California Sea Otters Need Kelp Cover To Recover
Ever since legal restrictions were imposed on maritime hunting practices in 1911, the populations of many aquatic mammals have recovered impressively, allowing species like the California sea lion and northern elephant seal to reoccupy their historical habitats. However, the sea otter population has not expanded to habitats outside of a small geographic range. Part of the reason sea otters struggle to expand spatially is that the geographic range they can occupy is limited by the amount of kelp cover available.
This study provides a novel perspective on the importance of kelp forest ecosystems for reducing threats to sea otters, and makes a strong case for human intervention to help sea otters expand their population range.
Kelp forests provide a source of prey for the sea otters, limit pathogen and toxin ingestion, shelter them from shark attacks, and serve as a nursery habitat for females rearing pups. When kelp cover is unavailable, two types of threats become more pronounced: the first are density-dependent threats, which are challenges that worsen as the population grows, like emaciation and interbreeding; the second are density-independent threats, which indiscriminately affect members of a population regardless of its size. Examples of density-independent threats include shark attacks and neurological disease.
This study aimed to understand how the absence of kelp cover is constraining California sea otters to small geographic pockets. To do this, the researchers collected data on 725 sea otter live strandings from 1984-2015 using records from stranding response teams at the Monterey Bay Aquarium. A live stranding is when an aquatic animal washes up on the shore and needs human intervention to return to its habitat. Stranding cause was determined for each sea otter. There were five stranding causes discussed most extensively in this study: neurological disease, shark bite, trauma (due to boat strikes, net entanglement, etc.), emaciation, and other.
By modeling interactions between the stranding causes and the available kelp cover, the researchers revealed four important findings: (1) stranding rates are increasing at the edges of the sea otter geographical range (i.e. range periphery) ; (2) emaciation is strongly associated with sea otter density, which is increasing in the center of the geographic range; (3) gaps in kelp cover correspond to high rates of shark bites, neurological disease, and other density-independent threats; and (4) trauma caused by human activities is no longer a substantial threat to sea otter populations.
These results indicate that sea otters are unable to expand their spatial range because there are gaps in kelp cover at the periphery of their range. These gaps contribute greatly to density-independent threats like shark attacks. Because sea otters cannot find food and are at greater risk of trauma and attack at the periphery, they concentrate in the center of their geographical range. However, this causes problems as well, because it increases sea otter density, resulting in increases in density-dependent threats like emaciation.
While the enactment of the International Fur Treaty has limited negative human impact on sea otter populations, there is more we can do to ensure these species are able to occupy their full historical range. Interventions to create more sheltered habitats and disperse sea otters to areas farther north may help sea otters repopulate these areas and expand beyond the limited kelp covers they are constrained to.