Why Southern Resident orcas harass porpoises

Why Southern Resident orcas harass porpoises

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For decades, Southern Resident orcas in the Pacific Northwest have harassed, and sometimes killed, porpoises without consuming them. Scientists have come up with some reasons for this behaviour.

Southern Resident orca harassing a porpoise
Southern Resident orca harassing a porpoise

A study in the Marine Mammal Science journal examines the question of why Southern Resident orcas in the Pacific Northwest harass and sometimes kill porpoises without eating them.

Such behaviour has been passed down through the generations and across social groupings, and has been recorded as far back as 1962.

The question of why comes to mind.

Not on the menu

Certainly not to eat them. Southern Resident orcas do not harass the porpoises to consume them.

These orcas, which consume salmon, are different from orcas that consume marine mammals like seals and porpoises. This is because their ecology and culture are different, according to Deborah Giles of Wild Orca.  

Three plausible reasons

To find the answer behind porpoise harassment, Giles, Sarah Teman of the SeaDoc Society (a programme of the University of California-Davis’ School of Veterinary Medicine), together with a team of collaborators, analysed 78 documented incidents of porpoise harassment from 1962 to 2020.

Three plausible explanations were uncovered:

  • Social play: Porpoise harassment may be a form of bonding, play or communication for these orcas.
  • Hunting practice: These orcas may regard porpoises as moving targets upon which to practise their hunting techniques.
  • Mismothering behaviour: In almost 70 percent of Southern Resident orca pregnancies, miscarriages occur or the calf does not survive long after birth; hence, the Southern Resident orcas may be attempting to care for the porpoises (which they perceive as weaker or ill).

At this stage, the reason behind porpoise harassment is yet to be confirmed. 

Salmon diet deeply ingrained

The survival of the Southern Resident orcas is linked to that of Chinook salmon, another endangered species. Describing it as “an amazing example of killer whale culture,” Teman added that “ we don’t expect the Southern Resident killer whales to start eating porpoises. The culture of eating salmon is deeply ingrained in Southern Resident society. These whales need healthy salmon populations to survive.”

Hence, the study also highlights the importance of conserving salmon populations both in the Salish Sea and throughout the whales’ entire range. An adequate supply of salmon is essential for the survival and well-being of Southern Resident orcas and for the overall health of the Salish Sea ecosystem.