Understanding what’s behind a shrinking population of endangered killer whale

With 73 individuals remaining, southern resident killer whales, or orcas, are the only endangered population of killer whales in the U.S. They spend the spring, summer and fall hunting for salmon throughout the inland and coastal waterways of Washington and British Columbia, and venture as far south as the coastal waters of Oregon and Northern California. Their territory overlaps with northern resident killer whales which hunt for salmon from Glacier Bay, Alaska to the central coast of Washington, and share a similar matrilineal social structure. Both groups face threats such as plummeting salmon stocks, pollution and increased maritime activity. But unlike the population of northern resident orcas, which has been steadily increasing, the southern resident population has declined over the past 20 years.

A team of international scientists has now revealed another possible clue to help explain their struggle for survival. They found that adult males spent more time hunting and catching salmon than females within the southern resident population, with the foraging pattern reversed in the northern population. They also found that southern resident mothers with calves spent less time hunting than northern resident mothers. Joining us now is Jennifer Tennessen, a senior scientist at the University of Washington Center for Ecosystem Sentinels, and the lead author of the study which was published this month in the journal Behavioral Ecology.

Note: The following transcript was created by a computer and edited by a volunteer.

Dave Miller: This is Think Out Loud on OPB. I’m Dave Miller. An aquatic mystery has been brewing off the coasts of Washington and British Columbia for decades now. That’s where two populations of orcas, or killer whales, make their home. The two groups are very similar. They both munch on a dwindling population of salmon and both have to contend with marine vessels and lingering toxins. But here’s the mystery – while the population of endangered southern resident orcas is the same as it was about 50 years ago, the northern resident population has steadily increased. Now, scientists think they might have more insight into why. Jennifer Tennessen is a senior research scientist at the Center for Ecosystem Sentinels at the University of Washington. She joins us to talk about this. Welcome to the show.

Jennifer Tennessen: Hi, thanks for having me.

Miller: I’m going to start with these two populations broadly. I mentioned a few of the similarities. What’s the overlap here? What do they have in common?

Tennessen: These are both fish-eating killer whale populations, so they eat primarily salmon. They really like chinook salmon, which are these really large fatty fish that can be up to three or four feet long. And they also tend to have really similar social structures as well. They are organized into matrilines, which are groupings of female matriarchs, either grandmothers or mothers, and then all of their related offspring. Both populations are structured that way. And [both] typically travel in similar habitats as well around coastal British Columbia and Washington, down throughout Oregon, and even as far south as Central California.

Miller: Before we get to this current research, what did scientists know already about the differences between these two populations?

Tennessen: The key difference is that they’ve really had opposite patterns of growth over the last 50 years. Both populations were exploited for the aquarium industry back in the 50s and 60s and members of both populations were extracted or some died in the process. Around the early 70s, both populations were at relatively low levels. Since then, the northern residents have really started to make a comeback and have grown somewhat steadily over the past 50 years. The southern residents though really haven’t exhibited any net growth over that same period of time. And so going into this study, we were trying to understand why these populations that live in relatively similar areas and behave similarly and are structured similarly, have really exhibited opposite patterns of growth.

Miller: What were the best theories for why?

Tennessen: There are some differences when you get into the different sources of disturbance in their habitats. They do have overlapping habitat. But typically the salmon runs in the northern resident habitat tend to be a little healthier. And also there tends to be more vessel disturbance in the southern resident habitat – more human activity, potentially more noise. There are more stressors that the southern residents are exposed to more often.

Miller: So let’s then turn to this new study. Where does the data for this study come from?

Tennessen: About 10 years ago, folks in Canada [at] the Department of Fisheries and Oceans, as well as NOAA Fisheries here in the US, began conducting two separate studies of northern and southern resident killer whales, looking at underwater hunting behavior using these really interesting tags that are suction cupped onto the top side, the dorsal side of killer whales. These tags paint this incredible underwater picture of what it’s like to go about your day as a killer whale. A lot of the research that we had previously done on killer whales relied on surface observations. Whales spend about 90% of their time underwater so relying on behavioral observations of about 10% of the daily activities of these animals – it’s kind of like trying to understand the world through a pinhole. These originally began as separate studies of each of these populations in isolation. But then about five years ago, we came together and decided to compare the foraging behavior, the hunting behavior, of these two populations, specifically by pooling this data that we had. These tagging studies have been conducted in similar ways and similar environments. So we were able to take advantage of that opportunity to really understand the key differences in the underwater behavior, and particularly focus on the patterns of hunting and the strategies that the whales seem to be employing when they engage in their hunting activities.

