Flexible diets are key to killer whale survival

Often referred to as resident orcas, killer whales are no ordinary predators of the sea. With their intelligence and strong matriarchal social structures, killer whales have learned complex foraging strategies that vary with regional prey resources, shaping their diet accordingly.

Researchers at the University of Washington set out to investigate whether social learning affects the dietary preferences of different killer whale populations.

The diet of a killer whale

Amy Van Cise is a UW professor of aquatic and fishery sciences, who began this study as a postdoctoral researcher with NOAA’s Northwest Fisheries Science Center.

“Killer whales are incredibly intelligent, and learn foraging strategies from their matriarchs, who know where to find the richest prey resources in their regions,” said Professor Van Cise.

“So we wanted to know: Does all of that social learning affect diet preferences in different populations of resident killer whales, or in pods within populations?”

In their research paper, the team describes the dietary intricacies of two resident killer whale populations, the Alaska residents and the southern residents.

What they found was a broad preference for salmon – more specifically Chinook, chum, and coho – among both populations. However, the timing of diet switchovers and the other supplementary fish species in their diets varied between the two populations.

Killer whale diet and survival

Southern resident killer whales are critically endangered, unlike other growing populations. The core purpose of this study was to compare the dietary patterns of the threatened whales with thriving populations to inform conservation efforts.

”We know that lack of food is one of the main threats facing the endangered southern resident killer whales,” said Professor Van Cise.

“We figured that if we could compare their diet to the dietary habits of a healthy and growing population, it might help us better understand how we can steward and protect this vulnerable population.”

Impact of human activities

Unfortunately, human activity has significantly disrupted the usual food supply for resident killer whales, particularly in Southern areas.

Pollution, construction of dams, and the historical capture of these whales for amusement parks have left the southern resident killer whale population plateaued at around 75 individuals.

Over the past decade, the research team collected fecal samples from both southern resident and Alaska resident killer whales. Their analysis revealed surprising differences.

The Alaska residents had more chum and coho salmon in their summer diet compared to the southern resident killer whale’s Chinook-heavy diet.

The vital role of dietary diversity

In addition, recent sample collections outside of the summer months unveiled an unexpectedly diverse diet for the killer whales.

Species such as sablefish, arrowtooth flounder, lingcod, Pacific halibut, and big skate were all featured in the diets of these whales.

This dietary diversity and flexibility seem to be essential for the thriving killer whale populations, hinting at possible strategies for supporting the endangered southern resident killer whales.

Prey availability across regions

The researchers also explored the influence of seasonality and regional prey availability on killer whale diets. It was discovered that seasonal shifts in fish populations could mold the feeding habits of resident killer whales, urging them to adapt their foraging techniques.

For instance, during the fall when Chinook salmon runs dwindle, southern residents have been observed turning to alternative prey sources, albeit not always in ample supply.

This seasonal adaptability emphasizes not only the resilience of these marine mammals but also the exquisite complexities of their relationship with the marine ecosystem.

The implications are profound, indicating that conservation efforts should consider seasonal prey patterns to bolster the sustenance of southern resident populations.

Broader ecosystem health

Addressing the food scarcity faced by southern resident killer whales goes beyond safeguarding a single species; it involves understanding and nurturing the interconnected marine ecosystems.

Efforts to protect these whales must extend to maintaining and improving the health of salmon populations, removing or modifying physical barriers such as dams, and mitigating pollution and climate impacts that disrupt these systems.

The study highlights the need for a holistic ecosystem approach to conservation, which could foster a sustainable environment for various marine species.

Key insights on killer whale diets

A deeper understanding of killer whale diets is key to aiding the recovery and conservation of these majestic creatures.

“While protecting key populations of Chinook salmon will always be vital to supporting the recovery of the endangered southern resident killer whale population, this study has taught us that we might need to think more holistically about how we can conserve the whole ecosystem of fishes that together make up the annual diet of predator populations like this one,” said Van Cise.

The research is a powerful reminder that the fate of these extraordinary creatures is intertwined with their ecosystem, and our actions play a crucial role in the balance.

The study is published in the journal Royal Society Open Science.

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