Cal­i­for­nia sea otter num­bers are up, accord­ing to the lat­est pop­u­la­tion sur­vey led by fed­eral, state and uni­ver­sity sci­en­tists. The rea­sons: more pups — and the addi­tion of San Nico­las Island sea otters to the pop­u­la­tion count.

Since the 1980s, U.S. Geo­log­i­cal Sur­vey (USGS) sci­en­tists have cal­cu­lated an aver­aged pop­u­la­tion index each year for the South­ern sea otter — Enhy­dra lutris nereis — a fed­er­ally listed threat­ened species found in Cal­i­for­nia. For the 2013 report, USGS lists the pop­u­la­tion index as 2,941 (data online). For south­ern sea otters to be con­sid­ered for removal from threat­ened species list­ing, 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.

Tin­ker, who super­vises the annual sur­vey, added, “Cer­tainly, sea otters have made an impres­sive recov­ery in Cal­i­for­nia since their redis­cov­ery here in the 1930s. But as their num­bers expand along California’s coast, they are fac­ing dif­fer­ent ‘grow­ing pains’ in dif­fer­ent locales. Our research part­ner­ship is inves­ti­gat­ing the fac­tors respon­si­ble for these local trends.”

Researchers from the USGS, Cal­i­for­nia Depart­ment of Fish and Wildlife’s Office of Spill Pre­ven­tion and Response, Mon­terey Bay Aquar­ium, Uni­ver­sity of California-​Santa Cruz and other insti­tu­tions col­lab­o­rate annu­ally to con­duct the sea otter sur­vey. The pop­u­la­tion index reported to USFWS is cal­cu­lated as mul­ti­ple year aver­ages of raw data from annual sur­veys, to com­pen­sate for year-​to-​year vari­abil­ity in obser­va­tion con­di­tions, and to give sci­en­tists a more reli­able pic­ture of sea otter abun­dance trends.

“We counted a record num­ber of pups this year, which led to the uptick in the 3-​year aver­age,” says USGS biol­o­gist Brian Hat­field, coor­di­na­tor of the annual sur­vey, “A high pup count is always encour­ag­ing, although the num­ber of adult otters counted along the main­land was almost iden­ti­cal to last year’s count, so we’ll have to wait and see if the pos­i­tive trend continues.”

There is a sec­ond rea­son for the higher pop­u­la­tion index reported this year. In 2013, the equa­tion for this pop­u­la­tion index was amended to add sea otters liv­ing at San Nico­las Island. One-​hundred-​and-​forty sea otters were intro­duced to the island in the 1980s as part of a USFWS recov­ery exper­i­ment, but most of them returned to the main­land, died, or sim­ply dis­ap­peared. USFWS com­pleted an exten­sive review of the translo­ca­tion pro­gram in Decem­ber 2012, result­ing in ter­mi­na­tion of the pro­gram. As a con­se­quence, sea otters at San Nico­las Island are no longer con­sid­ered to be an “exper­i­men­tal” pop­u­la­tion and will now be included as part of the California-​wide pop­u­la­tion index for south­ern sea otter recov­ery. The pop­u­la­tion at the island is now at 59 individuals.

Statewide Trends and Local Questions

USGS sci­en­tists also annu­ally update a data­base of sea otter strand­ings — the num­ber of dead, sick or injured sea otters recov­ered along California’s coast each year. In 2012, sci­en­tists from CDFW, USGS, Mon­terey Bay Aquar­ium and other insti­tu­tions came across a total of 368 stranded sea otters.

This strand­ing num­ber only accounts for sea otters that peo­ple find, and past research indi­cates that pos­si­bly less than 50% 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. In Elkhorn Slough, a new study sug­gests that sea otter appetites for crabs can improve the health of sea­grass beds. And at the south­ern end of their main­land range, researchers are observ­ing sea otter feed­ing and move­ment behav­iour to under­stand their slow south­ward expansion.

“Over­all trends are impor­tant, but they can mask prob­lems that may be affect­ing only a por­tion of the pop­u­la­tion,” says Lil­ian Car­swell, South­ern Sea Otter Recov­ery Coor­di­na­tor for USFWS. “These regional research projects help us under­stand the effects of local influ­ences, whether human-​caused or nat­ural, and inform the over­all south­ern sea otter recov­ery strategy.”

Sea Otter B-​roll, var­i­ous video footage (date: 30.06.2008) of sea otter researchers, sea otters in their habi­tat, and scenery in Cal­li­for­nia — USA:

Credit: U.S. Geo­log­i­cal Sur­vey; video pub­lished in the pub­lic domain.

Sur­vey Methodology

• The annual pop­u­la­tion index is cal­cu­lated from visual sur­veys con­ducted 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, U.S. Bureau of Ocean Energy Man­age­ment, and Santa Bar­bara Zoo.

• Sur­veys are con­ducted via tele­scope obser­va­tions from shore and via low-​flying air­craft, typ­i­cally from April through June. This year, the sur­veyed coast­line spanned Point San Pedro in San Mateo County, south to Rin­con Point near the Santa Barbara/​Ventura County line, and also included San Nico­las Island.

• The annual sur­vey was inter­rupted in 2011, when weather con­di­tions pre­vented the main­land survey’s completion.

(Source: USGS news release, 12.09.2013)