Tracking harbour porpoise Levi’s movements following his successful rehabilitation and release has been a little nerve-wracking for Chad Nordstrom, a researcher with the cetacean research program at the Vancouver Aquarium. Although Levi’s mileage and deep dives indicate an animal that is healthy, his presence in certain areas has caused some concern. That’s because he’s been tracked in areas where Bigg’s (transient) killer whales have also been seen.
And Bigg’s killer whales eat harbour porpoises.
Before Levi was released on September 10, 2013 in the waters of Saanich Inlet (where he was found stranded on a beach earlier in March), researchers attached a satellite-linked tag on his dorsal fin. This tag is giving researchers valuable data on where he goes and how deep he dives. Harbour porpoises haven’t been studied as closely as other marine mammals off the coast of B.C., so this information is proving to be significant.
According to Chad, in the last several weeks, Levi has been tracked in the vicinity of at least three groups of Bigg’s killer whales – first in Trincomali Channel on September 11, then in Sabine Channel on September 16 and finally, around Bowen Island on September 21. So far, Levi has managed to avoid becoming a meal.
We know about Levi’s close calls because of the data that has been collected by both Vancouver Aquarium scientists and citizen scientists. While Aquarium researchers have been tracking Levi’s movements in the Strait of Georgia, ordinary British Columbians have been reporting sightings of Bigg’s killer whales to the Vancouver Aquarium’s B.C. Cetacean Sightings Network, a joint collaboration with Fisheries and Oceans Canada.
Bigg’s killer whales exclusively eat marine mammals (which is different from resident killer whales that eat only fish), and their diet also includes harbour seals and Dall’s porpoises. Dr. Lance Barrett-Lennard, head of the Aquarium’s cetacean research program, says there are some ways that porpoises can avoid being eaten: head into the shallows (which are harder for Bigg’s killer whales to access) or “go cryptic,” (which means being motionless and “hiding” against the noise at the surface, since killer whales rely on sound for navigation and hunting). Of course, sometimes chases are inevitable, and Dr. Barrett-Lennard says in this case, porpoises might opt to deep dive and then try to swim off in another direction.
While it’s unclear if Levi has had to make use of any these strategies, it has brought the role of Bigg’s killer whales as predators in the Strait of Georgia into focus. The combined efforts of the tracking program and the Sightings Network will continue to provide researchers with information about Levi, as well as new insights on the secret lives of harbour porpoises.
Read about Levi’s post-release journey.
Written by Karen Horak, writer-editor, content and digital experience at the Vancouver Aquarium.