A pregnant killer whale from an endangered population in the U.S. Pacific Northwest that was found floating dead near Canada died of an infection linked to her nearly full-term fetus, according to preliminary necropsy results.

The body of the orca, an 18-year-old female known as J32 and named Rhapsody, was found floating in the Strait of Georgia, between Vancouver Island and mainland Canada, in early December, experts said.

She was hauled to shore, and a necropsy performed Dec. 6 concluded that Rhapsody had been close to giving birth to a female calf, but suffered "in utero fetal loss with secondary bacterial involvement," according to Fisheries and Oceans Canada.

"The fetus caused an infection that became systemic, and ultimately fatal," the report said.

The death brought to 77 the number of orcas in the southern resident Orca population, comprised of three pods of killer whales who live in the Puget Sound off the coast of Washington state and are endangered under both U.S. and Canadian law.

The Washington-based Center for Whale Research said it was not uncommon for southern resident orcas to venture north into the Strait of Georgia looking for their primary food source, Chinook salmon, which has been depleted due to overfishing.

The orcas also face severe environmental threats linked to pollution, said Ken Balcomb, the center's executive director .

"Environmental contamination is a serious issue for these whales, and it is exacerbated during times of nutritional stress when their toxic blubber is metabolized to maintain body functions," he said.

Rhapsody was the second orca death in as many months. In October, a weeks-old calf, the first baby born in more than two years, disappeared from its mother's side and was pronounced dead.

The southern resident orca population has reached an almost record low this year, down from 98 in 1995 and well over 200 in the 19th century, according to the Center for Whale Research and the Orca Network.

"There are only about a dozen reproductively viable females remaining in this population and very little possible recruitment to this cohort within the next few years," Balcomb said.

Killer whales are the largest member of the dolphin family and recognizable by black and white coloring and their size, with males weighing up to 22,000 pounds (10,000 kg). The marine mammals are highly intelligent and highly social, and communicate with each other using whistles and pulsed calls.