A young, sickly killer whale that scientists have been fighting to save has been spotted alive, after being declared missing by researchers on Monday.

The Center for Whale Research said that three-year-old orca known as J50 and her group became separated from the main pod for two days, somewhere in the Strait of Juan de Fuca.

The southern resident killer whales, which are so endangered there are just 75 individuals left, swim between Canadian and U.S. waters through busy shipping lanes to Seattle and Vancouver ports.

Scientists have now headed out to dart the whale with another shot of antibiotics.

J50 first received an unprecedented treatment at sea last month.

Scientists grew concerned after the whale lost more than 20 per cent of its body weight, and developed an indentation on its head, indicating a loss of fat stores.

J50 is part of a family group known as J-pod, which also includes the mother orca, who gained international attention for carrying her dead newborn calf in an apparent display of mourning that lasted 17 days.

J50 has been part of an unprecedented international rescue effort.

In August, a response team led by the NOAA obtained a breath sample from J50 and delivered a broad-spectrum antibiotic using a dart. The team decided on antibiotic treatment for the young whale despite the fact they did not have a diagnosis, because they said an infection was the most likely cause of her illness.

The team also took the unprecedented step of feeding the whale live Chinook salmon off a boat.

Last week, scientists determined a second dose of antibiotics may be necessary, after noting the whale still looked emaciated.

B.C.'s southern resident killer whales face a multitude of threats — including dwindling supplies of Chinook salmon, high levels of vessel noise and toxic contamination.

After J50 was declared missing by the Orca Conservancy earlier Monday, the National Oceanic and Atmospheric Administration (NOAA) said scientists remained hopeful .

"We all know J50 as tough and tenacious. One of the last sightings by DFO on Thursday reported that J16 and J26, her brother, were lagging behind most of J-pod by about three nautical miles, and J50 was lagging about a half-mile behind them. Sometimes she got closer, but she looked to be struggling to keep up."

According to the NOAA, the standard for determining the loss of a whale is to spot its family group "multiple times" without them.