Orca Prey Sharing and Aerial Photogrammetry

There were two talks at the Transboundary Workshop that really stood out to me. The first was a talk by John Ford, someone I was especially excited to see because he really pioneered killer whale acoustics, the field where I've done my research. This DFO scientist didn't talk about acoustics this time, though, but rather resident killer whale prey specialization on Chinook salmon.

We all know the whales prefer Chinook, but the extent to which they do is really phenomenal. At times, sockeye salmon will outnumber Chinook salmon 1000:1 in areas where the whales are foraging, yet there is virtually no evidence that the whales ever feed on sockeye. Chinook are probably preferred not only for their superior lipid content, but because they are abundant in nearshore waters year-round, whereas many other species are only abundant during brief seasonal migrations. (Young adult Chinook stay in nearshore waters while they grow, whereas other salmonid species go far offshore, outside of the resident orca's normal range.)

Whatever the reason, their preference for Chinook has really become a cultural one. Another cultural behavior the whales engage in is prey sharing, where one fish will be divided up among two or more whales. I knew this occurred, but Ford suggested in his talk that it may be occurring more often than we realized. He also had some absolutely stunning still images taken from underwater video footage they've recorded of the whales engaging in prey sharing. Imagine three whales all facing each other underwater, pulling apart a large silver fish and dividing it up among their family members. Moms will share fish they catch with their offspring, Ford explained, but juveniles will share fish with their mothers and other siblings, too.

Realizing that prey sharing may in fact occur on a regular basis, I figure that some of the circling behavior I see from the whales sometimes may actually be prey sharing in action. The whales are sometimes spread out in what looks like foraging when suddenly you'll see a momentary aggregation like this one - now I know what's probably going on:

Prey sharing in action?

The other talk I was really impressed by was John Durban from the Center for Whale Research, who talked about new research started last summer with the goal of assessing killer whale size and body condition. Basically, they photograph the whales from a helicopter, and knowing their altitude and the focal length of their lens allows them to scale their photographs and measure the size of the whales in the photo. This simple but elegant concept is known as aerial photogrammetry.

Okay, the above photo wasn't taken from a helicopter, but from the top of Lime Kiln Lighthouse, which is the closest to an aerial view I've had of the whales. But I would love to see them from the air!

In September of last year they ran 10 flights and were able to photograph 69 of the Southern Residents. From their photographs, they made three measurements of each individual - length, breadth (width just behind the dorsal fin), and head width (width near the blowhole - important because a malnourished whale often appears thin in this area, and it is believed that some of the 7 whales lost last year were malnourished).

The longest whale was not in fact J1 Ruffles, as many people believe. It was another adult male, L41 Mega, who measured in at just under 24 feet. The range for male whales was 21-24 feet, while the range for females was 18-21 feet. This puts our local killer whales in the mid-size range for killer whales worldwide. For instance, Antarctic orcas are smaller, whereas local transient orcas are larger. The largest killer whale every documented was 32-foot male of the coast of Japan.

The largest of the Southern Resident Killer Whales: L41 Mega.

The largest head width in proportion to body size came in the calves, specifically K42, which is a good sign that we have some robust youngsters out there! On the other hand, the smallest head width belonged to K14 Lea, the mother of K42, likely because she has a decreased body condition due to the energetic costs of keeping her calf well-nourished. The female with the next smallest head was L67 Splash, a female who was lost last year, supporting the theory that she was malnourished before she died.

This research technique has been used on cetaceans in other places, but there is really a unique opportunity here because it's the only place where you can do this on whales of known age and gender. Of course there is variation in measurements between different whales (ie, not all the females are the same size), so it would be difficult to say "a whale with head width less than X is malnourished". However, there is potential to gather longitudinal information on individual whales in this population. This means that over time, they may be able to assess the body conditions of each specific whale compared to data from that whale gathered in previous years. Cool stuff!