Why do killer whales target and attack porpoises instead of preying on them?

Scientists are investigating mysterious behavior.

Scientists have long been perplexed by the behavior of fish-eating killer whales in the Pacific Northwest. Killer whales have been seen harassing porpoises and sometimes killing them instead of eating them.

In a recently published study, marine mammal scienceCo-led by Deborah Giles of Wild Killer Whales and Sarah Teman of the Sea Dog Society, a program at the University of California, Davis School of Veterinary Medicine, the 60-year record between southern killer whales and porpoises in the Salish Sea We investigated the above interactions. You can better understand why they exhibit this behavior.

Southern killer whales are endangered, with only 75 individuals left. Their survival is closely tied to the fate of Chinook salmon, which is also an endangered species. seed. Without enough Chinook salmon, these whales are at risk of extinction.

“I’m often asked: Why don’t southerners eat seals and porpoises instead?” Giles said. “That’s because fish-eating killer whales have a completely different ecology and culture than marine mammal-eating killer whales, even though they live in the same ocean area. Therefore, interactions with porpoises We must conclude that there is another purpose, but this purpose has hitherto been speculative.”

Three plausible explanations

Scientists recorded instances of southern resident killer whales harassing porpoises as early as 1962, but the reason for this behavior remained a mystery for a long time. Giles, Teman, and a team of collaborators analyzed his 78 porpoise harassment incidents recorded from 1962 to 2020. This study suggests three plausible explanations.

• Social play: Harassing porpoises may be a form of social play for killer whales. Like many intelligent species, these whales may engage in playful activities to bond, communicate, or simply have fun. This behavior can benefit group coordination and teamwork.
• Hunting practice: Another hypothesis suggests that harassing porpoises may hone their salmon-hunting skills. Southern resident killer whales may view porpoises as moving targets to practice their hunting skills, even if they don’t intend to eat them.
• Abusive behavior: This theory suggests that the whales may be trying to care for porpoises that they perceive to be weak or sick. This is an expression of the whale’s natural tendency to help other dolphins in the group. Females have been seen carrying dead calves, as well as porpoises.

“Abusive behavior, also known to scientists as ‘left-behind behavior,’ may be due to children having limited opportunities to care for them,” Giles explained. “Our research found that malnutrition causes nearly 70 percent of southern killer whale pregnancies to miscarry or for the calves to die shortly after birth.”

salmon specialist

Despite these interesting insights, Giles, Teman, and their collaborators acknowledge that the exact reasons behind porpoise harassment may never be fully understood. .But what is clear is that porpoises do not have Part of the diet of southern resident killer whales. Southern killer whales have a diet so specialized in salmon that the idea of ​​them eating porpoises is highly unlikely.

“Orcas are incredibly complex and intelligent animals. We found that harassing behavior toward porpoises has been passed down through generations and across social groups. It’s a great example,” says Teman. “Still, we don’t expect southern-dwelling killer whales to start eating porpoises. The salmon-eating culture is deeply rooted in southern-dwelling societies. For these whales to survive, healthy salmon populations are essential. We need numbers.”

This study highlights the importance of protecting salmon populations in the Salish Sea and throughout whale range. Maintaining an adequate supply of salmon is critical to the survival and well-being of Southern killer whales and the overall health of the Salish Sea ecosystem.

affinity for play

The study was conducted at a time when distinct killer whale populations on the Iberian Peninsula drew international headlines for interacting, sinking ships three times off the coasts of Portugal and Spain. After all, southern killer whales and killer whales of the Iberian Peninsula are two distinct populations with different cultures. What the two have in common is an affinity for play behavior.

Reference: “Harassment and Killing of Porpoises by Fish-Eating Southern Resident Killer Whales (Orcinus orca) (‘Phocoenacide’)” Deborah A. Giles, Sarah J. Teman, Samuel Ellis, John K.B. Ford, Monica W. Shields, M. Bradley Hanson, Candice K. Emmons, Paul E. Cottrell, Robin W. Baird, Richard W. Osborn, Michael Weiss, David K. Ellifritt. , Jennifer K. Olson, Jared R. Towers, Graham Ellis, Dena Matkin, Courtney E. Smith, Stephen A. Laverty, Stephanie A. Norman, Joseph K. Gaydos, September 28, 2023 , marine mammal science.

DOI: 10.1111/mms.13073

This research was funded by Wild Orca and the SeaDoc Society. Additional partners include the University of Exeter, Fisheries and Oceans Canada, Orca Behavior Institute, National Oceanic and Atmospheric Administration, Cascadia Research Institute, Whale Museum, Center for Whale Research, Ocean Research College Academy (ORCA) at Everett Community College, and Gulf Zoology. , North Gulf Coast Marine Society, George Mason University, Institute of Marine Medicine.