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Scientists study the diets of top ocean predators as they change in response to their environment. Indeed, the quantity and what they eat can affect the functioning of ecosystems.
And while researchers know that killer whales, also known as orcas, are the ocean’s top predators, our understanding of their diets, particularly how much of each species they consume, remains incomplete.
This is especially true for remote populations that cannot be observed year-round.
But now there’s a way to recreate the precise diet of killer whales using just a sample of their skin and blubber. My research team has developed a promising technique that reveals the diet of these wild predators in the North Atlantic Ocean. Our study is published in the Journal of Animal Ecology.
Multiple Feeding Strategies
Killer whales are intelligent predators that are known to adopt specific hunting techniques, ranging from carousel feeding – cooperative herding and then feeding herring, to cooperatively creating waves that can dislodge seals from the pack ice. As a result, they can hunt almost any species, from fish to fur seals to blue whales, in all the oceans of the world.
Depending on their location and evolutionary history, different groups of killer whales have developed different ecotypes – unique diets and lifestyles. The most notorious ecotypes are the transient and resident killer whales of the eastern North Pacific.
These ecotypes have been widely studied for decades, as killer whales inhabit densely populated areas, allowing scientists to observe these individuals year-round.
In these populations, scientists have found evidence of “food cascades”, the effects of eating predators on the rest of the food web. Killer whales had top-down effects on the density of kelp forests; the killer whales drastically reduced the sea otter population, which caused the proliferation of sea urchins – the sea otters’ main food source – and decimated the kelp forests.
North Atlantic Mystery
Comparing different groups of killer whales around the world reveals that there is still a lot we don’t know about them. Figuring out what killer whales living in remote Arctic regions, such as Baffin Bay, Greenland and arctic Norway, eat is a challenge. Observing feeding events can be difficult in the volatile waters of the Arctic Ocean.
Early studies suggested that there were two types of killer whales in the North Atlantic: those that feed on marine mammals and the other group that eats fish and occasionally seals. However, the lack of data, combined with emerging evidence, has led scientists to propose removing this classification. There appears to be a more diverse range of diets in some North Atlantic populations.
Because of the challenge of collecting observational data, the researchers focused their efforts on the chemical signals they can measure inside the skin and blubber of killer whales. These chemical signals can be made up of lipids or stable isotopes that tell us what the whales are eating and their impact on the food chain.
Our technique measures the lipid composition of whale blubber and uses a computer program to recreate the most likely proportion of each prey species in an individual’s diet.
All that’s needed are multiple lipid “signatures” – representing the proportion of each fatty acid in the whale’s blubber – of killer whales and their potential prey.
Our open access research, recently published in the Journal of Animal Ecologyused a technique called quantitative fatty acid signature analysis to reveal the diet of nearly 200 North Atlantic killer whales.
We also measured lipid compositions in over 900 prey samples. The results showed a range of feeding strategies across the ocean. In the western North Atlantic, killer whales primarily ate other whales (such as large baleen whales, belugas, and narwhals); killer whales in Greenland preferred seals and in Norway they showed a preference for fish such as herring.
Using this technique, scientists can now estimate the exact percentage of different species in each whale’s diet. But what surprised us most was the level of variation between individual diets within each population.
In the western North Atlantic, individuals focus either on cetaceans – marine mammals like beluga whales and narwhals – or on seals. In the mid-North Atlantic, killer whales feed on whatever prey is available. And for the most part, killer whales in the eastern North Atlantic seem to eat a diet high in fish. Several individual killer whales in Norway and Iceland complement the fish with marine mammals.
Our study is the first and largest of its kind on killer whales, and our findings encourage us to further investigate killer whale diets on an individual scale. We now know that individuals within the same population may have different diets.
This not only translates into different contaminant exposures and health risks for these top predators, but also represents different feeding strategies across the Arctic Ocean.
This approach allows us to measure future changes in the diets of these predators and understand how they may impact Arctic food webs. Due to climate change, killer whales are gradually moving further into the Arctic.
Their presence and a potential increase in the consumption of Arctic species could alter the dynamics of ecosystems in the North. Further research using this technique on samples collected over a long period of time could allow researchers to detect changes in the diet and ecosystem of whales.
Anaïs Remili et al, Quantitative fatty acid signature analysis reveals a high level of dietary specialization in North Atlantic killer whales, Journal of Animal Ecology (2023). DOI: 10.1111/1365-2656.13920
Journal of Animal Ecology
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