Cambrian: Opinion

Deep-sea elephant seal secrets: What do they eat down there?

This Año Nuevo seal wears a satellite tag. Researchers need to recover the tag to extract full information about where the seal has been.
This Año Nuevo seal wears a satellite tag. Researchers need to recover the tag to extract full information about where the seal has been.

Young seals are on the beach for their Fall Haul-Out. That means they are hauling their blubber out on to the beach for a few weeks of rest.

September is the beginning of the juvenile takeover of the beach. Their numbers will increase through October, then gradually decline in November, until all are gone in December. The adult bulls arrive then for the breeding season.

The youngsters spend most of their time resting, but some spar in the surf. It’s all low-key, not the serious battles of the breeding season, December through February.

The adult seals are out at sea, feeding and gaining weight for the winter breeding season. Males and females feed in different places, presumably on different prey. Sarah Kienle, Ph.D. candidate at UC Santa Cruz, is gathering data to answer some of those questions.

Data points collected on satellite tags show the female seals wander around the ocean, what Kienle calls a drunkard’s walk: feeding a few days in one place, then wandering off to another place to feed for a few days. They dive continuously, staying down 20 to 30 minutes. Digital tracking equipment shows they zig-zag up and down at the deepest part of their dive, probably chasing and catching fish and squid. Their exact diet is not yet documented.

“We still don’t know what they are eating,” Kienle said.

Members of her lab are using stable isotope analysis, comparing the biochemical makeup of seal blood and whiskers with that of fish and squid found at the depths where the seals are hunting, to tease out what they are consuming.

Adult males feed along the continental shelf and slope. It’s shallower there, so they dive down to the bottom and eat what they catch there.

“They are so different, it’s hard to compare males to females directly,” she told a meeting of Friends of the Elephant Seals docents.

Electronic tags are the only way to know where these long-distance wanderers are spending their time. Satellite tags showed some seals that breed at San Benito Island off Baja California don’t migrate as far as their more northern cousins do. They stay within 500 kilometers of the island, half the distance Año Nuevo seals migrate.

“They stay at sea the same amount of time but don't have the transit cost of migrating so far,” she said. “None of the San Benito animals go all the way north to the Aleutian Islands.”

So, if they find enough to eat in southern waters, why do most of them migrate so far north?

Her research, and research done by others at UCSC, may give us insight into that and all the other unanswered questions about elephant seals.

“Feeding is easy to observe in terrestrial environments,” she said. “We know so much less about the marine environment. It’s difficult to study feeding in marine animals because they forage underwater on a wide variety of prey at extreme depths and even in the dark. There’s so much to learn. We have to be creative in how we study them. The questions with elephant seals are never ending.”

Check out the elephant seals being tracked at www.seaturtle.org and more information at costa.eeb.ucsc.edu.

Christine Heinrichs’ monthly column is special to The Cambrian.

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