A new University of Washington study explained why northern resident orcas’ populations are growing, but the neighboring southern resident orcas are declining, the UW announced on Friday.
Annual counts show that southern orca populations have fallen back to mid-1970s levels. Most pregnancies end in miscarriage or the newborn dies, said scientists.
Scientists said orcas may not be catching enough food and many elderly orcas who usually help younger generations have died.
With only 73 individual southern resident orcas left, conservationists are worried they may not survive.
Yet, northern resident orcas, who have a similar diet and overlapping territory, grew in population. Today there are more than 300, scientists said.
A new study by the UW and National Oceanic and Atmospheric Administration Fisheries showed that the two populations hunt differently.
“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,” said lead author Jennifer Tennessen, a senior research scientist at the University of Washington’s Center for Ecosystem Sentinels. “We also found that if their mother was alive, northern resident adult males hunted less, which is consistent with previous work, but we were surprised to see that southern resident adult males hunted more. Adult females in both populations hunted less if they had a calf, but the effect was strongest for southern residents.”
The five-year study showed scientists that although the two populations have some of the same territory and social structures, they should not be looked at as the same for research purposes.
“In the past, we’ve made assumptions about these populations and filled in the gaps when designing interventions, particularly to help the southern resident orcas,” said Tennessen. “But what we found here are strikingly different patterns of behavior with something as critical to survival as foraging. And as we develop management strategies, we really need to consider these populations differently.”
The study also showed the impact of elderly female orcas on their adult sons. For northern resident orcas, adult males with a living mother hunted less than adult males without a living mother. Scientists said this could be because the mother still provides food. But among southern resident orcas, adult males with a living mother 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. “Relatedly, southern resident matriarchs may be leading the group to areas where their adult sons may be able to capture more prey since healthier sons might be more successful at mating and passing along some of their mothers’ genes. We need more studies to determine what role the presence — or absence, for southern resident orcas — of matriarchs has on male foraging behavior.”
Future studies on the northern and southern resident orcas’ behaviors could shed light on more differences. Also studying Alaska resident orca populations, where salmon is generally healthier, can help pinpoint the causes of problems for southern residents, said Tennessen.
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