Why do some dolphin species, including the Southern Resident killer whales, kill harbor porpoise when they are neither prey, nor competition for prey?

That’s a question Wild Orca’s Dr. Deborah Giles has been seeking to answer for nearly two decades, and this week she had a front row seat to one such event.

On July 25, 2022, Wild Orca’s research team was collecting fecal samples for analysis for our Southern Resident Killer Whale Health Monitoring Program. “With the aid of scent detection dog Eba, our research is non-invasive,” said Giles, “this means we’re at a distance from the whales to avoid disturbing them, especially when feeding.”

What happened next, she said, was both thrilling and shocking. In a sudden explosion of water, several members of J pod approached the research boat at high speed. As the whales came just feet away, two were seen apparently shouldering a young harbor porpoise between them.

As the group surfaced next to the boat, Giles could see that the porpoise was still alive, but was being propelled now by several killer whales. They continued at speed away from the boat, yet close enough for Giles and the team to safely witness and document how the event played out, until the death of the porpoise some 35 minutes later.

Nearly 3,000 photos plus video footage aided in identifying the individuals involved at each key stage, to record the play-by-play in this fast moving, rarely witnessed event. “Porpicide” has been documented in fish-eating killer whales since the 1970s, and occurs in all three West Coast populations; also seen worldwide in bottlenose dolphins, and although the size difference is much smaller, the outcome is the same.

Giles continued, “As scientists, we’ve always wanted to understand what drives this behavior. Teaching hunting skills seems unlikely; this is not a species they need to equip their youngsters to handle, given that they don’t eat marine mammals. When it’s been possible to retrieve these porpoise, they’re often unmarked, suggesting they might die of anxiety or exhaustion.”

Giles is now able to add a unique eyewitness perspective to the record. “I’m still processing this event,” Giles explained. “This encounter was by far the longest and most interactive episode of phocoenacide I have personally observed. To be able to capture footage and stills to add to our collective body of knowledge is a dream, and that our team was able to do so in an unexpected, high stress moment, is simply astonishing.

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