L120 apparently died while its pod was in the open ocean off Washington or British Columbia, the Center for Whale Research said.
The pod was offshore for a week to 10 days, and the orca designated L-120 might have been lost in a storm in the middle of last week, Balcomb said.
The calf’s birth was heralded in September, as the South Resident population has dropped to 78, the lowest count in more than a decade.
The Puget Sound killer whales primarily eat fish, rather than other marine mammals. Offspring tend to stay with their mothers for life.
Fish runs are much stronger in Hood Canal than they were 30 years ago, Orca Conservancy said, but the South Resident Killer Whales haven’t rediscovered it since restoration.
When food is hard to find, it’s hard for lactating females to produce enough milk to support a calf, according to Orca Conservancy.
The Associated Press contributed to this story.