Miller: You know, it’s striking so many times over the years on this show, when we’ve talked to scientists, the data that you and many other people in all kinds of fields end up using comes sometimes by accident like this. I mean, obviously it wasn’t an accident to have researchers put these suction cups with sensors attached on different populations of whales. But it’s just by chance that people were putting these sensors on these two populations at the same time. And that could prove to be the basis for this important comparison.

You actually provided us with a recording that came from Marla Holt at NOAA Fisheries so we can hear what one of these whales sounds like. This is from a southern resident killer whale, a southern resident orca. Let’s have a listen and you can tell us what we have heard.

[SOUND OF ORCAS COMMUNICATING INCLUDES CLICKS AND CALLS]

Miller: How much do you know about what’s being communicated there?

Tennessen: People often debate, is there a language or will we ever crack the code of killer whale communication. We really can’t say. We know that killer whales produce a variety of sounds. They produce these social sounds to stay in contact with each other. It’s likely that these sounds also communicate identity, sort of like a badge, and that they can discriminate who’s making the call just by the information conveyed in that sound. There’s also group cohesion. You have to remember that they’re traveling in these often dark underwater environments where visual cues don’t work at close range or at longer range. They can be spread out a few miles and still be in contact using these calls [that] often serve the purpose of keeping the group together while apart.

They also use sounds that almost sound like clicking or I think they sound like rattling a can or bouncing a basketball. And these sounds are echolocation clicks, which are used to find fish. They swim at the surface and they emit these sounds repeatedly, sometimes slowly, sometimes faster. And then they listen for the echoes from those sounds that they sent out to return back to them. The way they return back and interpret that information tells them what fish is ahead of them, how big it is, how far away it is, if there’s a rock in the way. There’s so much information that they can glean just from the echoes of those sounds. As they get closer to the fish, they start producing clicks that are faster. There’s shorter spacing between them and then ultimately, they make these buzzes right at the end that we don’t really understand fully the function of these buzzes, but they’re related to that final phase of capturing a fish.

Miller: The buzz means, “I’m about to eat you, salmon.”

Tennessen: Yeah, I think so, but it doesn’t stun them. For a while the theory was that buzzes may stun prey. We see buzzes in other toothed whale species as well, like sperm whales. But it’s not exactly clear what the buzz does because sometimes they miss the fish. We hear a buzz, but then we don’t see movements that are indicative that a fish was captured. So we don’t exactly know what that buzz does, but we know that it means the killer whale is right there, almost ready to eat it.

Miller: So when you compared all this data from these two different populations of killer whales, what did you find that was most surprising?

Tennessen: We found that there were these interesting differences in who was catching the most fish in each population. In the northern resident population, we found that females were doing the majority of capture and males were doing less. That’s consistent with previous research and with the theory that males are larger and require more calories. I should back up and say that in killer whales – fish eating killer whales, specifically – fish that are captured are often shared among family members. That’s one of the important ways that the family meets its energetic requirements.

Miller: Meaning, the hunting killer whale would just store the salmon in some part of their mouth and then spit them out for their family to eat when they bring them back?

Tennessen: Basically. It’s really interesting. They catch a fish at depth, sometimes more than 300 feet deep, and they bring that fish back to the surface. Then they kind of shake their head around and rip it apart, and toss pieces to family members. We don’t have a great sense of how often that happens. But we know that it makes up a significant component of the fish that they’re consuming – that they’ve either captured a fish and shared it, or they’ve received a piece of fish from someone else in their family. So we found that in northern residents, females were capturing more fish than males, which made sense. But in southern residents, we found the complete opposite. We found that males were catching more fish than females, and that kind of goes against evolutionary theory, as far as we understand.

Miller: So how have you tried to make sense of these findings?

