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Deadly Sea Lion Mystery Draws Biologists to Remote Island in Search of Clues

By Nadia Drake 08.16.13 9:30 AM

Sea lions come to Point Bennett each year to breed and birth pups. They’re joined on the island by northern fur seals, the darker mammals in clumps.

SAN MIGUEL ISLAND, Calif. – It’s late June, and San Miguel Island’s white sand beaches are filled with barking sea lions. More than 100,000 of them. The marine mammals have come to this windy, remote island to breed and give birth – a rowdy, stinky summer extravaganza that last year, enigmatically, ended in disaster.

When the sea lions converged on this most westerly of southern California’s Channel Islands in May 2012, as they do every spring, there was no hint of anything amiss. A year later, thousands of pups – perhaps as many as 70 percent of the newborns – were dead. The struggle to survive led desperate pups from their sandy nursery into the churning, dangerous sea, long before they were ready.

Between January and June, five rescue centers along the southern California coast, from Santa Barbara to San Diego, took in more than 1,500 stranded pups – five times more than normal.

San Miguel island is the farthest west of the northern Channel Islands, off the coast of California.

And those are just the ones that survived the journey of more than 50 miles. Many thousands more died on the islands, or along the way.

What happened is still a mystery, but investigating scientists have come to suspect that an unexpected shift in the sea lions’ food source is to blame. Now, as a new generation of pups are being born here, a different question arises: Has the danger passed, or are this year’s pups in peril too?

I went to San Miguel Island to try to find out.

The conduit between the sea lions and me is Sharon Melin, a biologist with NOAA’s National Marine Fisheries Service. Melin has been studying the San Miguel Island sea lions (Zalophus californianus) for more than two decades, tracking their population trends and health. In the summer, she comes to the island for about three months, working at a sun- and wind-powered research station perched on the bluff above Point Bennett.

It’s here, at the westernmost tip of the island, that the bulk of the island’s sea lions converge, nearly 50,000 of them. The breeding colony is among the largest in the world, and the island’s population accounts for just over half of all sea lions in California. They come here because the waters normally brim with fish, and because San Miguel is remote and undisturbed: Located about 60 miles from Ventura harbor, the island’s nine campsites host fewer than 200 visitors per year.

A 7-mile trek separates Point Bennett from the campground at the island’s eastern end, a hike that must be made with a park ranger or naturalist guide (in part because the U.S. Navy can’t guarantee the island is free from unexploded ordinance — leftovers from when the military used San Miguel as a postwar bombing range). On the seaward side, the pristine, white beach is surrounded by deep, blue-green waters that hide dangerous rocks and have sunk many a ship.

In other words, it’s not trivial to get here.

On this briefly sunny day in late June, the sea lions – joined by northern elephant and fur seals – have reached peak pupping season. They’re generating quite a racket on the beach, several hundred feet from where we stand. I’m glad the wind is blowing the smells offshore, as I’ve heard the colony can reek like a dumpster full of funky fish on a warm day.

Despite their aroma, California sea lions are charismatic and intelligent, capable of logical reasoning and dancing to disco tunes. Males can grow to weigh 800 pounds. Pups, who weigh around 16 pounds at birth, stay with their mom for almost a full year, during which they learn to swim and fish on their own, and she begins incubating another pregnancy. By the time a pup is weaned, it should weigh about 60 pounds.

Some of the pups washing ashore last year weighed less than half that.

Once, about a century ago, this beach on San Miguel would have been nearly empty: Seal and sea lion populations were driven to near-extinction by an endless appetite for their fur and oil; for the “trimmings” from large bulls (the bits that make a bull a bull, if you follow), which were sold as aphrodisiacs in Asian markets; and for their meat, which often ended up in pet food. Sea lions were, and still are, disliked by fishermen because they interfere with salmon and other fisheries. Just before the Marine Mammal Protection Act was passed in 1972, there were fewer than 12,000 sea lions on the entire island.

Melin tells us to keep a low profile and speak softly, because the sea lions are skittish and will abandon the beach for the surf if they sense us.

