A pod of endangered killer whales living in the waters off Washington State has added to its group the first newborn orca in more than two years, and it's a girl, researchers say.

Ken Balcomb of the Center for Whale Research in Washington first spotted the calf on Dec. 30.

Later, center volunteers in boats captured photographs of the newborn near Vancouver Island.

"[They] took photographs and confirmed that the baby was still there," Balcomb said. "And the baby actually rolled over and showed them the underside, and we confirmed it is a female, so that's wonderful news."

It's encouraging news for the pod, coming at a time when the numbers of southern killer whales have dropped to a historic low, researchers say.

The female baby, given a pod identifier name of J50, takes the population to only 78 animals, they say.

"Girls are the ones that have babies and we're running real short on whales that are able to produce babies lately," said Balcomb.

A pregnant female of the southern resident orca population, known as J32, was discovered dead off Vancouver Island's east coast last month.

Researchers said they determined her fetus had died and she subsequently died of a bacterial infection.

"The loss of J32 was a disturbing setback," said Brad Hanson, a wildlife biologist with the National Oceanic and Atmospheric Administration. "We lost a lot of reproductive potential."

That's why the researchers are so pleased by the birth of J50.

The mortality rate among killer whale calves is high, Balcomb says. Around 35 to 45 percent don't survive past their first year, but since J50 has survived her first week, he says he's hopeful.

Researchers say they're having trouble determining which of the females in the pod is the mother. They suspect it could be J16, a mother with three surviving offspring, or her 16-year-old daughter, J36, who hasn't had a calf recorded yet.

"It quickly became the subject of mystery because [the calf] was swimming alongside a female whale that is estimated to be 43 years old -- beyond the age calculated for reproductive senescence in these whales," the research center said.

"Sometimes it takes a few encounters and some time to sort these things out because these whales are very caring for one another, and baby-sitting is not unusual, especially with grandmothers," it said.

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