by Todd McLeish
The results of the 2016 survey of sea otters in California found 3,272 otters, including nearly one hundred animals at San Nicholas Island, with the remainder along the central coast bounded by Point Conception in the south and Half Moon Bay in the north. The problem is clearly not at San Nicholas Island, where sea otter numbers have increased annually by 16 percent in recent years. Food is abundant there, and where food is abundant, populations can grow quickly. Because the island is small, however, Tinker anticipates that the San Nicholas Island population growth will begin to slow soon after the abundant and easy-to-access food becomes depleted. But he’s not worried about what is happening there. The big worry has been the mainland population, where there has been no measurable growth over the last five to seven years. About 80 percent of all southern sea otters are found in the central part of the coastal range, between the northern edge of the Monterey peninsula and the resort community of Cayucos, just north of Morro Bay. The otter population is quite dense along that 115-mile stretch, yet it hasn’t grown in twenty-five years.
“That population is at dynamic equilibrium,” said Tinker. “You might get a couple of very productive years and it will go up a bit, but then you’ll get less productive years and it will go down a bit. It’s at carrying capacity.” That means that given the food and habitat available, the environment in that stretch of coastline already has the maximum number of otters it can sustain. And in this particular case, the limiting factor is food. There isn’t enough food available there to provide for a larger population of sea otters. And as a result, it is virtually impossible for the otter population to grow there. Even if it were possible to remove some of the sources of mortality, like disease or boat strikes, the otters would die of something else because there just isn’t enough food to sustain them.
However, at the northern and southern extremes of their range, where otters have only recently begun to recolonize, they are found in lower densities and food is relatively abundant. “What ultimately limits sea otters in a given area is their prey,” Tinker said. “That’s the definition of an apex predator. They’re not limited by animals above them on the food chain. They’re limited by the trophic levels below them.”
While the otter population had experienced strong growth at the edges of its coastal range as the animals expanded into new areas, that growth stopped in the early 2000s. That’s when mortality from shark bites skyrocketed in those two regions, limiting range expansion to the north and south and eliminating population growth in the only parts of the coastal range where population growth was even possible. According to Tinker, there has always been a high level of shark-bite mortality in the northern part of the sea otter range between Santa Cruz and Half Moon Bay because large numbers of great white sharks are attracted to nearby Año Nuevo Island to feed on the massive herd of elephant seals that congregates there each winter. That area had typically been a region dominated by male sea otters, so while otter mortality from sharks slowed overall population growth, it didn’t have a significant effect on long-term population trends, since few females were killed there. It’s the females that drive range expansion and population growth. But when shark-bite mortality increased at the southern end of the range, where both males and females were being killed, otter numbers began trending downward. Tinker described it as “a new nucleus of shark mortality” that began near Pismo Beach and expanded from there. Whereas the population in the southern third of the otters’ range had previously been growing at 5 to 6 percent per year, it began declining at 2 percent per year after 2005, with nearly 70 percent of all otter deaths in the region caused by sharks. Tinker said the sea otters in the area of the Santa Barbara Channel were big and healthy, reproductive success was high, and food was abundant. If it weren’t for the sharks, that area could have grown at rates comparable to San Nicholas Island, fueling growth in the statewide population despite the no-growth region in the middle part of the range. “Right now there’s a 30 percent chance that an otter of any age in any one year is going to die from a shark bite,” Tinker said. “That’s just too high. When you have that level of risk from one source of mortality, you cannot really sustain population growth.”
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UNFORTUNATELY, there is little that can be done about shark-bite mortality. Like sea otters, great white sharks are protected in California waters, and no one really has any idea how many white sharks even live there. Unlike sea otters, which spend much of their lives at the surface of the water, sharks never need to surface, so counting them is extremely difficult. But Chris Lowe, director of the shark lab at California State University in Long Beach, is convinced that great white shark numbers are increasing. At nursery areas in Southern California, he has observed a steady increase in baby white sharks over the last ten to fifteen years. He thinks that a 1994 law banning gillnet fishing within three miles of shore has reduced the number of sharks killed by commercial fishermen and is boosting the overall population. He also thinks improved water quality and improved management of coastal fisheries have led to more fish being available as prey for juvenile sharks as well as for seals and sea lions, whose populations have exploded since the 1970s.
Does this purported white shark population growth explain why so many sea otters are being killed by sharks? Maybe. Lowe says one hypothesis is that the increasing number of subadult sharks—those that have reached nine feet in length—begin to realize that they can prey on larger animals than the flatfish and stingrays they ate as juveniles. In their naïveté, they take a bite of a sea otter. It may also be that otters are wandering farther offshore in search of food, making them more accessible to adult sharks. Or perhaps it’s adult sharks moving back to their preferred feeding grounds closer to shore after human conflicts had previously pushed them away from the coast. No one really knows which it is, or if it’s something else entirely.
