
Significance
This study will provide information on the foraging behavior of one of the Southern
Oceans prominent apex predators. Global warming will likely affect oceanographic features
such as currents, eddies, and ice edge characteristics. This three-year study will most likely not
demonstrate affects of global warming on elephant seals but it can provide data that can be used
as a baseline in the future. With the information obtained from this study on the mechanisms
seals are using to find prey, I will be able to model what affects global warming will have on this
species.
In addition to providing habitat for many animals, the Southern Ocean plays an important
role in global climate. Oceans absorb about half the heat from the sun with the top 3 meters
holding more heat than the entire atmosphere. Ocean currents then carry the heat away from the
warmer tropics towards the polar regions. Recent studies have found that temperature data
collected in the Southern Ocean is better at predicting rainfall in Australia than the Southern
Oscillation Index (CSIRO Marine Research). Currently many resources are being devoted to
developing models on the Southern Ocean’s role in global climate change. The temperature and
salinity profiles collected by the seals will be collecting data in regions and seasons that are
unlikely to be sampled in more traditional ways. I will share these data with oceanographers
who are working on creating models to predict the effect of global climate change.
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Bornemann, H., M. Kreyscher, S. Ramdohr, T. Martin, A. Carlini, L. Sellmann and J. Plötz.
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