Foraging ecology of Antarctic Pinnipeds
The lab has conducted two main ongoing studies of the foraging ecology and physiology of pinnipeds in the Antarctic
Winter foraging ecology of Crabeater seals diving along the Western Antarctic peninsula
This research project was part of the Southern Ocean Global Ocean Ecosystems Dynamics Program (SO GLOBEC), which was designed to elucidate shelf circulation processes and their effect on sea ice formation and Antarctic krill (Euphausia superba) distribution, and to examine the factors that govern krill survivorship and availability to higher trophic levels, including seals, penguins and whales. These goals are approached through a moored instrument program, broad-scale physical, biological, and chemical oceanographic surveys, process-oriented investigations, and modeling studies focused on austral winter processes in the region west of the Antarctic Peninsula.
Co-PI's on the project were Daniel Costa, University of California Santa Cruz and Daniel Crocker, Sonoma State University
Project Summary: Within the Southern Ocean, marine coastal and pelagic environments are characterized by seasonally high productivity that supports large numbers of breeding marine predators. However, in contrast to early work which suggested that primary productivity was unusually high and the food chains short, robust, and highly efficient, more recent work has suggested both that the overall levels of primary productivity are lower, and that food webs are more complex. In addition, it has become clear that the Antarctic marine environment experiences considerable inter- and intra- annual variability. Consequently, available prey resources for vertebrate predators can be both spatially and temporally patchy. In the face of such unpredictability, most marine predators appear to rely on oceanographic features, such as frontal systems, thermocline depth, and bathymetry to concentrate or aggregate their prey in such a way to enable effective foraging. However, while the importance of these physical features has long been recognized, the interactions between environmental features, prey abundance, and predator distributions and foraging strategies are only poorly understood.
Crabeater seals offer a unique opportunity to better understand the foraging strategies utilized by marine predators in the face of meso- and fine-scale ecological variability. Within the seasonal pack ice zone crabeater seals prey almost exclusively on Antarctic krill (Euphausia superba). Yet despite their reliance on a locally abundant and spatially heterogeneous prey resource, crabeater seals are the most abundant Antarctic pinniped (c. 15 million). Thus, the crabeater seals have clearly evolved behavioral patterns that are optimized to exploit prey that are patchily distributed within both space and time. Therefore we predict that studying how crabeater seals are dispersed within and below the seasonal pack ice will likely reveal those specific strategies used by seals to optimize foraging success. Two such strategies include the close proximity of haul-out and foraging sites, and the reliance on environmental features to aggregate their select prey. Therefore this work focused on understanding aspects of ocean structure that are correlated with specific behaviors, identifying how dependent seals are on these oceanographic features to concentrate prey, and determining how crabeater seals optimize their behavior to accommodate seasonal or annual fluctuations in oceanographic features and the distribution of prey.
Specifically, this project was designed to determine the distribution and foraging behavior of adult female crabeater seals (Lobodon carcinophagus), and simultaneously assess the impact that oceanographic features and prey aggregations have on the foraging strategies employed. To do so, we used a combination of satellite-linked tracking, specialized dive recorders, isotopic tracers, and concurrently and/or remotely collected information on the distribution and abundance of krill. We tested the following hypotheses: 1) that foraging locations are correlated with environmental features known to influence krill abundance; 2) that seasonal shifts in foraging behavior track krill movement patterns; and 3) that crabeater seal foraging behavior conforms to that predicted by optimality models. In addressing these hypotheses, this research enabled us to link biological (prey composition, distribution, and abundance) and physical characteristics of the foraging environment with the foraging behavior and strategies of a free-ranging marine vertebrate predator. Because crabeater seals are the most abundant pinniped in the Antarctic, their foraging activities undoubtedly have a large impact on the Antarctic marine ecosystem. Therefore, understanding both how crabeater seals select their foraging locations and prey, and how alterations in environmental conditions and krill abundance may impact crabeater seals is essential to our understanding of Antarctic ecology.