Perhaps worst of all, however, is that the sharks don’t even consume the otters. Rather than gaining the abundant nutrition from eating otters, the sharks just chomp down on them—Tinker called it “taking a taste”—and spit them out. “We’ve never seen a partially eaten sea otter, and no white shark biologist has ever found otter remains in a white shark,” explained Tinker. “It looks like they’re just taking a single bite. We have thousands of otter carcasses that have been bitten by sharks, and there’s never been any tissue missing. It’s always a single bite.” Presumably, the sharks bite the otters to determine whether they are something the sharks want to eat, like a seal, and after that first bite the sharks decide they don’t want any more. A population of white sharks apparently does the same thing to seabirds in Australia. In the case of sea otters, the animals end up with a semicircular pattern of puncture wounds on both sides of their body. The sharks also leave scrapes along otter bones—telltale markings from their serrated teeth—and occasionally chips of shark teeth are found in otter carcasses. Both confirm to pathologists that great white sharks, and not another species, caused the damage. If the otter does not die immediately from acute trauma, it dies days or weeks later of an internal infection as a result of seawater entering the body cavity through the punctures. Several sea otters implanted with data recorders have been bitten by sharks, and when the data was retrieved, it showed that at the moment the animals were bitten, they experienced an instantaneous drop of four degrees in body temperature. Most probably went into shock and died as their core temperature continued to plummet.
So without any other options, Tinker said the best that can be done about the increase in shark-bite mortality is to keep an eye on it and try to learn from it. No one in the marine-science world would advance a plan to control or reduce the population of great white sharks to protect sea otters, even if it were legal and possible. And there is no way to keep the otters from areas where sharks are known to feed. So even though sharks are now the leading cause of death among otters at the northern and southern ends of their range in California, Tinker said the focus of sea otter conservation should s
till be on understanding and reducing the sources of mortality that can be controlled, like boat strikes, pollutants, and pathogens caused by human activities. “That’s not going to solve the shark problem,” he said, “but it will certainly mitigate the overall level of mortality.”
Chris Lowe agrees. It would be a different story if one of the animals were an invasive species, but a native shark preying upon a native otter is entirely natural. “There may be some losers in this situation, but every time humans try to intervene in these kinds of situations, we really screw things up,” he said. “These animals have been coexisting for millions of years, so my guess is that the sea otters will eventually get smarter.” That’s assuming that limited food resources don’t put them further at risk.
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THE PHENOMENA of shark-bite mortality at the edges of the sea otter’s range and food limitation in the middle of the range are relatively recent explanations for the lack of growth in the otter population in California. But growth was slow long before those issues came to the fore. For almost their entire careers Tinker and Estes have been trying to understand why that has happened, and they think they finally know. They describe their conclusion as a paradigm shift in the way they think about how and why populations grow. But it’s not a particularly satisfying answer to those of us who love to solve mysteries. The answer, they now say, is that the population hasn’t been growing slowly after all; it has been growing at the speed at which it is supposed to grow.
According to Tinker, there was never a problem with the California population of sea otters. Nothing out of the ordinary was happening to them to keep their numbers low when compared to otters in other regions. Despite years of detective work, there was never really a mystery that needed solving. The tool that the scientific community needed to understand the situation was the science of spatial ecology. That’s because the reason California growth rates are so much lower than elsewhere has everything to do with the space the otters are living in. After fifteen years of intensive monitoring of individual sea otters in California and Alaska, Tinker found that survival rates and growth rates were similar if he assumed that habitat and food availability were similar, too. But they are not.
If you imagine a map of the central coast of California, from San Francisco south to Santa Barbara, it’s a long, somewhat straight line with habitat for sea otters extending only a couple hundred yards offshore. Compare that to Southeast Alaska, including the 1,100 islands of the Alexander Archipelago, and you’ll see why space is so important to understanding sea otter growth rates. Although the linear distances—as the crow flies—are quite similar in California and Southeast Alaska, the space available for sea otters to live in Alaska is many times greater because of all the habitat bordering the islands and bays and inlets that make up its geography. Tinker describes the space for the otter population in California as a long, narrow thread of habitat compared to the sponge of Southeast Alaska.
“So when sea otters are expanding along this thread of habitat,” he said, “very quickly most of the otters are going to be stuck in the middle and the only areas where it’s going to be growing are the two ends of the thread. But when you’re growing in a sponge like Southeast Alaska, it’s actually spreading in all directions.” In addition, the sea otters in California started from one small group in Big Sur, while those in Alaska started from six small groups relocated from the Aleutian Islands. “If you think of it like a paper towel and you’re dropping water at six different locations, it’s going to spread over that whole paper towel really fast. In California, though, you have a really long, narrow thread of paper towel, and when you drop just one spot of water in the middle, it’s going to spread very slowly north and south. The otters in any one place are no sicker or poorer, they have babies at the same rate, they live and die at the same rate. It’s just that in California they quickly reach carrying capacity, and in Alaska it’s still a growing population.”
