Where do the elephant seals go?

Seals and sea lions are such a fixture of life along the coast that it's surprising how little we know about their movements. Researchers at Hubbs-Sea World are starting to fill in the picture, with some amazing findings.

by Katy Koster
f you've spent much time on California beaches, chances are you've seen seals and sea lions, playing in the surf or basking on the rocks. The truth is, these ubiquitous mammals spend substantial periods at sea each year, but their whereabouts and behaviors are relatively unknown.
Scientists at the Hubbs-Sea World Research Institute are trying to change that. Using the latest in satellite technology, they have been studying the diving behaviors and foraging migrations of northern elephant seal bulls for the past several years.

California sea lions and northern elephant seal populations have increased substantially along the California coast during the past several decades. The sea lion's increase has been quite apparent to sport and commercial fishermen, and more recently each winter, to those living along the coast of Washington, where the marine mammals have invaded the waterways of Puget Sound to feast on migrating salmon. Near Pier 39 in San Francisco and in Monterey Bay, large numbers of male sea lions have hauled out and caused damage on private boat docks and fuel barges.

Return of the elephants

The rapid repopulation of California's waters by northern elephant seals has been much less conspicuous, however, attracting only minor public attention. Commercial hunters exterminated northern elephant seals from California waters in the l9th Century - not until the 1950s did elephant seals return to the Southern California Channel Islands to breed.
Today, San Miguel Island, lying about 70 miles west of Los Angeles, hosts the world's largest northern ele-phant seal colony. Seals congregate there in winter to breed and again in the spring and summer to molt, but they are rarely seen during the ten months or so when they are at sea. Indeed, their whereabouts for most of their lives are unknown.
In 1987, Hubbs scientists began studying the diving behaviors and foraging ecologies of elephant seals at San Miguel Island. They document the depths and duration of the seals dives, the amount of time seals spend resting at the surface between dives, and the sequential patterns of dives.

Hi-tech tracking

To track the seals, the scientists employ small electronic instruments called time-depth recorders (TDRs). The original TDRs were large devices, about 6 by 8 inches in size and weighing over 2 pounds. The newest units are much improved, consisting of a 6-inch long cylinder about 1 1/2 inch in diameter. The TDR's are glued to the hair of seals just before they leave the rookeries in late February. The housings for the TDRs must be pressure-tolerant to protect the unit at depths to which seals were diving - 1,500 to 2,500 feet, with the deepest recorded dive to about 5,000 feet.
The TDRs provide information in two ways. First, the unit records and stores data internally. The unit is retrieved when the seals return to San Miguel Island in July to molt, and the data is dumped for analysis. Second, the unit is able to send a small amount of data to a satellite in polar orbit when the animal surfaces to breathe. The satellite then retransmits the data to a ground station in France, and on to a computer terminal at the Institute's headquarters on Mission Bay. Scientists can determine the whereabouts of an animal in the middle of the Pacific Ocean within minutes of that transmission.
Adult male elephant seals dive continuously while at sea for periods of 120 to 150 days in spring and early summer; it is a rare event for a seal to spend more than five minutes at the surface between dives that average 25 minutes. Because of limitations of previous satellite systems, this pattern of infrequent and brief surface activity has made it difficult to provide detailed movement information. However, recent improvements in satellite technology now allow scientists to get locations of pelagic (open sea) animals as well as dive duration and dive-depth measurements.

On the road

Analysis of the data gathered details some astounding movements. Although routes differ slightly, northern elephant seals travel north to the Gulf of Alaska and the Aleutian Islands in approximately 40 days. They forage there for another 40 - 50 days, then take 40 - 50 days to return to San Miguel Island. Elephant seals can cover more than 5,000 miles during annual migrations.
Diving patterns reveal uninterrupted diving for four to five months. The records suggest that the seals forage continuously (diving to depths of about 1,500 feet) during north- and southbound transits. Their infrequent and brief surface appearances, as well as their offshore distributions when at sea, explain why so few elephant seals have ever been seen away from land-based breeding and molting sites despite their rapidly growing numbers.
Institute studies were the first to document the pelagic distribution and movements of northern elephant seals and demonstrated the utility of this technology for studying the migrations and foraging ecologies of marine mammals in general.
Studies are also conducted to examine annual variability in foraging migrations and the extent of overlap in distributions of males and females while at sea. Adult males and females separate - for reasons as yet unknown - during two annual migrations covering 10,000 - 12,000 miles each year.
Information gained from Institute research provides baseline data for sound ecology and conservation programs, and may provide the foundation for legislation and environmental management decisions that protect oceanic resources for generations to come.

Katy Koster, a San Diego resident since 1985, lives in Normal Heights and works in Public Relations at Hubbs-Sea World Research Institute, a non-profit marine research foundation. The information in this report is based primarily on the work of Dr. Brent Stewart.