The Mystery of What’s Killing Sea Lions in California

The Crime and the Victims

It’s a murder mystery on a grand scale. The story has only recently been publicized yet few people seem to have heard it. It has taken almost 40 years to unravel the details. I learned about the story during a visit to the Marine Mammal Center in Sausalito, California. The Center is the largest marine mammal rescue center in the world. It is here that a dedicated corps of volunteers, veterinarians, research scientists and others followed a trail of suffering to discover what amounts to a particularly shocking crime. A crime that reverberates along a broad swath of the California coastline.

The victims are California sea lions. The murder weapon is a deadly substance known as DDT, an insecticide originally developed in the lead up to WWII to protect troops from insect-borne diseases. Its effectiveness in killing insect pests led to the chemical’s wide disbursement for almost three decades until it was banned due to findings that it had adverse environmental effects. DDT is a lipid soluble toxin and, as such, accumulates in an animal’s fatty tissues. DDT was eventually banned in the US due to findings that it had adverse environmental effects.

The Murder Weapon

DDT has been classified as a probable human carcinogen. But in California sea lions, DDT has been shown to have a direct link to cancer. The result of three decades of research has shown that one in four adult autopsied California sea lions had urogenital carcinoma. This frequency is the highest prevalence of this type of cancer in any mammal including humans. Cancer rarely occurs in wild animals. Like the proverbial canary in the coal mine, this tale is both tragic and a cautionary for the fate of the ocean and the threat to humans.

With the mystery of what is killing the sea lions solved, the questions becomes: how did the sea lions ingest the DDT? The source of the DDT is directly related to the ban. A Los Angeles Times article provided the answer. Barrels of DDT were discovered by a University of California Santa Barbara scientist using a deep-sea robot. There are an estimate of upwards of 500,000 barrels in the deep waters off the Channel Islands, an eight-island archipelago located about 20 miles off the coast of Los Angeles, California. This is a primary breeding area for the sea lions and even in the non-breeding season they congregate by the hundreds, hauling out on the beaches and rock.  

The Lasting Effects

While sea lions do not directly ingest the DDT, they eat fish and squid that eat creatures that do. The level of DDT in the tissues bioaccumulates or increases with each animal eaten up the food chain. The sea lion, the top predator, is thus the recipient of the largest bioaccumulation. A humbling thought is that humans eat some of the same seafood that sea lions eat. The DDT continues to be exist in a toxic form. The chemicals haven’t broken down in any significant way since the barrels were dumped. It is unclear whether the toxins will ever be harmless so there is no way of knowing how long the sea lions or any sea creatures are going to be exposed.  

The Heros

What is known for certain is that the MMC staff and its more than 1300 active volunteers rescue sick and injured animals as well as abandoned pups. They make every attempt to rehabilitate and release the animals back in the wild. Deceased animals are autopsied and the findings are shared with the public and tissues are made available to researchers, all with the hope of promoting the health of the ocean and her creatures. The Center operates 24 hours a day, 365 days a year,  answering more than 10,000 calls each year from people reporting sick, injured, and abandoned marine mammals. The MMC is equipped to work with seals, sea lions, dolphins, sea otters, and on occasion whales and sea turtles. They try mightily not to turn any marine mammal away. Their reputation is so stellar that a marine mammal may be transferred to them for veterinary care. And, because of their size and experience, the Center is the preeminent teaching veterinary and research hospital for those wishing to study marine mammals.  

How You Can Help

The mystery has been solved thanks to the tireless and enduring work of these ocean stewards. It, indeed, is inspiring. Should you be in the San Francisco area, it is well worth a trip to the Marine Headlands to visit the Center. You can learn more by visiting the Marine Mammal Center website or donate directly through the link on Women Mind the Water.