Why do two-thirds of killer whale pregnancies fail?

July 3, 2017, 8:27 a.m.
Southern resident orca breaching in the Pacific Northwest.
Photo: Monika Wieland Shields/Shutterstock

Living in Washington's Puget Sound is a special group of orcas. The Southern Resident killer whales are a genetically distinct population, and they are considered critically endangered with only an estimated 78 individuals left.

The number of Southern Resident orcas has been dropping for years, with few new births occurring to replace individuals that have died. In fact, only around two-thirds of pregnancies come to term.

The reason why confused researchers until now. Using dogs trained to track down orca scat, the researchers have been collecting scat samples for six years. They analyzed the DNA and hormones held within the 350 samples gathered over the years and the results were published in PLOS ONE this week. The culprit is food scarcity.

"Low availability of Chinook salmon appears to be an important stressor among these fish-eating whales as well as a significant cause of late pregnancy failure, including unobserved perinatal loss," note the study authors, which include Sam Wasser, founder of the Conservation Canines program of the Center for Conservation Biology at University of Washington which provides the scat-sniffing dogs so important to the research.

"However, release of lipophilic toxicants during fat metabolism in the nutritionally deprived animals may also provide a contributor to these cumulative effects. Results point to the importance of promoting Chinook salmon recovery to enhance population growth of Southern Resident killer whales," the authors write.

The problem comes down to not enough food, and as females burn what fat stores they have during pregnancy, they release toxins that are then absorbed by the fetus. How much of each factor drives the failure of pregnancies is still unclear.

But what is obvious is that as the the salmon disappear, it's only a matter of time before the orcas disappear too.

"Many species of Chinook salmon along are listed as threatened or endangered due to factors including loss of habitat from urban development, dams, fishing, pollution and competition from non-native fish," reports The Washington Post. "[I]mproving those Chinook salmon runs could help save Puget Sound orcas. Reducing pollution and other factors could increase the number of salmon, providing orcas with more food."

In other words: Save the Chinook, save the Southern Resident orcas.