During the past decades, Southern resident killer whale populations in the Pacific Northwest have declined substantially, leading researchers to look for causes of this worrisome trend. Some of the threats that have been identified include fluctuations in salmon prey, exposure to toxic pollutants, and disturbance and noise from ships and other vessels.
Now, by combining modern genomics with decades of field observations, an international team of scientists has found that the small size and isolation of the endangered population of Southern residents in the Pacific Northwest have led to high levels of inbreeding. This phenomenon has significantly contributed to their decline.
“For a long time, we’ve struggled to understand why this population has consistently lower survival and birth rates than other killer whales in the region, and this research highlights a strong link between inbred individuals and increased risk of death,” said study co-author Eric Ward, a statistician at the Northwest Fisheries Science Center. “It’s a view we’ve never had before, and it begins to fill a gap in our understanding.”
Killer whales reproduce for the first time at around ten years of age and reach their reproductive prime in their twenties. However, the researchers discovered that, compared to the least inbred whales, highly inbred ones had less than half the chance of surviving through those years to reach 40. While females with low levels of inbreeding usually live long enough to produce an average of 2.6 offspring during their lifespan, those with high levels of inbreeding produce an average of only 1.6 offspring during their significantly shorter lives – which is insufficient for animal populations to remain stable or increase.
Moreover, since inbreeding significantly limits the population’s growth and recovery, the whales become much more vulnerable to a variety of other threats, such as disease, limited prey, or exposure to contaminants and human disturbances.
“The whales are not necessarily dying of inbreeding itself, they are dying prematurely because inbreeding has set them up to be more vulnerable to diseases or other problems,” said co-author Bradley Hanson, a research scientist at the Northwest Fisheries Science Center. “We need to minimize the potential for those factors to have an impact.”
“It would be a mistake to see this as inbreeding alone causing the decline. Over the last 50 years, this population has been impacted by multiple stressors, and the relative impact of various threats on the Southern resident population has fluctuated through time. These combined impacts, coupled with the mating system of killer whales—where only a few males are contributing to the gene pool—may have made inbreeding a more significant threat in recent years,” Ward concluded.
The study is published in the journal Nature Ecology & Evolution.
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