Orcas living off the west coast of the US may be inbred to the point of stunting the population’s recovery. Researchers found these inbred orcas live shorter lives, and suspect the females are dying before they reach peak breeding age.
Compared with other orcas (Orcinus orca), this group, known as the southern resident population, is strange. For starters, its members live year-round in the North Pacific Ocean, while most other orcas migrate around the world. They also only eat fish, avoiding mammals such as sea lions that most orcas include in their diet.
And unlike their cousins, the southern resident population is declining. By the 1970s, commercial fishing and water contamination had depleted the number of orcas around the globe. Habitat and hunting protections have since helped the species make strides toward recovery, but the southern resident population isn’t following suit.
“A lot of the population sizes have tripled among killer whales in the North Pacific,” says Marty Kardos at the US National Oceanic and Atmospheric Administration (NOAA). “The southern residents are one of the few exceptions.” This group consisted of 71 individuals in 1974, peaked with 97 animals in 1996 and now has numbers in the 70s.
Kardos and his colleagues sequenced the genomes of 100 living and dead killer whales from the southern resident group. This gave them a picture of the group’s genetic diversity: the more similar their genomes are, the more likely they are to be inbred.
When they compared the genomes of these whales with those of 50 other orcas from around the globe, the researchers found southern residents had the highest level of inbreeding and the lowest level of genetic diversity. They examined individual orcas’ genomes and lifespans, revealing that inbred animals died earlier. Because female killer whales don’t reach peak breeding age until around 20 years old, Kardos suspects that female whales are dying before or during their most reproductive years.
It is uncommon for orcas from different groups to mingle with each other, says Kardos, largely due to their different social structure. “They eat different things and they have different languages,” says Kardos. But some breeding between different populations has happened in the past and “does have the potential for happening again”, he says.
Until then, Kardos says we should focus on giving southern orcas the conditions to thrive by protecting marine areas and limiting pollution, commercial fishing and shipping traffic.