Northern elephant seals (Mirounga angustirostris)
Female northern elephant seals undertake a long foraging migration in the North Pacific each year, building a reserve for subsequent months spent fasting on land while giving birth, nourishing a pup, and breeding. Using data from the Tagging of Pacific Predators project (www.topp.org), we identify an area of high female northern elephant seal density during their annual 6-8 month foraging migration, indicating it is an area of special importance for life history stages of this species.
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Many wide ranging marine animals have an amphibious life history. For example, sea turtles, seabirds, sea lions, and seals spend part of their lives feeding at sea, and part of their lives on land, breeding, caring for young, or molting. In the North Pacific, the northern elephant seal is a wide-ranging top predator with such a life history. Female northern elephant seals haul out on the beaches of the North American west coast twice yearly: once to give birth, nourish young, and breed, and once to molt (Stewart and DeLong 1996). They fast completely during this time. Following each terrestrial visit, northern elephant seals return to sea to feed. They thus undertake a double foraging migration each year - a “short migration” following breeding and a 6-8 month “long migration” following molt. Until recently, it was thought that northern elephant seals were restricted to coastal waters during their migrations; range-maps indicated that elephant seals venture no farther than the continental shelf (Riedman 1990). From the mid-1980s, an advance in tracking technology for obtaining fine-scale data on animal movements allowed for an amazing discovery.
During the long migration, female northern elephant seals travel half way across the Pacific, feeding almost entirely in habitats beyond national jurisdiction. They spend more time in some places than others. A robust tracking dataset from 2004-2007 allowed for the identification of multi-individual high-use areas and an analysis of their persistence from year to year. The ecological significance of these areas to northern elephant seal life history is reflected in the energy required to sustain their time on land and ensure pup survival; 48% of their body energy is lost during lactation, and “body reserves obtained during biannual foraging migrations are the most important determinants of reproductive effort in female elephant seals” (Crocker et al. 2001).
How the area of special importance for life history stages of northern elephant seals was identified
Geographic areas of ecological significance to wide-ranging marine predators can be delineated in several ways. Marine researchers increasingly use techniques that provide a utilization distribution - the relative frequency of locations of an animal or group of animals in a particular area during a given time frame (Van Winkle 1975). The utilization distribution is a probabilistic model describing the relative amount of time that an animal or group of animals spends in any place (Seaman and Powell 1986) and has been particularly useful for identifying areas highly used (or visited) by many individuals (BirdLife International 2004), variably called core areas, high-use areas, and hotspots. Kernel density estimators have emerged as the most commonly used technique for utilization distribution estimation (Worton 1989, Kernohan 2001; BirdLife International 2004; Laver and Kelly 2008), and is the technique we used here.
We used the open source software “R” and the kernel density utilization distribution function in the R-package AdeHabitat (Calenge 2006) to calculate and map the utilization distribution of 55 females tracked during their post-molt, long migration in 2004-2007. We focused our analysis on the at-sea portion of the female elephant seal migration (July–November), omitting migration corridors to and from haul-out beaches. Migration corridors are primarily located within Exclusive Economic Zones; most females move quickly through them.
We used location data collected from satellite tags deployed as a part of the Tagging of Pacific Predators project (see Sources of Data for more information). Raw Argos satellite positions include a range of inaccurate positions (i.e. positions falling on land, or positions farther away in distance than the animal could possibly have traveled in a given time period). Traditional methods of processing Argos data to remove erroneous positions include simple speed, distance, and angle filters. A more robust approach is a state-space model, a Bayesian statistical approach to accounting for measurement error and estimating the most probably movement pathway (Jonson 2003). We used a state-space model to process raw Argos satellite positions, obtaining the best location estimates given the error distribution of our data. Tracks were interpolated to 4 positions per day.
Utilization distributions were calculated for each individual and averaged across years (Figure 1), and within each year (Figure 2), producing a series of volume contours encompassing the area within the average animal spends a given percentage of time. The 95% contour indicates the area where tagged elephant seals spent 95% of their time at sea or, where one is 95% likely to find a tagged elephant seal during the study period. Areas of high use are relatively persistent from year to year (Figure 2). Areas of likely significance were identified using the 50% utilization contour for the utilization distribution averaged across 2004-2007 – those areas of average concentrated use within the home range.
Figure 2. Yearly high use open ocean areas of female northern elephant seals during their long migration, July-November, 2004-2007.
Sources of data
We used data collected by the Tagging of Pacific Predators project (www.TOPP.org) and provided courtesy of Dr. Daniel P. Costa, University of California, Santa Cruz. TOPP began in 2000 as one of 17 projects of the Census of Marine Life, a 10-year, 80-nation endeavor to assess and explain the diversity and abundance of life in the oceans. TOPP researchers from eight countries began venturing into offshore waters, remote islands, and along rugged coastlines to attach satellite tags to 22 different species of top predators that roam the Pacific Ocean. As of 2007, they have tagged more than 2,000 animals, including elephant seals, white sharks, leatherback turtles, squid, albatross and sooty shearwaters.
Molted females were tagged in May-June, 2004-2007 with Wildlife Computers Argos satellite transmitters. Only complete tracks (females that returned to land following their long migration) were used for this analysis: 14, 17, 15, and 9 individuals in 2004-2007 respectively (total n=55).
To understand patterns of space-use in years not sampled and under environmental conditions not included in the sampling period (for example, El Nino events), tracking data may be combined with previous publications, expert opinion, habitat models, and at-sea sighting and observation data when available.
Aarts (2008) summarized a number of important statistical and technical considerations when using tracking data to identify important habitats for marine predators. Considerations in applying kernel density estimators to tracking data were reviewed by Kernhohan et al. (2001), Getz and Wilmers (2004) and Laver and Kelly (2008).
In marine environments, habitats move, and they can do so on decadal, yearly, monthly, even daily and hourly time scales. Habitat use and patterns of animal movement are influenced by spatial and temporal resource availability, physiological limits, predator avoidance, and human disturbance. Large-scale episodic phenomena such as the El Niño Southern Oscillation (ENSO) can have large effects on where northern elephant seals spend their time and on reproductive success and pup survival (Le Boeuf and Crocker 2005).
Because use of space by marine predators can be dynamic, the best tracking datasets for identifying areas important to the life histories of these animals will span multiple years under a range of ecological and anthropogenic influences and include a sample size large enough to be robust to individual variation. The TOPP northern elephant seal datasets is one of the best in this regard, but even five years of very good data about a relatively small subset of the total population may be few when placing these results in a context of climatic and population level phenomena.