Sea Otters

 Resource Brief - March 2009

Importance/Issues

Sea otters (Enhydra lutris) are important mammalian members of the nearshore community throughout the north Pacific, and are the only marine mammal that relies exclusively on shallow or intertidal macro-invertebrates as prey. Sea otters were selected as a SWAN vital sign because they are a textbook example of "keystone" carnivore. By consuming 'grazers,' the animals that feed on kelp, sea otters dramatically change the structure and complexity of their ecological community resulting in communities characterized by diverse and abundant algae and relatively few large grazing invertebrates such as urchins. Other well documented sea otter mediated predation effects include reduced biomass and size distributions of many large and conspicuous invertebrates, such as clams, mussels, urchins, and crabs. Sea otters tend to be relatively sedentary in comparison to other marine mammals; eat large amounts of food; have an incidence of disease that is correlated with contaminants; and have broad appeal to the public. In September 2005, the Western Alaska Stock of sea otter, which include the Katmai NP, but not the Kenai Fjords, was federally listed on as threatened. There are three major components to sea otter monitoring in the nearshore. The first relates to estimating the distribution and density of sea otters. The second to estimating the age composition of dying sea otters, and the third relates to their role and response to the structure and function of nearshore communities. Because sea otters often respond to vessels in ways that reduces detection and they can occur in shallow water many km offshore, skiff surveys provide biased estimates of abundance. Consequently we employ a small float equipped 2 passenger aircraft to conduct aerial surveys. Typical ages at death in sea otter populations consist of relatively large proportions of very young (<2 yrs) and old (>8 yr), and relatively few prime age individuals. Monitoring ages at death provides a powerful and efficient means to model survival over time. Because of their complete reliance on nearshore marine invertebrates as forage, the sea otter provides a mammalian analog to the black oystercatcher, to view the nearshore zone from the perspective of a predator, as well as allowing us to use the recognized relations between sea otters and their prey to allow inferences between the otters and their prey populations. Specifically, over the long-term the sampling design will facilitate inferences relating density of sea otters to their diet (as reflected by species, number, and sizes consumed) and to direct measurements of their prey populations.

Sampling Design

To estimate sea otter abundance and distribution, systematic aerial surveys of transects within marine habitats between the shoreline and the 100 m bathymetric contour are sampled. Methods to estimate detections, based on prolonged searches of a sub-set of those transects provides correction for otters diving, or otherwise not detected on transects.

Collection of sea otter carcasses that become beach-cast provides an estimate of the age composition of animals dying in the population. Age specific likelihood models can be used to evaluate how changes in survival contribute to changes in the age distributions of animals that are found dead on the beaches. Systematic searches of depositional beaches or haul-out sites will be conducted annually to obtain estimates of ages at death based on cementum annuli in teeth.

Because sea otters are relatively shallow divers, most of their foraging is close to shore, and because they bring their prey to the surface for consumption, their diet is relatively easy to determine through direct observation with high resolution telescopes. The purpose of collecting data on sea otter diet is to quantify foraging success and intensity as well as the types, number, and relative sizes of prey being eaten by sea otters. A stratified random sampling design will be used to obtain foraging data and to examine differences in foraging and prey species recovered between regions, years, and the interaction between regions and years. These data provide information on the status of sea otter populations (amount and types of food obtained per unit effort) as well as the relative abundance of prey items (as measured indirectly by sea otter foraging success and prey selection).

Progress and Current Efforts

In 2007, between 18 and 28 June we completed an aerial survey of sea otter distribution and abundance at Kenai Fjords NP. The estimated population size was 1,511 with an average density of 1.02/km2. Due to a lack of aircraft and pilot availability we did not complete the sea otter aerial survey of the Katmai NP in 2007. It is being rescheduled for 2008.

In 2006 we collected sea otter carcasses from the Katmai coast during spring and summer. Accessing beaches during spring was difficult and only one carcass was obtained. During the summer we located several offshore islands where sea otter apparently hauled out during winter and carcasses were relatively abundant. In 2007 we also obtained relatively high numbers of carcasses from these same islands. In 2007 we did not locate any shorelines at Kenai Fjords NP where sea otter carcasses were similarly abundant and only obtained one carcass. Additional effort may be required to increase sample sizes of carcasses at Kenai Fjords NP

In 2006, sea otter foraging data was collected in association with three of the five rocky intertidal intensive monitoring sites along the Katmai NP coast. Sea otter abundance was low at the two remaining sites and at the selected Ninagiak site, precluding acquiring sea otter forage data. Two observers spent about 21 hours (42 observer hours) acquiring feeding data from 65 forage bouts and about 500 dives. Diet was numerically dominated by bivalves, primarily clams, but included a diverse array of other invertebrates including, stars, urchins, crabs, chitons, snails, octopus. In 2007 we obtained comparable sea otter forage data from those same sites at Katmai NP. In 2007 we initiated collection of sea otter forage data associated with the sheltered rock intensive intertidal monitoring sites along the Kenai Fjords NP.

USGS biologists observing foraging sea otters with high resolution telescopes to determine foraging success, composition and size of prey, and number of prey items recovered, KATM 2006.