It is estimated that the worldwide population of sea otters once numbered between several hundred thousand to over one million before being nearly hunted to extinction by fur traders in the 1700s and 1800s. Sea otters finally gained protections with the signing of the International Fur Seal Treaty of 1911, and became listed under the Marine Mammal Protection and Endangered Species Acts in the 1970s. Worldwide, numbers have slowly recovered but still stand far below original population numbers. While sea otters are vulnerable to natural predators, their populations are significantly impacted by several human factors as well.
Conflict with Humans
Direct conflict with humans, such as shootings and entrapment in fishing traps and nets pose a major threat to sea otter populations. Since sea otters eat many of the same shellfish humans like to eat, such as sea urchins, lobster and crab, they often find themselves in the same areas fishermen like to harvest. Some shell fishers view sea otters as competition and a threat to their economic gain. Many fishermen use fishing gear that can entangle sea otters and cause them to drown.
Whether intentionally or unintentionally, sea otters who find themselves too close to a fisherman’s harvest are often harmed or killed. Fortunately, the number of sea otters deaths from human conflict is slowly decreasing as a result of their protection under the Endangered Species Act  and increased regulation of fishing nets.
Oil spills from offshore drilling or shipping are an immense threat to sea otter populations. When sea otters come into contact with oil, it causes their fur to mat, which prevents it from insulating their bodies. Without this natural protection from the frigid water, sea otters can quickly die from hypothermia. The toxicity of oil can also be harmful to sea otters, causing liver and kidney failure as well as severe damage to their lungs and eyes.
A historic example of the impacts oil spills have on sea otters is the 1989 Exxon Valdez oil spill in Alaska’s Prince William Sound, which killed several thousand sea otters. Sea otters are still threatened by events like this because countries around the northern hemisphere continue to ship and drill for oil throughout the Pacific and along coastal areas that sea otters call home. Because their numbers are low and they are located in a rather small geographic area compared to other sea otter populations, the California sea otter is especially vulnerable, and could be devastated by oil contamination.
Pollution on land runs off into the ocean, contaminating the sea otters’ habitat . This can jeopardize their food sources, as well as harm them directly. Sea otters are often contaminated with toxic pollutants and disease-causing parasites as a result of runoff in coastal waters. In California, parasites and infectious disease cause more than 40% of sea otter deaths. Hundreds of sea otters have succumbed to the parasites Toxoplasma gondii and Sacrocystis neurona, which are typically bred in cats and opossums. Scientists have also reported the accumulation of man-made chemicals, such as PCBs and PBDEs, at some of the highest levels ever seen in marine mammals.