VANCOUVER - The death of a single wild animal is not usually significant, but for an endangered species of killer whales the loss of a young female has some experts worried that the population may reach a point where it stops growing.

There are only 80 killer whales among the southern residents - a clan of orcas that live in the waters off southern British Columbia and Washington State - and the death of each female is a lost opportunity to increase the pod.

Ken Balcomb, senior scientist for the Centre for Whale Research, said the recently deceased orca called J28 follows a trend of females dying either late in pregnancy or not long after giving birth.

“This has got to stop,” he said. “The population is not going to recover if we don't have reproductive females.”

J28 gave birth to a male calf in October last year.

Researchers noticed something was wrong last January, Balcomb said, when she began losing weight.

The 23-year-old orca died in October near the Juan de Fuca Strait separating Vancouver Island from Washington state.

Her new-born calf also looked thin, and Balcomb said his survival without a mother was unlikely.

J28's body was not recovered so the cause of death remains uncertain, but Balcomb said he suspects an inadequate food supply and toxins are to blame.

Killer whales have been found to carry high levels of toxins in their blubber, the result of pollutants in the water and in their food.

The whales - and their neighbouring northern residents which ply the waters off B.C. and Alaska - rely predominantly on chinook salmon but also eat chum and coho.

Balcomb said in years chinook and other fish stocks are poor, the orcas are forced to metabolize their blubber, subsequently releasing toxins into their blood and organs.

Hunger is particularly problematic for pregnant orcas that need extra food to carry their babies to term, he said.

Another female orca died over the summer, and more than 50 per cent of pregnancies end in miscarriage.

But Lance Barrett-Lennard, director of cetacean research at the Vancouver Aquarium, said aerial photos he has been collecting on southern residents don't show the appearance of starving whales, despite a poor chinook run this year.

Barrett-Lennard said the photos, collected in partnership with the U.S. National Oceanic and Atmospheric Administration, provide accurate information on the orcas' body composition.

Images captured in September found that although the orcas were thinner compared with images captured in Sept. 2015, they appeared to be in generally good condition.

“Most of them are not emaciated by any means,” he said. “(J28) was the outlier, she was the unusual one.”

That doesn't rule out the possibility that J28 died as a result of an illness triggered by toxins.

John Ford, a research scientist for Canada's Fisheries Department, said there are a lot of uncertainties about how toxins affect orcas, but most researchers believe it suppresses their immune systems making them more susceptible to diseases.

The effects of toxins also appears to vary between pods, which leaves researchers with even more questions on how to protect them, Ford said.

Efforts to increase salmon stock and other environmental protections that include monitoring and restricting shipping traffic and industrialization of the coast are in place to give the orcas a better chance at survival, Ford said.

“All you can do is make their habitat better for them,” Ford said.

While the population of the southern residents is down, Ford said they aren't in crisis yet with their numbers stronger than they were in the 1970s when they dwindled to only 71 orcas.

Northern residents are faring even better, with a population of about 300.

But Balcomb said the death of J28 should sound an alarm that stricter measures are needed to limit fishing and environmental degradation of rivers and waterways, including the installation of dams, to protect the orcas' food supply and prevent more deaths.

“It is a human problem, but the whales are suffering from the effects of these problems,” he said.