Sea otters are threatened by oil spills. The oil mats their fur, so it can’t insulate them and the otters perish from hypothermia. The Exxon Valdez disaster in 1989 killed several thousand in this way. Runoff water containing agricultural fertilisers from land can cause blooms of harmful algae on the sea surface. These can produce toxins that otters ingest, potentially leading to sickness and even death.
Pollutants and parasites from land pose a risk to sea otters. Parasites usually found in cats and opossums can cause disease. In California this is responsible for 40 per cent of otter deaths.
Sea otters eat shellfish that are also commercially fished, unfortunately putting them in competition with human fishermen. Otters can get tangled in fishing gear or hit by the increasing boat traffic.