Elephant Seal Movements: Some Frequency Based Studies

• Main Authors: David R. Brillinger, Brent S. Stewart
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Key words; Diving; Elephant seal; Fourier analysis; Marine mammal; Migration; Parallel boxplots; Periodogram analysis; Point process; Spectrum; Starplot; Time series; Baja; Gulf of Alaska; Pacific; San Miguel; Elephant Seal; Alaska; Aleutian Islands
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.51.5747; http://www.stat.berkeley.edu/~brill/Papers/rebrape.ps

This paper focusses on the case of a real-valued time series (depth). Others in a series of papers, (Brillinger(1996), Brillinger and Stewart (1996a)), are concerned with the bivariate case (of latitude and longitude). 2 The data and some descriptive statistics The primary data set for marine vertebrates that is studied here is for the northern elephant seal (Mirounga angustirostris ). This species breeds on offshore islands and at a few mainland sites along the coasts of California - and Baja California (Stewart and Huber, 1993; Stewart et al., 1994). Adults are ashore briefly in winter to breed and again in spring (females) or summer (males) to molt but spend the remainder of the year, 8-10 months, at sea foraging. They each make two precise, long-distance migrations each year between islands in southern California and offshore foraging locations in the mid-North Pacific, Gulf of Alaska and along the Aleutian Islands covering 18,000 to 20,000 km (surface movements alone) during the double migrations (Stewart and DeLong, 1995). The seals dive continually during these migrations. The dives average 20 to 40 minutes long (longest = 2 hours) and 350 to 650 meters deep (deepest = 1560 meters) and are only separated briefly for 2-3 minutes while the seals are at the sea-surface breathing (e.g., DeLong and Stewart, 1991; Stewart and DeLong, 1995). See Figure 1 for the track of one migration and the record of six successive days of diving. In the top graph a great circle path has been superposed for reference. The data studied refer to vertical movements, recorded at 30 second intervals, throughout the period at sea by a microprocessor-controlled event-recorder which was harmlessly glued to an elephant seal's hair at San Miguel Island off Southern California (e.g., Stewart a.