Risks of migration
Researchers study how elephant seals avoid predators on long Pacific swims
The northern elephant seals that overwinter on California beaches each year then go for a long swim, spending seven months migrating halfway to Japan and back to gorge on squid and fish.
By studying their movements, researchers at UC Santa Cruz have gained new insight into how the 1,000pound marine mammals behave during their epic sea journey and the risky choices they make between feeding and resting to avoid becoming supersized prey themselves.
“There’s no kelp forest, there’s nothing for the seals to hide under or in. They’re just exposed,” Roxanne Beltran, assistant professor of ecology and evolutionary biology at UC Santa Cruz, said of the seals’ time spent in the open ocean. “Darkness is the only thing that they have to hide in. They can use light strategically to make themselves more safe.”
Beltran and fellow researchers attached satellite tags and timedepth recorders to 71 adult female northern elephant seals at Año Nuevo State Reserve on the San Mateo coast before the animals made their annual 6,000mile migration. In a study published Wednesday in Sci
ence Advances, they found that the animals’ behavior was riskier at the beginning of the journey, when they had gone months without food. They took fewer risks as they became fatter, which was measured by the seals’ buoyancy in the water.
“They’re strategically taking more risks when they’re at their skinniest and they need to make up for lost time, basically,” Beltran said. “And what that means is feeding during the night, when the prey are closer to the surface and feeding is more efficient, and the result of that is they have to rest during the day when they’re more vulnerable to predators.”
Though the researchers don’t know that much about how the seals interact with predators in the open ocean, they believe their main predators are sharks and killer whales — on shore, many seals show signs of shark attacks. Because those predators are visual hunters, the seals are at their most vulnerable during daytime and close to the ocean’s surface, which is why they spend about 90% of their time during the entire sevenmonth migration underwater holding their breath.
The researchers found that the seals dive for an average 23 minutes at a time to fish or rest, only taking a breath at the surface for 2 minutes in between. They might dive down 1,300 feet to fish at night and about 2,000 feet during the daytime, while they typically rest at around 1,200 feet.
Resting dives, when they “fall on their backs and drift through the water like falling leaves,” Beltran said, was the period that researchers could measure the animals’ fat levels by their rate of buoyancy. While skinnier animals tend to sink, fatter ones float upward.
The research found that while they are still skinny, the seals fish more at night and rest more during daylight hours, which is riskier because they’re more vulnerable to predators. But as they get fatter, they can prioritize resting at night for safety.
The findings confirm longheld ideas among biologists that less stressed, fatter animals take fewer risks. But monitoring terrestrial animals’ fat levels in the wild isn’t really possible, which is why having the ability to track the elephant seals’ buoyancy in the ocean is a breakthrough, Beltran said.
Beltran was joined by several collaborators, including Daniel Costa, director of UC Santa Cruz’s Institute of Marine Sciences, which has studied elephant seals for decades.
Each year from December to March, female northern elephant seals head to the coasts of Baja California and California, including at Año Nuevo and Point Reyes National Seashore, to give birth to their pups, nurse them for about six weeks and breed again. The cows then go on a short foraging trip that lasts about 70 days and return to shore to molt for about 90 days before embarking on their long migration.
Male elephant seals travel similar distances but stay closer to Washington and Alaska, where they likely feast on coastal fish species during the migration. But it takes female elephant seals two months just to get to their foraging grounds, where they gravitate especially toward small, very fatty deepwater lantern fish.
“We think elephant seals have this sort of unique deepdiving, smallfisheating niche out in the middle of the ocean,” said Beltran.
By the time they reach their fishing grounds, the seals have dropped about 36% of their weight and are down to 20% body fat. Within 100 days of heavy feeding, they shoot back up to 35% body fat and reach around 1,000 pounds, Beltran said.
Beltran describes the animals’ behavior during the migration as “lightscapes of fear,” a phrase she based on the scientific theory called the landscape of fear, which says that animal movement in the wild is dictated by the sense that predators are lurking everywhere. In the case of elephant seals, they lurk most in the sunlight or shallower waters.
“We took that idea — risk in twodimensional space — and extended it to include both threedimensional space with depth in the ocean and also with time,” she said. “What we know is animals experience very different conditions based on not just time but space in three dimensions in the oceans. This idea captures all of that together, which is what excites me so much.”