Sea otters live in coastal waters 15 to 23 meters deep and usually never go more than a kilometer from the coast. They are most often found in areas sheltered from the strongest ocean winds, such as rocky coasts, dense kelp forests and coral reefs.
Although most closely associated with rocky substrates, sea otters can also live in areas where the seabed consists primarily of mud, sand, or silt. It is thought that there were between 150,000 and 300,000 sea otters in the past.
In about two-thirds of its native range the species is currently at various levels of recovery, with high population densities in some areas and threatened populations in others. Currently, sea otters are present in stable populations in parts of the Russian east coast, Alaska, British Columbia, Washington State and California, with signs of recolonization in Mexico and Japan.
Population estimates made between 2004 and 2007 give a grand total of about 107,000 specimens. Their presence affects the ecosystem more profoundly than their size and number would suggest. They keep the population of some benthic herbivores under control, especially sea urchins.
Sea urchins graze on the underside of kelp stalks, causing them to detach from the substrate and thus cause their death. Loss of habitat and nutrients supplied by kelp forests has profound cascading effects on marine ecosystems.
Areas of the North Pacific lacking sea otters often turn into barren urchin carpets, with no kelp forests. Kelp forests are extremely productive ecosystems, as they sequester carbon dioxide from the atmosphere through photosynthesis.
Sea otters may therefore help mitigate the effects of climate change through their cascading trophic influence. The reintroduction of sea otters to British Columbia has led to dramatic improvements in the health of coastal ecosystems, and similar changes have been observed in places where the otter population has recovered, including the Aleutian and Commodore Islands and California's Big Sur coast.
However, some kelp forest ecosystems in California have thrived even without sea otters, since sea urchin populations appear to have been kept in check by other factors. The role of sea otters in maintaining kelp forests has been observed to be more important in open coastal areas than in more sheltered bays and estuaries.
Sea otters also make their influence felt in rocky ecosystems dominated by mussel carpets, removing these molluscs from rocks. This allows other sessile species to find space to adhere to the reefs, thus increasing species diversity.