Puget Sound orca whales struggle with inbreeding as scientists search for answers

Apr 23, 2022, 6:06 AM


Orca Whales in Puget Sound-Olympic (credit U.S. Forest Service via Flickr)

(credit U.S. Forest Service via Flickr)

With only 73 southern resident orca whales remaining — 25 to 30 of which are potentially reproductive females — about a third of them with the potential to rebound the population are not reproducing, causing inbreeding among the population with the potential for long-term adverse genetic expression.

The most obvious mitigating factor for southern resident population health is a shortage of wild Puget Sound Chinook salmon, listed under the Endangered Species Act.

The confounding problem is that the period during which the whales in question were most fertile was between 2010 and 2015 “when Chinook abundance was fairly high,” as Dr. Eric Ward, a statistician with the National Oceanic and Atmospheric Administration (NOAA), put it to a state board tasked with Puget Sound environmental recovery.

“This was a time when the Columbia River saw … some of the highest returns in the last 70 or 80 years … As we look at population rate as a whole … an average [birth success rate] for a 20-year-old female was … 20% … That’s fallen to about 5% [among the southern resident orca whale population]. We’ve seen big decreases in fecundity for the last decade,” Ward continued.

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The southern resident population data was compared with four other orca populations throughout the Pacific Northwest, and flat or similar declining population trends were observed. However, “it doesn’t explain all of these quirky demographic mysteries that are happening with this population,” Ward added, referring to the declining pregnancies of healthy females.

NOAA tracked the genetic history of the southern residents against Alaskan-resident orcas, going back roughly 100 generations. Their population size, an approximate 500 whales, remains relatively consistent, until “75 generations ago [when] those two diverge. Around 30 generations ago, we see … [the southern resident] population experiencing a substantial population decline and we do not have an ecological explanation for that pattern right now,” Martin Kardos, NOAA research geneticist, said.

That decline is partially to blame for “southern residents [having] by far the highest inbreeding among the five populations we have sequence data from,” Kardos added.

Among the fallout of the inbreeding are “alleles that cause protein structure to change or to destroy the function of proteins.”

The boards discussed potential solutions. Historically, if inbreeding is observed in a population, scientists can selectively capture animals from separate populations to artificially stimulate genetic diversity.

“That obviously would be extremely difficult to do. The million-dollar question is, will mitigating environmental factors help? For example, will supplementing pray for killer whales … at all alleviate inbreeding depression. The answers to that is extremely complicated and unknown,” Kardos offered.

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Puget Sound orca whales struggle with inbreeding as scientists search for answers