Here is a killer whale fact for you: despite decades of intensive research, we still do not know for sure what killer whales eat in most regions of the world. Killer whales are top predators, and the amount and types of animals they eat can greatly impact a whole ecosystem. To figure out how killer whales can affect communities, we first need to know what they like to eat and how much of it they eat.
This article summarizes the findings of the third chapter of my doctoral thesis: an open-access peer-reviewed study just published in the Journal of Animal Ecology. We used lipid analyses to reveal the diets of North Atlantic killer whales in unprecedented detail.
Killer whales in the North Atlantic remain understudied
Scientists have been studying the feeding habits of killer whales for decades, but there is still much to learn. In the past, scientists relied mostly on watching killer whales feed and hunt in the ocean to figure out what they eat. However, this method only tells us what they ate at that specific moment and doesn’t give us a clear idea of what they eat over time. This is especially true for remote populations (think the Canadian Arctic and Greenland) because these observations at sea are generally rare.
At first, scientists thought that there were only two ecotypes of North Atlantic killer whales: one that eats fish and seals and another that eats marine mammals. But due to a small sample size and emerging evidence of different feeding strategies in several populations, scientists suggested retiring these two ecotypes and focusing on getting more data from the rest of the North Atlantic.
So how do we measure killer whale diets?
To better understand what these predators eat, scientists can use chemical signals in killer whale skin and blubber biopsies. These methods involve measuring ratios of isotopes or ratios of lipids called fatty acids in the whales’ skin and blubber. This gives scientists insight into the whales’ diets over a long period of time, which can be difficult to observe directly, especially when they are in remote areas.
That’s where quantitative fatty acid signature analysis (QFASA) comes in. This statistical model allows scientists to estimate the relative contributions of each prey species to a predator’s diet. The method involves measuring the fatty acid composition of the killer whales (in their blubber) and their potential prey. This enabled our team to estimate the exact proportion of different prey species in the whales’ diets. To do so, we collected 200 killer whale skin and blubber biopsies (through dart sampling). Then we measured the lipid composition of the 200 North Atlantic killer whales and over 900 potential prey items, ranging from fish to seals to cetaceans.
Killer whale diets vary across the ocean
We found that killer whales have different diets throughout the North Atlantic. For example, killer whales in the Eastern North Atlantic (Norway, Faroe Islands, Iceland) mainly feed on fish, specifically herring (for more details, check out the infographic below). Meanwhile, those in the Central North Atlantic (Greenland) prefer to consume seals such as ringed, harp, hooded, and bearded seals. Finally, killer whales in the Western North Atlantic (Eastern Canadian Arctic and Eastern Canada) tend to prey on other whale species, such as belugas and narwhals in the Arctic and baleen whales and porpoises in Eastern Canada.
But diets also vary between individuals in the same population!
However, we were surprised to find significant differences in the diets of individual whales, even within the same populations. The results showed that individual killer whales within several populations exhibit different feeding preferences. For instance, some Eastern North Atlantic killer whales supplement their fish diet with marine mammals. At the same time, those in the Central North Atlantic show a generalist feeding pattern, consuming all available prey. Finally, individual killer whales in the Western North Atlantic tend to focus on either cetaceans (narwhals and belugas in the Arctic; baleen whales, and porpoises in Eastern Canada) or seals.
We need to focus our efforts on the individuals
This study is the first and largest of its kind on North Atlantic killer whales. It has given us valuable information about how these animals eat in unprecedented detail. It is also the first study to quantify diets in Canadian killer whales. This is important because as killer whales move into the Arctic due to climate change, their eating habits could impact local ecosystems.
The study also showed us that each killer whale has a unique diet, which could expose them to different pollutants and health risks. This is why we need to do more research on individual killer whales instead of grouping them into two ecotypes. Our study supports the retirement of the two past North Atlantic killer whale ecotypes. The next step for us is to measure pollutant concentrations within the same whales to understand how these different diets impact their health throughout the North Atlantic. Stay tuned for more killer whale knowledge!
If you want to know more about the study, feel free to get in touch! On a more personal note, I would like to thank the 15 other authors for this study, all from different countries. Their support and kind feedback have helped me so much while writing this third chapter of my doctoral thesis!
You can find the video recap I created for our paper here:
Sources and further reading:
- Foote, A. D., Newton, J., Piertney, S. B., Willerslev, E., & Gilbert, M. T. P. (2009). Ecological, morphological and genetic divergence of sympatric North Atlantic killer whale populations. Molecular Ecology, 18(24), 5207-5217.
- Lefort, K. J., Matthews, C. J., Higdon, J. W., Petersen, S. D., Westdal, K. H., Garroway, C. J., & Ferguson, S. H. (2020). A review of Canadian Arctic killer whale (Orcinus orca) ecology. Canadian Journal of Zoology, 98(4), 245-253.
- Breed, G. A., Matthews, C. J., Marcoux, M., Higdon, J. W., LeBlanc, B., Petersen, S. D., … & Ferguson, S. H. (2017). Sustained disruption of narwhal habitat use and behavior in the presence of Arctic killer whales. Proceedings of the National Academy of Sciences, 114(10), 2628-2633.