Southern Resident Killer Whales: Mystery Behind Orcas Harassing, Killing Porpoises Without Eating Them Revealed

Killer whales, also called orcas, have been considered by scientists lately as the world's top ocean predator, with the title that previously belonged to great white sharks. Based on reported anecdotal and scientific evidence, orcas can hunt, intimidating, and killing the said white sharks with the purpose of eating their livers for needed nutrients. Killer whales also hunt other sea creatures, including fish, squids, seabirds, and other marine mammals or whale species.

However, a mysterious behavior by fish-eating orcas, called Southern Resident killer whales, of killing porpoises for no reason, was discovered in previous years. Now in a new study, researchers in the United States and their colleagues revealed they unraveled the mystery as to why Southern Resident orcas harass and kill porpoises without eating them. The findings add to the existing evidence that large marine animals have complex emotional and social intelligence.

Apex Predator of the Ocean

Being the apex or top predators of the world's oceans, killer whales are found worldwide with an abundant population in the cold waters of Antarctica, Alaska, and Norway. They are also found in tropical and subtropical waters of Earth, according to the National Oceanic and Atmospheric Administration (NOAA).

There are approximately 50,000 orcas worldwide, with around 2,500 of the giant marine mammals living in the eastern part of the North Pacific Ocean. However, the global orca population has declined in recent decades, with some species being endangered, the NOAA Fisheries says.

In the US, Southern Resident killer whales are the only endangered orca population in the country, with their population ranging from central California to southeast Alaska, according to the US government agency.

Harassment and Killing of Porpoises

Despite the declining population of the endangered Southern Resident killer whale (Orcinus orca), the large predators continue their fish-hunting activities, where they prefer to kill adult Chinook salmon. However, killer whales also have time to kill porpoises for reasons not related to consuming them.

In the study published in the journal Marine Mammal Science on September 28, researchers have determined that members of different pods of the Southern Resident orca species, labeled as (J, K, L) have been observed harassing and killing porpoises without eating them. The research team analyzed 78 cases of the O. orca species harassing porpoises between 1962 and 2020, with 28 porpoises dying from the incidents.

The team asserted that the orca pods could be engaging in such behavior as a form of "social play" or a mistaken hunting practice that emanates from their natural mothering behavior, according to the study. In terms of participation, both male and female Southern Resident orcas participated in the porpoise harassment, with juvenile killer whales being the most active.

Based on the findings of the scientific paper, evidence suggests that Southern Resident killer whales also exhibit their playful behavior even while hunting their prey. Regardless, the fact remains that these marine mammal predators can inflict damage or causing death to their small prey even in social play.