The southern resident calf, known as L-120, was first spotted in August in Puget Sound, but it hasn't been seen with its mother during several sightings of L-pod over the weekend.
Ken Balcomb, a biologist with the Center for Whale Research in Friday Harbour, Washington, says the calf would be unable to survive without its mother.
"The baby is no longer with her, and at six or seven weeks of age it isn't able to survive on its own. A theory would be that the mother was unable to provide enough nutrition in terms of milk," said Balcomb.
The pod lost another calf, born to the same mother, in 2012, according to a statement released by the Orca Network.
"In February of that year the the body of L86's second offspring, three-year-old female L112, washed up at Long Beach Wash. with indications of death by severe acoustic trauma.'
Decades of suffering
Balcomb says the pod is now down to 78 members, the lowest it's been in more than a decade and he suspects a poor Chinook salmon run could be to blame for malnutrition among members of L-pod.
Research conducted in recent years has shown that southern resident orcas depend almost entirely on Chinook salmon for sustenance, with a diet of Chum salmon during fall months when Chinook are especially scarce, according to the Orca Network.
"This orca clan has suffered episodic food deficiency for many decades, as Chinook salmon runs were depleted by habitat destruction, excessive harvest and dams from Alaska to California.
"They were also routinely shot at for decades and over 50 were captured or killed for theme parks during the 60s and 70s, followed by wanton disposal of persistent toxins into Puget Sound that continue to impair fetal development and immune responses, especially when the whales can't find sufficient food."
"We haven't treated these magnificent orcas well at all. As a society we are not successfully restoring this orca community despite the many warnings and legal declarations," said Howard Garrett of Orca Network in the statement.
"Our challenge is clear: bountiful salmon runs must be restored and protected or we won't see resident orcas in the Salish Sea in coming years."
This summer Springer, the first orca ever successfully re-introduced into the wild was spotted with a young calf. Springer is part of the northern resident orca population which live in the waters around the northern end of Vancouver Island.