State of the Sea Lions

U.S. Population Sits at Around 250,000 Animals, Half of Which Call San Miguel Island Home

California sea lions
Richard Salas

In an upcoming lecture on September 14, wildlife biologist Dr. Sharon Melin will entertain and educate the public about those wonderful marine mammals, the sea lions. As Californians, we are blessed with a plethora of sea creatures living just outside our doors. The observant Californian might notice that sea lions are particularly prevalent, and for very good reason. Forty percent of the United States’ sea lion population lives just off the coast, on San Miguel Island, the only place in the world where four species of sea lion and seal breed in one place. And the population continues to grow.

Research on San Miguel Island has been ongoing for 40 years, and Dr. Melin has been participating for the last 25. In a recent phone conversation, she stressed the importance of continued research on sea lions, because researchers view them as “the indicators of ocean and human health linked to us in terms of the environments that they use,” said Melin. “They are an assimilation of what they are getting out of their environment.” This is, of course, the same environment that gives rise to our fishing industry. If the sea lions aren’t healthy, then our coastal communities are most likely also suffering.

Before the Marine Mammal Protection Act was enacted in 1972, the sea lion population was woefully small. Decades of exploitation — for the fur trade, the Asian market (which considered “trimmings” of whiskers and testicles of males to be aphrodisiacs), and even the use of sea lion meat as pet food — left very few sea lions for biologists to study. Post-1972, the population grew at a remarkable 5 percent each year until the 1990s, when growth leveled off at a steady annual one percent. The most recent census in 2007 left the United States’ population at a hefty 250,000 sea lions. These numbers give Melin what she referred to as “a pretty good idea of the overall population health and the health of the ocean current.”

With this steady increase in sea lion population — and it is no small increase, considering that bulls grow up to 900 pounds and nearly eight feet in length — new management challenges are surfacing. Melin discussed issues that arise when sea lions compete with endangered salmon species, sink people’s boats, and even take over whole docks. “We never had these problems before,” she said.

Part of the reason for the recent growth in sea lion ruckus is that they lack natural predators in the area. Killer whales and great white sharks do pose a threat to them farther north, but they cannot be seen as population managers. Diseases, however, are impacting the population in place of natural predators. Primarily it is hookworm that kills the pups, acting as a “population regulator.” In adult females, it’s demoic acid outbreaks, which are caused by a biotoxin found in algae consumed by fish, their primary prey. In adult males, it’s leptospiroris, bacteria that affects the kidneys.

While these diseases naturally slow population growth, other contributing population-regulators are climatic factors including El Nino, which disrupts the ecosystem roughly every five years, changing the availability of food and causing many animals to starve.

Despite these perils faced by sea lions, recent trends in population growth indicate the strong health of California populations. In fact, according to Melin, since sea lions are doing so well (and since they are considered the primary indicator of ecosystem health), then almost all marine mammals are doing well.

But there are still changes. Cancer is on the rise in sea lions, especially in females, and rates are higher than for any other wild population. Researchers do not yet know whether cancer in sea lions is endemic or comes from the environment. It appears as an outbreak in the urogenital region of female sea lions. Ovarian cancer and testicular cancer are also on the rise. Although researchers are still getting to know the disease, Melin said, “[There are] still a lot of contaminants in the water; PCBs and DDT, sewage outfall, agricultural run-off, etc.,” which could be contributing factors to the new types of cancer found in sea lions.

As for human-caused mortality and recent attention given to plastic bags and their detrimental effect on sea life, Melin said that it was, in fact, not consumed plastic bags that pose the greatest threat to sea lions. They rarely or never eat plastic bags (though they do swallow small rocks for unclear reasons), but rather get tangled up in them. Both adult and juvenile sea lions often become caught in old fishing nets and in the bands used to hold six packs of cans and other packaging plastics. Furthermore, the incidence of sea lions caught in gills nets is also on the rise, despite a moratorium on their use.

Part of what makes sea lions so interesting to researchers is their large wild population. Though they are not in captivity, they remain available for observation and study at all stages of their life cycle, making them more available for disease research than fish that migrate frequently. As Melin continues her research on the sea lions of San Miguel Island, she said, “The future challenge will be in conserving and managing the expanding population.”

Melin’s September 14 lecture is part of the ongoing lecture series “From Shore to Sea” put on by Channel Islands National Park with support from Channel Islands National Marine Sanctuary. The series is meant to further understanding of current research on the Channel Islands and their surrounding waters. Lectures continue the second Wednesday of the month through November of this year at 7 p.m. in the Channel Island National Park Robert J. Lagomarsino Visitor Center at 1901 Spinnaker Drive in the Ventura Harbor. Lectures are free and open to the public.


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