Research Products to date
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Weddell seals as autonomous sensors of the winter oceanography of the Ross Sea
Co-PI's: Dan Costa, University of California Santa Cruz & Eileen Hofmann, Old Dominion University
Project Summary: Marine mammals of the Southern Ocean have evolved diverse life history patterns and foraging strategies to accommodate the extreme fluctuations in the physical and biological environment. Our understanding of how their foraging behavior and habitat utilization responds to such changes is complicated by the need for information about this variation at scales relevant to the predator, and under environmental conditions that are not conducive to traditional oceanographic sampling schemes. In light of ongoing climate change and the dramatic shifts in the extent and persistence of sea ice in the Ross Sea, it is critical to understand how Weddell seals, Leptonychotes weddellii, a key apex predator select and utilize foraging habitats. Recent advances in satellite-linked animal–borne conductivity, temperature and depth (CTD) tags make it possible to simultaneously collect data on seal locations, their diving patterns, and the temperature and salinity profiles of the water columns they utilize. In other ecosystems, such data have revealed that marine predators selectively forage in areas where currents and fronts serve to locally concentrate prey resources, and that these conditions are required to sustain populations. These CTD tags have proven particularly useful for elucidating important physical correlates of foraging habitats of species that utilize high latitude, remote, and/or ice covered environments, and in regions or times where remote sensing and more traditional ship, AUV, and drifter datasets are extremely limited, such as during Antarctic winters. Weddell seals will be studied in McMurdo Sound and at Terra Nova Bay, Ross Sea and will provide the first new data on Weddell seal winter diving behavior and habitat use in almost two decades. Understanding winter behaviors is critical because this is when adults recoup mass loss during the summer breeding season and gain condition necessary for breeding the next spring. We will track seal movements and diving patterns from fall through spring, identify focal foraging areas and characterize those areas with respect to physical variables (temperature, salinity, water mass, bathymetry, ice extent, distance from shoreline and ice edge, etc). We use GLMM, information criterion, and other modeling approaches to identify physical and temporal features that influence habitat selection and diving behaviors, and incorporate information on animal physiological status to assess foraging success. The relationship between an animal’s diving behavior and physical habitat has enormous potential to enhance monitoring studies and to provide insight into how changes in ice conditions (due either to warming or the impact of large icebergs, such as B15) might impact individual time budgets and foraging success. The second thrust of this project is to use the profiles obtained from CTD seal tags to model the physical oceanography of this region. Current mathematical models of physical oceanographic processes in the Southern Ocean are directed at better understanding the role that it plays in global climate processes, and the linkages between physical and biological oceanographic processes. However, these efforts are limited by the scarcity of oceanographic data at high latitudes in the winter months; CTD tags deployed on animals will collect data at sufficient spatial and temporal resolution to remedy this lack. These data are required to better understand key features such as the properties of the ocean mixed layer, the formation of sea ice and Antarctic Bottom Water, and variability in the Antarctic Circumpolar Current (ACC) and associated ocean fronts.
Intellectual Merit and Broader Impacts:This project is a unique multidisciplinary endeavor bringing marine biologists and physical oceanographers working together to understand the physical oceanography of the Ross Sea, and the impact that the ocean environment on a resident apex predator, the Weddell seal. The information collected will be used to address three fundamental questions: 1) Are the behavior and movement patterns of adult Weddell seals associated with particular oceanographic features during the winter months, 2) how might climate change influence these features and impact seal foraging success and population health, and 3) what hydrographic currents and water masses characterize the Ross Sea during the winter months and how are these linked to the broad scale hydrography of the Southern Ocean. The project will contribute to two IPY endorsed initiatives: MEOP (Marine Mammals as Explorers of the Ocean Pole to Pole) and CAML (Census of Antarctic Marine Life). In addition, the highly visual nature of the data and analysis lends itself to public and educational display and outreach, particularly as they relate to global climate change, and we have collaborations with undergraduate and graduate training programs, the Seymour Marine Discovery Center, and the ARMADA program to foster these broader
First field work will be in January 2010
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Last modified 10/28/2009