There are a few tiny pups amidst the big, furry bodies on the beach. It’s warm enough that heat rising from the sand creates a haze that partially blurs the mammals, but it’s still pretty easy to distinguish the sea lion pups from the smaller and darker fur seal pups, and super easy to discern which lumps are the massive, 1-ton gray elephant seals. As we watch, some of the males wrestle and defend their territories, awkwardly charging at and chasing rivals away, while others lazily reach up to scratch an ear with a flipper, or snuggle into the sand (see video, below).

Every now and then, mothers scramble to retrieve little pups that have waddled off. Barks and bellows ring out from the sea lions, while the elephant seals emit deep, rumbling croaks that sound like a drunken uncle belching at the dinner table.

The raucous, grunty scene before me looks like an unfiltered glimpse of animals thriving in their natural environment.

You wouldn’t know anything was amiss.

Melin and I duck behind a sand dune to get some relief from the wind, and she tells me about what’s been happening here. Most days, she stays at Point Bennett and counts the animals from this spot, starting in the early morning. Now, it’s mid-afternoon, and though she’s just talked for hours with a group of campers who made the trek to Point Bennett, Melin patiently answers my dozens of questions.

Whatever caused the mass stranding snuck up on the colony quietly and quickly.

When researchers wrapped up last summer’s field season and left the island in early August, the colony looked fine.

“We had no signal that there was anything wrong. It wasn’t until we finished our summer season, and came back out in September — suddenly we had these skinny pups,” Melin said. She and her colleagues had returned to weigh the pups and tag 300 of them, something they do every year to track pup mortality rates. The skinny pups were a bad sign, but she and her colleagues didn’t yet realize how bad.

“Sometimes they can have a moment like that when they’re really little, but when we come back in February they’re fine,” Melin said.

Some of the rescued sea lions were sent to rehab centers up north, like this little pup at The Marine Mammal Center in Sausalito. (Ingrid Overgard/TMMC)
This year, they weren’t.

Winter trips to the island produced more observations of pups in bad shape. Then, tiny, starving sea lions began washing up on the mainland, overwhelming rescue centers and perplexing scientists. In March, the federal government declared the stranding an Unusual Mortality Event, sent funds and investigators to southern California, and convened a task force that is still working to figure out what happened.

By late spring, the stranding had stopped.

Although Melin and her colleagues haven’t finished processing all the data, so far, their estimates suggest that only about 30 percent of last year’s pups survived.

Some of those that did survive are sticking around their mothers for an anomalously long time and are still at the Point Bennett rookery.

“That’s not normal,” Melin said. “Most of the time they’re not allowed to be here.”

Despite having been fed by mom for longer than usual, the yearlings on the beach below us are still pretty small – as are some of the pups that were successfully weaned earlier this year. That doesn’t bode well for their survival. Even a few pounds of weight can make a huge difference, especially if conditions offshore aren’t optimal for youngsters, who can’t yet dive deeply or swim far in search of fish. “They’re really going to have a struggle,” Melin said.

This year’s group of pups is also showing the effects of the spring.

At first, it seemed there might be a simple explanation.In June, when I visited, it looked like only half the normal number of pups had been born. Now, the tally is a little better: It’s about two-thirds of normal. The lower birth rate, Melin says, probably reflects that fact that it’s difficult for sea lion mothers to simultaneously maintain a pregnancy and nurse a pup when food is hard to find. In those situations, mother sea lions will sooner abort a pregnancy than abandon a pup; and instead of hanging around the rookery, those pupless females may visit briefly to breed, which is why there are fewer females here as well.

But there are some encouraging signs. Despite the low birth rate, this year’s San Miguel pups look healthy, and their mortality rates are normal, so far.

In late July, after I’d visited, Melin and her colleagues traveled to San Nicolas Island, about 65 miles to the southeast – site of another large Channel Islands sea lion rookery. There, Melin’s preliminary observations suggest the colony is in slightly worse shape than San Miguel’s, with only half as many 2013 pups as normal. Observations made earlier in the spring by NOAA scientist Mark Lowry also suggest that the San Nicolas colony was harder hit than others.

