Things are looking up, but not too far up, for the threatened southern sea otter (Enhydra lutris nereis), according to an annual survey of the animal just concluded by the US Geological Survey (USGS).

Telescope counts from onshore coupled with aerial surveys performed by USGS scientists have, for the first time, seen the marine mammal's population push beyond a key threshold number that would allow the US Fish and Wildlife Service (USFWS) to remove the sea otter subspecies' current "threatened" status under the US Endangered Species Act (ESA).

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While that initial news is good for the otters, it comes with a few caveats.

First, the animals will need to repeat their strong showing for two more years before the USFWS could perform a formal review of their conservation status.

Second, officials are still concerned that there aren't enough otters at the northern and southern ends of their surveyed range, which runs from California's Pillar Point in San Mateo county in the north to Rincon Point, close to the Santa Barbara/Ventura County line, in the south and also includes a distinct population at San Nicolas Island.

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Predation may be behind the troubling numbers at the range ends.

"We are still seeing large numbers of stranded otters near the range peripheries, a high percentage of which have lethal shark bite wounds," said California Department of Fish and Wildlife biologist Mike Harris, in a statement.

Population growth at the range ends is considered key to the species' health.

"These are the portions of the population that typically fuel the colonization of new habitats," said USGS research ecologist Tim Tinker.

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Still, the trends overall are pointing up. According to the USFWS, the three-year running average of the annual census count has climbed from 2,939 in 2013 to 3,272 with this latest count.

The population index would need to surpass 3,090 for three straight years in order for the otter to receive consideration for removal of its threatened status under the ESA.

The USGS said increased populations in the center of the range account for the recent higher number, in part due to a more bountiful food supply, thanks to a "boom" in sea urchins off northern and central California.

The San Nicolas Island population, meanwhile, is experiencing a boom of its own. Begun with a relocation program at the island site in the late 1980s, sea otters numbers there are running a mean growth rate increase of 13% per year over the last 10 years.

"The sea otters at San Nicolas Island continue to thrive, and some may eventually emigrate to and colonize other Channel Islands in southern California," said the USGS' census coordinator Brian Hatfield.

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"The population index has exceeded 3,090 for the first time, and that's encouraging," said Lilian Carswell, USFWS southern sea otter recovery coordinator. "But sustained population growth will require range expansion, which means that sea otters will somehow have to get past the shark gauntlets near the ends of the current range."

"Over the longer term," she added, "it's not just sea otter numbers we're after but the restoration of ecological relationships in the ecosystems where sea otters and other nearshore species co-evolved."

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