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Behavior and Population Biology |
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In 1994, WERC scientists began a study to document the behavior and population biology of the Washington sea otter
population, which had been reintroduced (translocated) to the state from Amchitka Island, Alaska in 1969 and 1970.
Research was needed to understand how behavior and life history vary with population status, and to help evaluate the
status of other populations, particularly the threatened southern sea otter (Enhydra lutris nereis) population in
California.
Radio telemetry and visual observations of marked individuals are being used to compare foraging, social behavior, and demographic parameters among Washington, California, and Amchitka Island, Alaska sea otter populations. Thus far, in Washington, 92 sea otters have been captured and 63 have been implanted with radio transmitters. Observations of prey taken, dive times, surface time, and success/failure ratios can indicate the status of a sea otter population. Sea otters are suited for this kind of research because they generally consume their prey at the water's surface. |
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| When sea otters reoccupy historical habitat they often prey on macroinvertebrates such as red sea urchins. Photo by R. Mattison. | |
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In Washington, there exists a unique opportunity to compare foraging behavior within the same population but in two
different types of habitat. Some sea otters are foraging in areas where the species became reestablished following
reintroduction in 1969 and 1970. As of 2000, those habitats have been continuously occupied by sea otters for thirty
years. But the sea otter is also expanding its range beyond those areas of early reestablishment to habitat unoccupied
by sea otters since the end of the 19th century, when this species had been hunted to the brink of extinction due to
the maritime fur trade. These "new" habitats have been occupied by otters for less than 5 years.
In surveys of new habitat use, red sea urchins (Strongylocentrotus franciscanus) were the otter's principal prey, accounting for about 68% of the prey taken in 1997. By 1998, the percentage had decreased to about 38%, suggesting urchin abundance was decreasing in these areas. In areas reoccupied by sea otters for decades, red sea urchins account for less than 1% of the prey taken, and species such as snails, clams, and crabs are the most commonly taken prey. WERC scientists predict sea urchins will comprise a smaller and smaller proportion of the sea otter's diet in the newly-occupied habitat in Washington, ultimately reaching proportions comparable to areas occupied for decades. As a result the structure of the nearshore community will approach that of areas occupied for decades. Large exposed macroinvertebrates, such as sea urchins, will be restricted to cryptic habitats providing refugia from sea otter predation, and nearshore macroalgae will flourish. Preliminary data collected from subtidal surveys near Tatoosh Island support this prediction. Urchin densities have declined and there is some evidence of new algal recruits in transects previously dominated by sea urchins. |
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