These problems would be more normal during an El Niño year, when warm ocean waters suppress the cold, nutrient-dense upwellings that fuel ecosystem richness. In 1997-1998, a strong El Niño battered the sea lion colony, causing changes in their diet and mortality rates that were measurable for as long as four years afterward. In 2009, a more localized warm water patch settled offshore in the spring, just as the sea lion pups were weaning, producing mortality rates that are similar to this year’s.

The difference is 2012 and 2013 are not El Niño years. If anything, the California Current appears to be in the midst of a slightly cooler spell. Water quality, salinity, and oxygen levels aren’t askew either. In other words, there’s nothing obviously abnormal in the ocean that could explain what happened.

At first, it seemed there might be some other simple explanation.

Because the stranding only affected sea lions, and not seals and other animals that live in the same area and have similar diets, scientists thought the sea lion pups may have succumbed to a parasite or pathogen that didn’t affect adults or other species in the same way. But no obvious culprit has been identified.

Now, Melin and others suspect that the sea lion pups starved when their mothers had a harder time finding food. The rationale is pretty simple: The pups that washed up in Southern California recovered when rescuers give them plenty to eat. But why would the sea lion moms have had such a hard time feeding their young?

Sea lions are opportunistic feeders. Depending on what’s available, they’ll feast on anchovy, sardine, pacific hake, pacific mackerel, jack mackerel, short belly rockfish, or market squid.

“When one species is not as abundant, another one takes its place,” said Mark Lowry, who studies sea lion diets by analyzing the bony remains in their poop. Different diets are sometimes easy enough to detect, Lowry says. “When they eat just market squid, it’s real runny. When they eat fish, it’s like a regular poo.”

Every three months, Lowry goes to San Nicolas and San Clemente Islands, where he bags poop samples on the beaches. Then he filters through the poo, recording what he finds. Decades of data, gathered by both Lowry and Melin, suggest that sea lions tend to eat their regular fish species in good years and turn to squid and different fish species during El Niño years.

Ultimately, Lowry is hoping to use his fishy-poo observations to write a computer program that will take new data and tell him when the sea lion population is stressed, like during an El Niño year. He has a freezer full of frozen feces, going back several years. Those samples could contain crucial clues about what, exactly, the sea lions were eating before, during, and after the stranding event. But for now they’re just sitting in Lowry’s freezer until he can find funding to do the analysis.

Scientists are also hoping to find out where the sea lions are hunting for fish. This spring, Melin and her colleagues attached satellite tracking tags to 13 female sea lions: Seven with healthy pups, six with skinny pups. The researchers are looking for any differences between the hunting grounds of the two groups that might point them toward the problem.

Soon, scientists will be able to match those data with observations gathered by research ships that regularly sail up and down the California coast, monitoring the abundance of fish populations and collecting oceanographic data. Their most recent observations aren’t available yet, but several lines of evidencepoint to a steady decline in fish stocks in recent decades.

Since the early 1970s, some fish populations have decreased by as much as 70 percent – including the ones that sea lions like to eat, such as hake, rockfish, sardine, and anchovy.

“To me, it’s a cause for worry,” said Tony Koslow, former director of the CalCOFI program, at Scripps Institution of Oceanography, which sends ships out four times each year to collect oceanographic and biological data.

Sardine and anchovy, which normally cycle in abundance, are both low now, a combination that strains the food resources available to predators like sea lions, Koslow says. Though the cause of the decline is still unknown, it doesn’t seem to be the result of overfishing. “At this point, it’s hard to point the finger at us,” he said.

And so, the sudden spike in sea lion deaths are still a mystery. Whether the cause is something oceanographic, a pathogen that has managed to evade detection for months, or fish populations slipping below a crucial threshold – or perhaps some combination of these factors – is still unclear.

“There’s something in the environment that the sea lions picked up that we didn’t, and we’re still trying to sort out what that is,” Melin said.

As Melin and I step out from behind the dune, I turn to look again at the lively scene on the beach. All seems well, at least for now. Whatever secret hides beneath the waves offshore is invisible today, a menace tucked beneath an almost impossibly emerald sea. As I head back to camp, the sea lions’ barks and bellows follow me, just as the summer fog rolls in and hides the beach from view.

Source: http://www.wired.com/wiredscience/2013/08/island-sea-lion-pupdate/

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