Scientists develop a model to reveal wild killer whales’ diets

A new study presented a method to reconstruct killer whales’ diets using the lipid composition of their blubber. By measuring these lipids, called fatty acids, in the killer whales’ fat and those in their potential prey, scientists can estimate the abundance of each prey species in the whales’ diet. This new method may hold the key to unlocking the secrets of killer whale diets as they gradually invade the Arctic due to climate change. Let’s find out how researchers did it.

Lipids transfer from the prey to the predator

Researchers used a model called QFASA (Quantitative Fatty Acid Signature Analysis). The approach relies on a simple principle; fatty acids get transferred from the prey to the predator with predictable modifications. These fatty acid modifications or transformations depend on many factors: the type of predator, the type of tissue, etc. Until now, scientists were missing an essential piece of the puzzle: calibration coefficients. These numbers account for the transformation of fatty acids by killer whales after they ingest their prey. To be able to obtain these numbers, scientists would need to conduct captive feeding trials that are challenging and costly to implement. But scientists from this study found a way to measure these lipid transformations.

Captive individuals helped scientists calibrate their model

The researchers got access to full blubber samples from deceased captive individuals fed a long-term diet; a fixed proportion of different fish species. These killer whales died in captivity years ago, but vets who conducted their necropsy saved some tissues, like blubber, for future studies. Scientists calculated the calibration coefficients by measuring fatty acid compositions in these killer whales and the fish they consumed in captivity. The research team then tested the model on the same individuals to see if the missing piece (i.e.: the calibration coefficients) allowed them to estimate their known diets. Their model predicted the correct diets with great accuracy.

Blubber is a stratified tissue

Killer whale blubber varies in lipid composition across its depth, adding another layer of complexity to the model. Yet, researchers could fine-tune the model to the outer blubber (closest to the skin), which is of particular interest because it is the tissue ecologists collect when they take skin biopsies from wild whales. They addressed this issue by calibrating the model to each depth of the blubber, by cutting each blubber sample into ten equal layers. Then they demonstrated that the model can be used on biopsies that only collect the outer layers of the blubber. The last step for them was to prove the model works on wild individuals.

Testing the model to reveal Greenandic killer whales’ diets

After they confirmed that the model functioned on captive killer whales, scientists applied it to wild individuals. To do so, they collected full blubber samples from subsistence-harvested killer whales in East Greenland. From the model, they estimated that wild killer whales from Greenland mainly consumed seals and mackerel. This information was consistent with the stomach contents available for the same individuals. Therefore, they could confirm that this technique works on wild killer whales.

Why is this important?

Until now, ecologists could only measure fatty acids and compare the profiles qualitatively, allowing them to differentiate between feeding habits. Still, they could not apply this quantitative technique to biopsies to reconstruct whale diets in detail. This new tool can provide incredible details on killer whales’ long-term diets and estimate the percentages of consumed species. It may also help trace climate change-driven changes in wild populations’ diets, using only blubber biopsies, which are minimally invasive sampling techniques. This is especially important for remote populations scientists cannot observe year-round.

Can we use it on other species?

The researchers explained that they could most likely apply this quantitative analysis to other toothed whale species like narwhals, belugas, and pilot whales because their blubber structure is similar to killer whales. The study thus provides ecologists with a new tool to study the diets of toothed whales and they will need to test it on new species.

What comes next?

Scientists from this study are now applying the technique to multiple killer whale populations across the North Atlantic. Their goal is also to understand how diets differ between and within populations.

Sources

Remili, A. et al. Validation of quantitative fatty acid signature analysis for estimating the diet composition of free-ranging killer whales. Sci Rep 12, 7938 (2022). 10.1038/s41598-022-11660-4

The recap of the study’s findings in English, French and Danish — Credit: Anaïs Remili

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