When Tinker plugged a bunch of habitat maps and numbers into his complicated computer model, beginning with one hundred sea otters in California in 1938, the model concluded that in seventy-five years there should be three thousand otters in California. Which there are. And when he did the same thing for Southeast Alaska beginning with the numbers from 1964, when the animals were relocated there, the model indicated that there should be twenty thousand otters after forty years. And that’s about right, too.
There is only one way to make the California population of sea otters grow at rates approaching those in Southeast Alaska, he said. And that would be to establish additional new populations at other sites along the coastline that do not presently have any sea otters. The California population would have grown in a similar fashion to Southeast Alaska’s if it had started from multiple populations like the Southeast Alaska population did. And it still could, if a decision were made to introduce additional populations in Northern and Southern California. “In the first ten years you wouldn’t see any difference, but over fifty years you will end up with twice as many otters as you would otherwise,” Tinker said.
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AS TINKER CONTINUES to fine-tune his research into the population dynamics of sea otters, one of his doctoral students is investigating their little-known sensory system. Sarah McKay Strobel said that sea otters don’t appear to see or hear or smell much better than most other marine mammals or their close relatives in the Mustelidae family, but their tremendous efficiency at finding the enormous quantity of food their metabolism requires every day suggests that they have unique abilities that may be as yet undiscovered. Although the animals are relatively easy to observe and study at the surface, what they do underwater and how exactly they detect and find food is uncertain. Strobel studied the sensory systems of fish as an undergraduate, and she hopes she can use that experience to identify how sea otters sense their environment. “The world only exists in the way we sense it,” she said. “Humans are biased toward the visual, but sea otters may sense the world very differently.”
Strobel is hoping that a captive sea otter named Selka, formerly known at the Monterey Bay Aquarium as Otter 595, will provide the answers she seeks. Selka became separated from her mother and stranded on a beach when she was just one week old, in July 2012. She was raised for several months by two surrogate mothers at the aquarium—the first of which died suddenly of a brain tumor before Selka was weaned—and it was later determined that Selka could not be released back into the wild. So she was introduced to Strobel and is living in captivity in an outdoor tank originally built by the California Department of Fish and Wildlife to hold rescued seabirds and marine mammals following an oil spill. Strobel is conducting a series of behavioral tests designed to reveal the effectiveness of each of Selka’s senses.
I visited just days after Strobel and Selka had been introduced, so they were just getting to know each other. Selka was exploring her new environment, a twenty-foot-diameter tank filled four feet deep with seawater pumped from the ocean a quarter mile away. Hanging from a walkway across the tank was a mass of artificial kelp that researchers refer to as “car wash kelp,” the same material that hangs from the ceiling at automatic car-wash centers and is slowly dragged across your car to scrub the dirt from your vehicle. As I watched, Strobel poured a bucket of ice into a small kiddie pool at the edge of the tank, which prompted Selka to jump out of the water. It was feeding time. Hidden amid the ice were shrimp and large pieces of clam meat, part of the otter’s training in how to find food.
Propped on the edge of the pink pool not five feet from where I stood, Selka was a stunning beauty. Her lustrous mahogany coat shone from every angle, reminding me of my grandmother’s mink stole. Not a hair was out of place—even her whiskers appeared well coiffed—unlike some wild otters that often look more like Albert Einstein than Jennifer Aniston. And she almost seemed to pose to make sure I noticed her stylishness.
Despite the abundant food before her, Selka didn’t appear to be in any rush to di
g in. She knew she had no competition for the delicacies concealed beneath the ice, so she deliberately pawed through the cubes, selected the item she desired, and carefully placed it in her mouth before proceeding to the next one. Once, she even yawned between bites, briefly showing off her incisors, which appeared unnecessarily large for the requirements of the meal. The longer she took to eat, however, the longer she was out of the water and baking in the hot sun, where she could quickly become overheated. She seemed to sense the approaching discomfort, so she slowly slid over the edge of the shallow kiddie pool and sprawled on her belly in the ice while continuing to search for the rest of her hidden meal. As she luxuriated in the refreshing tub, forcing her head and shoulders beneath the ice cubes—and once even rolling over to do a quick backstroke—I could almost sense her feeling of relief.
Soon enough, Selka had found and consumed every bit of her food and leapt back into her large blue tank. She was purposely not offered molllusks, since she hadn’t yet learned how to open them. “Older otters are like a machine at finding food,” said Tinker. “A two-year-old like Selka might take four or five minutes to open a crab, something an older otter could do in thirty seconds. It emphasizes their specialization skills,” which Selka had not yet developed.