Tennessen: We know that the northern resident population is relatively healthy. They are threatened – they were listed threatened in the early 2000s – but they’ve been growing steadily since. They’re on a good path forward. The behavior that we see in the northern residents, we think, is consistent with what you might expect in a relatively healthy population where females ought to do the majority of the prey capture and help their adult sons and adult grandsons meet their own requirements, in terms of taking in calories. But we think that potentially what we’re seeing in southern residents may be a population that’s so fragmented in terms of mortality, the loss of key individuals of each family group, that the roles that would potentially make sense if they were doing better don’t necessarily make sense anymore. Or they may just not be able to engage in those same roles. For example, we found that females with calves across both populations were less likely to engage in hunting than those that didn’t have calves. And that makes sense. It takes a lot of energy and time to nurse calves. And typically that is done at the surface. So it makes sense that you might see females with calves kind of engaging in fewer foraging opportunities and maybe relying more on other members to bring food back for them. But what we saw in southern residents was that during the time that the tags were on, we didn’t see any hunting behavior at all in these females with calves. We’re thinking that potentially males have needed to step up to the plate a little bit more in order to account for these females engaging in fewer and fewer hunting opportunities.

Miller: So it sounds like your hypothesis is that this difference in hunting is more a response to population pressures and dwindling population than a reason for that population drop.

Tennessen: I think that’s right. And we’re still trying to figure out exactly what the implications of these different strategies may be. It could be that in the short term, this solution of males catching more prey than females, may help the population get the calories that it needs or at least get enough calories to continue to kind of get by. But in the long term, it seems to me that a strategy that relies heavily on these larger males who require more food to bring back more prey may potentially take a toll on these males in the long run. And we are seeing these reproductive males dying prematurely. We’re seeing that across all age and sex groups in southern residents.

Miller: Perhaps because they have not evolved over millions of years to do this much hunting, this much moving around, and they’re now acting differently than their bodies are used to. I’m curious – not too long ago we talked about changes in the migration patterns of bowhead whales. This was further up, in the Arctic, because of climate change. Is anything preventing the southern resident population from just going further north [and], in a sense, becoming northern residents?

Tennessen: That’s such a good question. We don’t really understand what is keeping them in this area. In some ways we scratch our heads every year when they return to their core summer habitat in the San Juan Islands and around Vancouver Island. It would make sense to go up to Alaska where the salmon are much more abundant, but for whatever reason they don’t. They’re long-lived animals and the female matriarchs, the grandmothers, tend to lead the groups to key places to hunt. It may just be that for whatever reason they’ve learned throughout their lifetimes that these areas are the best places to go. And so they continue to come back.

Miller: I can’t help but think humans aren’t that different in a lot of ways. Year after year, hurricanes may batter the Gulf Coast and there are some people who don’t live there and say, “why you’re going to rebuild there? Climate change is just going to make these storms worse, go somewhere else.” The whales are doing the same thing; they’re saying this is home.

Tennessen: Absolutely.

Miller: I’m curious, what you’ve learned from this study, and there’s more on the way – my understanding is that one of the next things you’ll be releasing will be more information about whale activity at night – does this provide more information about better conservation efforts for these southern residents?

Tennessen: What we’re trying to do ultimately is to really understand what factors are most related to foraging success. If we can tease apart the oceanographic variables, environmental factors, factors related to human disturbance, then we can start to develop management that specifically tries to increase opportunities for foraging. A long term solution, of course, is to increase the number of salmon, to restore salmon habitat, and to really help salmon runs reach the levels that they used to be. But that won’t happen overnight. In the meantime, recognizing that these whales still need to eat every day, how can we better understand the factors that increase their chances of successfully catching fish? And how can we promote those opportunities through management?

Miller: What are you most excited to study next?

Tennessen: Right now we have this really fun study that we recently completed where we tagged these whales with these similar suction cup tags overnight. We’re starting to get a good picture of what they do at nighttime. And previously, we really had no idea. We could listen on hydrophones that were mounted underwater. But we didn’t understand how they were moving, if they were catching fish. By putting these tags on and allowing them to stick on overnight, we’re starting to analyze data that’s giving us a much better picture of [whether they] can shift their hunting behavior to nighttime when there’s fewer boats and less noise. In the next few months, I think we’re going to have a better answer to that question. So stay tuned.

Miller: We will. Jennifer Tennessen, thanks very much.

Tennessen: Thank you.

Miller: Jennifer Tennessen is a senior research scientist at the Center for Ecosystem Sentinels at the University of Washington. She’s a part of the team that recently published a paper that provides more insight into the differences between endangered southern resident orcas and the orca population just to the north that are faring better.

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