A blue whale, the largest animal on Earth, has been spotted off the B.C. coast by researchers with the Department of Fisheries and Oceans (DFO).

The rare sighting happened as they were conducting a survey on July 7.

"In the presence of greatness ... we documented a blue whale off SW Haida Gwaii," tweeted DFO researcher Jackie Hildering.

"The photo does not do the size of the whale justice," she wrote.

How big is big?

Blue whales are the largest animal known to ever exist. They can be up to 30 metres long and an adult female can weigh up to 180 tonnes.

"This is the equivalent of 2¼ logging trucks at 80 metric tonnes and twice as heavy as any dinosaur," Hildering wrote.

Hildering and her team spotted the blue whale, and some sperm whales, from a coast guard ship while researching endangered and threatened turtles, sharks and cetaceans — the group of aquatic mammals that includes whales.

There have been fewer than 50 sightings of blue whales off B.C. by DFO since surveys began in 2002. Their numbers diminished largely because of commercial fishing in the North Pacific between 1910 and 1965, when at least 9,500 of the animals were killed.

The B.C Cetacean Sightings Network says it is unclear how many blue whales remain because of both the large area they travel and low sampling rates.

Since the end of commercial whaling in 1966, human threats to the blue whale population have included collisions with ships, increasing whale-watching activity, entanglement in fishing gear, and pollution, according to DFO.

The blue whale is listed as endangered and protected under the Species at Risk Act.

For more on the future of the southern resident killer whales, you'll want to listen to a new CBC British Columbia podcast. Killers: J pod on the brink is hosted by Gloria Macarenko, and the first episode comes your way July 18.

You can subscribe now, wherever you get your podcasts.