Killer whales have been spotted in Vancouver's harbour twice in the last two weeks and are expected to return.

Two separate groups have been spotted swimming into the harbour to snack on seals. A group of five, a mother and her four sons, was seen most recently on Tuesday.

The whales swam through the harbour under the Second Narrows Bridge and up into Indian Arm.

"It's been many years since we've seen that happen," said Lance Barrett-Lennard, director of the Marine Mammal Research program at Ocean Wise.

If killer whales enter the harbour, they tend to go as far as the Vancouver Convention Centre then turn around and head back out, he added.

"I was quite pleased to see that they actually ventured up into Indian Arm, a place that they would have used quite frequently a hundred years ago."

Seals are popular prey for Bigg's killer whales and there are plenty of them in the harbour. As long as the seals stick around and the harbour isn't too crowded with boats, Barrett-Lennard says it's likely the whales will continue to return.

Seeing whales in the harbour is a fairly rare occurrence, he said.

Watch video of the whales swimming in unison past an ocean-going freighter

A group of five Bigg's killer whales belonging to the T049A pod were spotted on Tuesday, April 23. Transient killer whales feed on marine mammals, unlike resident killer whales, which feed mostly on Chinook salmon. 1:47

In the last few years, there have been one or two sightings each year of whales swimming under the Lions Gate Bridge, he added.

Bigg's killer whales are listed as a threatened species. Due to healthy populations of its primary prey — seals and sea lions — B.C.'s population is recovering, with a current population of more than 300.

This is in contrast to resident killer whales, which mostly feed on chinook salmon and have seen their numbers dwindle.

For anyone interested in catching a glimpse of the whales in Vancouver harbour, they can often be seen from along the seawall.

Once they come in, they tend to stick around the harbour for a few hours, Barrett-Lennard said.

"I think that the whales returning to Vancouver harbour is a sign that the environment is improving in the inner harbour a bit for them," Barrett-Lennard said.

"I'm encouraged by that."