How can killer whales, known for their isolation, depend on interactions with other populations?

Ford noted that decades of genetic data have shown the Southern Residents to be genetically distinct — reflecting cultural and behavioral differences — and that this distinctiveness contributed to their listing under the Endangered Species Act. Over time, though, he suggested there has been a tendency to treat them as more isolated than they really are, and results from recent decades indicate they may be less isolated than once thought.

One open question is whether they are becoming less isolated than before. Ford pointed to observed range changes as possible evidence: the Southern Residents are spending less time in their historic Salish Sea home range and more time on the outer coast, especially off Washington, where they are more likely to encounter other populations. Northern Residents, meanwhile, are ranging farther south than was understood 20 or 30 years ago. Both populations spend time off the northern Washington coast and around Vancouver Island. Shifting ranges create more opportunity to interact — which does not prove they do, but means the opportunity has increased.

Is it realistic to think the Southern Residents may interbreed with another population?

Globally, Ford explained, killer whales tend to live in fairly small populations but show some degree of occasional interbreeding with other populations. He cited a 2024 study of tropical killer whale populations that are small yet more genetically diverse than some larger, higher-latitude populations — apparently because they interbreed more. Some Alaska killer whale populations likewise interbreed with other groups more frequently.

For the Southern Residents, Ford said two lines of evidence suggest interbreeding occurred historically and perhaps more recently. Methods used in the 2023 inbreeding paper to reconstruct effective population size over time show a large increase roughly 20 to 50 generations back — a pattern also seen in Alaska residents and Bigg's (transient) killer whales — that likely marks a period of more interbreeding rather than genuinely enormous populations. Their calculations suggest the maximum time the Southern Residents could have been genetically isolated is on the order of 500 years, which is not long in evolutionary terms. A parentage analysis published 10 to 15 years ago, covering Southern Residents and some Alaskan populations, indicated possible interbreeding between the two groups; the lower-resolution methods mean it could be a false positive, but the new paper raises the possibility of more recent interbreeding.

You also say interaction may come in the form of competition for prey. Is that increasing?

Ford noted the diets of several of these populations are similar: the Southern Residents eat fish, mostly salmon, as do the Northern and Alaska Residents. They do not all target the same salmon stocks and there are seasonal differences, but they share some of the same larger stocks. Salmon leave Pacific Northwest rivers such as the Columbia, largely head into the North Pacific, and return along the coast as adults, where different killer whale populations prey on them. As understanding of these populations' diets improves, Ford said, it is becoming clear that other killer whale populations also eat a great deal of Chinook salmon and may be out-competing the Southern Residents for it.

This matters more, he added, because those other populations have largely grown over the past 30 years while the Southern Residents have not. The Northern Resident population has roughly tripled over that period. Research estimating how much salmon each population consumes suggests the totals are large; with 300-some Northern Residents and multiple hundred Alaska Residents competing for the same resource against far fewer Southern Residents — and being less inbred — the other populations may simply be more successful. It becomes, in his words, a numbers game.

You outline scenarios where the Southern Residents disappear by 2100. Is inbreeding driving that?

The models Ford referenced, some done at the Northwest Fisheries Science Center, take an "if present trends continue" approach: if the Southern Residents keep the same average birth and death rates as recent decades, the demographic prediction is decline. That does not mean extinction by 2100, but it points in that direction, and other published models reach similar conclusions. Ford noted the 2023 inbreeding paper showed that if the population were not inbred — if every individual had the survival rates predicted for the least-inbred whales — that trend would reverse and, all else equal, the population would grow. That, he said, indicates inbreeding is a fairly big problem for the population.

Ford emphasized there is not much people or managers can do to encourage interbreeding directly. But given the spatial overlap with other populations and what killer whales do elsewhere in the world, he said it is not implausible that some interbreeding could occur in the future simply through increased opportunity — and if it did, he believes it “would be really beneficial to the population.” He recommended exploring population models to examine such scenarios: occasional interbreeding that leaves a still-distinct Southern Resident population with higher survival and less inbreeding, or heavier interbreeding in which the population effectively merges with, for example, the Northern Residents. If the Southern Residents continue to decline while the Northern Residents grow, there could gradually be more of the latter and fewer of the former — still fish-eating killer whales, but different ones — a shift that might be hard to notice because it could happen very slowly.