The body of a young killer whale was discovered Thursday near Vancouver Island by residents of Courtenay, B.C., who towed it to shore.

Its death is troubling, said marine mammal researchers from the Vancouver Aquarium, as it marks the fourth recent death in the endangered southern resident population.

Two of the whales, L100 and L53, are presumed to have died sometime this summer, while the calf L120 died about eight weeks ago.

Now, only 77 southern resident killer whales remain in the Salish Sea.

The whale found Thursday was identified as 18-year-old J-32, according to Paul Cottrell, a marine mammal co-ordinator with the department.

The whale that was found dead on Thursday , J-32, had been carrying a full-term fetus, the necropsy team reported. (Ken Balcomb of the Center for Whale Research)

"It really tugs at the heart strings. It's a magnificent animal. Sixteen feet or so — so it's likely a juvenile. So, it's terrible and we want to figure out what the cause of death was here and how this animal died," Cottrell said.

According to the Orca Network, J-32, which was nicknamed Rhapsody, was thought to be in the late stages of pregnancy last summer judging by her wide girth, which was visible when she breached.

"A necropsy Saturday led by Dr. Stephen Raverty will reveal if she was indeed pregnant and hopefully will find the cause of death. She was believed to have died in the past 24 to 48 hours," said the statement posted by the Orca Network.

Most contaminated marine mammals in world

It’s always a concern to lose an individual from a small population of endangered animals, such as the southern resident killer whales, said marine mammal scientist Lance Barrett-Lennard.

"But this is even more distressing because it was a female of reproductive age," he said. "Her probability of contribution to the recovery of this population was very high, and her years of maximum importance to the population were still to come.”

It's believed L120, a seven-week-old southern resident orca shown here, died sometime in August or October because of a shortage of chinook salmon, according to researchers. Now another whale has died and researchers are worried about the continued health of the resident population. (Orca Network)

​Vancouver Aquarium pollution researcher Peter Ross said there are a number of factors that might have contributed to the whale's death.

"We have long been concerned about very high levels of endocrine-disrupting pollutants in these whales, reduced food supply — notably chinook salmon — and noise and disturbance,” said Ross.

“The planned necropsy will hopefully provide early clues about why this young female died, but more detailed insight will come from laboratory analysis for pathogens, sign of illness and contaminants."

The aquarium said it was Ross's research that determined the southern resident killer whales are among the most contaminated marine mammals in the world.