Southern orca decline may be tied to hunting behaviors

Orcas populations were decimated in the early 20th century due to their capture for display in aquariums and theme parks. Thankfully, that practice ended in the 1970s, and many populations have been slowly recovering. 

However, the southern resident orca population in the Pacific Northwest and British Columbia has dwindled back down to their mid-1970s levels, with only 73 orcas left. Much of this is attributed to numerous pregnancies ending in miscarriage or newborn death.

The situation is perplexing because while the southern population is declining, the northern population is increasing, with 300 current individuals. In a new paper published in the journal Behavioral Ecology, researchers from the University of Washington and NOAA Fisheries have proposed that this contrast could be due to differences in hunting behavior.

“For northern resident orcas, females were hunting and capturing more prey than males. For southern resident orcas, we found the opposite: The males were doing more hunting and capturing than females,” explained study lead author Jennifer Tennessen.

Dams affecting the salmon population and other human disturbances could be the cause of these shifts, especially with young mothers.

“In both populations, a mother with a young calf foraged less than other females, possibly due to the risk of leaving the calf temporarily with ‘a babysitter’ – another adult – while she hunts, or because of the time demands of nursing a calf,” said Tennessen. “But for southern resident females, which are more prone to disturbance and stress from vessel traffic, there was an outsized effect: Our study found no instance of a southern resident female with a young calf who successfully carried out a hunt.”

The experts also uncovered differences in female orca behavior towards their sons. In the northern population, adult males living with their mothers hunted less than those without mothers. However, researchers confirmed the opposite in the southern population – adult males with mothers hunted more.

“These unexpected differences left us scratching our heads. It is possible that southern resident adult males could be sharing with other members of their group, including their mothers, to help out, especially since an adult male’s survival is strongly linked to his mother’s survival,” said Tennessen.

“Understanding how healthy populations behave can provide direction and goals for management of unhealthy populations. Future comparisons to healthy fish-eating orca populations could help us understand whether the divergent behavior we’re seeing in the southern residents is indicative of a population trying to survive.”

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By Erin Moody, Earth.com Staff Writer

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