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The recov­ery of south­ern sea otters appears to have taken an upturn, accord­ing to results from the annual Cal­i­for­nia sea otter sur­vey released by the U.S. Geo­log­i­cal Sur­vey (USGS) on 17 Sep­tem­ber. Yet despite an over­all increase in sea otter abun­dance, sharks have been “tak­ing a bite” out of the por­tion of the pop­u­la­tion that could fuel expan­sion into new areas.

“There’s much more to the story here than the main find­ing would sug­gest,” said Dr. Tim Tin­ker, a research ecol­o­gist who leads the USGS sea otter research pro­gram, “We are look­ing into var­i­ous fac­tors that may be affect­ing the sur­vey results, includ­ing a boom in urchin abun­dance from Big Sur to Mon­terey that may explain the uptick in num­bers in the range cen­tre, and high lev­els of shark bite mor­tal­ity that are likely respon­si­ble for con­tin­ued declines at the north and south ends of the range.”

This year’s sur­vey results sug­gest an increas­ing trend over the last five years of almost 2 per­cent per year and the pop­u­la­tion index, a sta­tis­ti­cal rep­re­sen­ta­tion of the entire pop­u­la­tion cal­cu­lated as the three-​year run­ning aver­age of cen­sus counts, has climbed to 3,054 from 2,711 in 2010. The growth is accounted for by an unex­pected jump in num­bers in the cen­ter of the sea otter’s range, an area that spans from Mon­terey south to Cambria.

“It appears that the high pup counts from the last few years might be trans­lat­ing into higher num­bers of juve­niles and adults in the cen­tre of the range,” said Brian Hat­field, the USGS biol­o­gist who coor­di­nates the annual cen­sus. “This makes sense if there are sig­nif­i­cantly more sea urchin prey avail­able to them in those areas. How­ever, our long-​term cen­sus data sug­gests the ele­vated num­bers of otters seen dur­ing this sur­vey along the Monterey-​Big Sur coast may not persist.”

While the pop­u­la­tion index con­tin­ues to trend upward, the north­ern and south­ern sub­sets of the pop­u­la­tion con­tinue a neg­a­tive five-​year decline, drop­ping 2 per­cent and 3.4 per­cent per year, respec­tively, num­bers con­sis­tent with increased shark bite induced mor­tal­ity in these same areas. The increase in white shark bites became evi­dent after 2005 and now appears to be impact­ing the growth and expan­sion of the pop­u­la­tion at the periph­eries of the range, as described in a recent pub­li­ca­tion.

Since the 1980s, USGS sci­en­tists have com­puted the annual pop­u­la­tion index and eval­u­ated trends in the south­ern sea otter (Enhy­dra lutris nereis), a fed­er­ally listed threat­ened species found in Cal­i­for­nia. For south­ern sea otters to be con­sid­ered for removal from threat­ened species list­ing under the Endan­gered Species Act, the pop­u­la­tion index would have to exceed 3,090 for three con­sec­u­tive years, accord­ing to the thresh­old estab­lished under the South­ern Sea Otter Recov­ery Plan by the U.S. Fish and Wildlife Ser­vice. To reach its opti­mum sus­tain­able pop­u­la­tion level under the Marine Mam­mal Pro­tec­tion Act, the num­ber of ani­mals that will result in the max­i­mum pro­duc­tiv­ity of the pop­u­la­tion, keep­ing in mind the car­ry­ing capac­ity of the habi­tat and the health of the ecosys­tem, the south­ern sea otter pop­u­la­tion would have to reach a much higher bar, prob­a­bly at least 8,400 ani­mals in Cal­i­for­nia accord­ing to the Recov­ery Plan.

Trends in sea otter deaths and shark bites
In addi­tion to con­duct­ing the annual sur­vey, USGS sci­en­tists also annu­ally update a data­base of sea otter strand­ings which tal­lies the num­ber of dead, sick or injured sea otters recov­ered along California’s coast each year. In 2014, sci­en­tists from Cal­i­for­nia Depart­ment of Fish and Wildlife (CDFW), USGS, Mon­terey Bay Aquar­ium and other insti­tu­tions recov­ered or doc­u­mented a total of 386 stranded sea otters.

This strand­ing num­ber only accounts for sea otters that peo­ple find, and past research indi­cates that pos­si­bly less than 50 per­cent of sea otters that die in the wild end up on the beach. But efforts are made to exam­ine each reported sea otter car­cass, and a sub­set of fresh car­casses are sent to the CDFW Marine Wildlife Vet­eri­nary Care and Research Cen­ter, where sci­en­tists con­duct necrop­sies to deter­mine the pri­mary causes of death and iden­tify fac­tors that may have con­tributed to the death of each animal.

Data from both liv­ing and deceased sea otters con­tin­ues to shed light on sea otter pop­u­la­tion ecol­ogy in dif­fer­ent parts of the Cal­i­for­nia coast. For exam­ple, a high pro­por­tion of sea otter car­casses recov­ered between Cayu­cos and Pismo Beach in recent years have white shark– bite wounds, a poten­tial expla­na­tion for the down­ward trend in sea otter num­bers in that area.

“Before the early 2000s we did not see very many shark bit­ten otters south of Mon­terey,” says Mike Har­ris, a biol­o­gist with CDFW, “but in the last few years, shark bite cases have become very com­mon and now explain about 70 per­cent of the total strand­ings in this area.”

The sea otter sur­vey and strand­ing pro­grams are just one part of a larger research pro­gram inves­ti­gat­ing sea otters and their role as preda­tors in coastal ecosys­tems. In Elkhorn Slough, a recent study sug­gests that sea otters’ appetite for crabs can improve the health of sea­grass beds, and USGS sci­en­tists are col­lab­o­rat­ing with biol­o­gists from the Mon­terey Bay Aquar­ium, the Elkhorn Slough National Estu­ar­ine Research Reserve, Uni­ver­sity of Cal­i­for­nia Santa Cruz (UCSC) and CDFW to study the pop­u­la­tion in this unique habi­tat. And a new study near Mon­terey by UCSC and USGS, in col­lab­o­ra­tion with Mon­terey Bay Aquar­ium, will inves­ti­gate how sea otters are respond­ing to a glut of sea urchins that may be in part a result of the loss of sea star preda­tors from wast­ing disease.

Sur­vey Method­ol­ogy
- The annual pop­u­la­tion index is cal­cu­lated from visual sur­veys con­ducted via tele­scope obser­va­tions from shore and via low-​flying air­craft along the Cal­i­for­nia coast­line by researchers, stu­dents and vol­un­teers from USGS, CDFW’s Office of Spill Pre­ven­tion and Response, Mon­terey Bay Aquar­ium, UC Santa Cruz, USFWS, and U.S. Bureau of Ocean Energy Man­age­ment.
- This year, the sur­veyed coast­line spanned from Pil­lar Point in San Mateo County, south to Rin­con Point near the Santa Barbara/​Ventura County line, and also included San Nico­las Island.

Mini doc­u­men­tary: Every­thing a sea otter enthu­si­ast needs to know! Katie joins a sea­soned nat­u­ral­ist out on the water to get the facts

{Source: Otter501 YouTube channel)

More infor­ma­tion on sea otters includ­ing a web cam can be found at the web­site of the Elkhorn Slough Foun­da­tion and Elkhorn Slough National Estu­ar­ine Research Reserve.

Elkhorn Slough otter cam high­lights:

(Source: USGS news release, 17.09.2015)