Endangered Killer Whales Known for Isolation May Depend on Their Interaction with Other Populations

Endangered Southern Resident killer whales are known for the tight-knit family structure that isolates them from other killer whale populations. In 2005, NOAA Fisheries listed the whales as a Distinct Population Segment under the Endangered Species Act due to their unique genetic legacy and endangered status. The 74 whales remain at risk from insufficient prey, environmental contaminants, disturbance and inbreeding.

In a recent study , scientists report that a fifth factor-interactions with other killer whale populations-may determine their future. They say such interactions may take three forms: competition for prey, sharing habitat, and interbreeding.

We asked lead author Michael Ford about the findings.

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

If you look back over multiple decades, we've known for a long time that there are genetic differences between the Southern Residents and other populations, reflecting cultural differences, behavioral differences, and so forth. All those things showed they had become genetically distinct, and contributed to their listing under the Endangered Species Act.

On the other hand, I do think that over time, there's been a tendency to treat them as more isolated than they really are. And some of the scientific results that have come about over the last several decades indicate that maybe they are not quite as isolated as we once thought they were.

One question is, are they becoming less isolated than they used to be? There is some evidence this might be occurring, particularly based on some of the observed range changes different killer whale populations have exhibited.

The Southern Residents are spending less time in what was considered their primary home range in the Salish Sea. They are spending more time on the outer coast, especially the Washington Coast, where they are more likely to encounter other killer whale populations. We've also learned that the Northern Residents are spending more time further south than was understood 20 or 30 years ago. Both of these populations, Northern Residents and Southern Residents, spend a fair amount of time off the northern Washington coast and the waters around Vancouver Island.

These shifting ranges result in more time and space to interact than maybe we appreciated some time ago. It doesn't mean that they do interact, but it means they at least have increased opportunities to do so.

Is it realistic to think the Southern Residents may interbreed with another population, as you discuss in the paper?

If you look at killer whales globally, they have a general pattern that they live in fairly small populations, but do have some degree of occasional interbreeding with other populations. For example, a paper that came out just last year looked at tropical killer whale populations. These populations tend to be very small, but are actually more genetically diverse than some of the larger populations at higher latitudes.

Part of the reason is they seem to mate, or interbreed, more with other populations. So they manage to maintain these small populations, but remain genetically diverse through interbreeding. That also happens with some other populations. Some Alaska killer whale populations also interbreed with other groups more frequently.

In terms of whether that's ever happened to the Southern Residents, there are two little bits of evidence that suggest it occurred historically and perhaps more recently. In the inbreeding paper we published a couple years ago, we used some methods that allowed us to look back at the effective size of the Southern Resident population over time.

If you go about 20 to 50 generations back, you see this big increase in effective population size. From a genetic perspective, the population had the diversity of a much bigger population. You also see that in the Alaska resident populations and in the transient, or Bigg's, killer whales. The reason is probably not that populations were enormous 20 to 50 generations ago, but rather, it marks a time when there was more interbreeding.

We did some calculations that suggested that the maximum time the Southern Residents could be genetically isolated is probably on the order of 500 years or so. This is not super long in the scheme of things.

There was another paper from other researchers 10 or 15 years ago that did a parentage analysis including both Southern Residents and some Alaskan populations. That paper showed some potential interbreeding between those two groups.

There's some skepticism about that result because the methods used were not super-high resolution. So it's very possibly a false positive, but we do mention in our paper at least the possibility that there could have been some more recent interbreeding.

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

We've known for some time that the diet of a number of these populations is very similar. The Southern Residents prey exclusively on fish and mostly salmon, as do the Northern and Alaska Residents. They do not all eat the same stocks of salmon, and there are seasonal differences in prey preferences, but they do eat some of the same larger stocks.

The salmon in the Pacific Northwest leave their rivers down here, like the Columbia River, and for the most part take a right turn and head up into the North Pacific. They face predation as juveniles. And then when they come back as large adult fish, they come back along the coast, where different killer whale populations prey on them. As we get a better understanding of the diet of killer whale populations in the North Pacific, we learn some other killer whale populations are also eating a lot of Chinook salmon, and potentially out-competing the Southern Residents for that resource.

This becomes a little more important when you realize that these other populations, for the most part, have grown significantly over the last 30 years. The Northern Resident population has basically tripled in size over the same time period that the Southern Resident population has not grown. There are some research papers indicating that, if you estimate how many salmon each of these killer whale populations consume, they're eating a lot of salmon.

Since there are so many more of them, and perhaps also because they're less inbred than the Southern Residents, these other populations may be more successful. It becomes a numbers game when you have 300 some-odd Northern Residents and multiple hundred Alaska Residents competing for the same resource with far fewer Southern Residents.

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

The models that we refer to, some of which were done at the Northwest Fisheries Science Center, all tend to take an if-present-trends-continue type approach. So if the Southern Resident population continues to have the same birth and death rate averages as the last several decades, the simple demographic prediction is the population will decline. It doesn't mean that it will be extinct by 2100, but it is heading in that direction.

Other published models come to similar conclusions. One of the things we showed in the inbreeding paper was that if the population was not inbred, if all the individuals had the survival rates that you'd predict for the least inbred individuals, that trend would reverse. All else being equal, the population would grow.

So that does suggest that inbreeding is a pretty big problem for the population.

One of the points we make in this paper is there's not a lot that we, as people or managers, can do to encourage interbreeding. However, if you look at the overlap spatially with other populations and what other killer whale populations are doing worldwide, it's not implausible there could be some interbreeding in the future between the Southern Residents and other populations purely based on increased opportunities. And if there was, I think that would be really beneficial to the population.

We recommend exploring population models, for example, to examine some of those scenarios. There are scenarios where interbreeding could be helpful to the population, either as occasional interbreeding that might result in the population remaining a distinct Southern Resident population, but with higher survival and less inbreeding. There are other potential scenarios where there's a lot of interbreeding and the population effectively merges with, say, the Northern Residents.

If they do continue to decline, and if the Northern Residents continue to grow, there could be more and more Northern Residents and fewer and fewer Southern Residents. There would still be fish-eating killer whales, but they'd be different. It may be hard to tell it's happening because it could happen very slowly.