Killer Whale Diet & Hunting
What it takes to be an apex predator
Killer whales follow a strict seafood diet: they "see" food and eat it! Bad jokes aside, orcas are generalist feeders who have been recorded to prey on over 140 different species and are the only cetaceans to hunt marine mammals. Killer whales have no natural predators (except humans) and their co-operative hunting strategies allow them to prey on such a wide range of species, even great white sharks! (Ford, 2009, Reeves et al., 2017)
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As previously mentioned, the three killer whale ecotypes all specialize on different types of prey. In this section, I will give an overview on the different dietary preferences and hunting strategies for resident, transient and offshore killer whales and I will also discuss the growing negative impacts associated with their diets.
Resident Killer Whales
The Fish-Feeders
The Prey
To the resident killer whales, fish are food, not friends. They hunt and consume various teleost prey, primarily pacific salmonids. They show a strong preference for Chinook Salmon (Oncorhynchus tshawytscha) (pictured bottom right), yet they also prey on sockeye salmon, pink salmon, and chum salmon. This strong preference for chinook is interesting because they are the least abundant salmon species in their habitats, although they are large and have higher caloric content than other salmonids (caloric analysis is under energetics). Chinook salmon are most common in the fall during salmon migration along the shores of British Columbian and Washington, where many residents often aggregate in these areas during this time.
(Ford and Ellis, 2006; Ayres et al., 2012; Ford and Ellis, 2013)
Hunting
The hunting strategies of residents is not as co-operative as one might think given their strict social structures. When foraging, members of the pod and matriline disperse into smaller groups across wide areas. Each of these smaller groups travel in a similar direction and use calls to echolocate prey and co-ordinate between groups. Resident killer whales do not appear to co-operatively herd or capture prey, although some older individuals will break up their prey to share within matrilines. (Ford and Ellis, 2006; Ford and Ellis, 2013)
Chinook Salmon (Oncorhynchus tshawytscha)
(NOAA. 2021. https://www.fisheries.noaa.gov/species/chinook-salmon#overview)
Transient Killer Whales
The Mammal Eaters
The Prey
Transient killer whales specialize on endothermic prey, mainly other aquatic mammals. Common prey of transients includes steller sea lions (Eumetopias jubatus) (pictured below) harbor porpoises, dall's porpoises, harbor seals, and even dolphins and minke whales are also consumed. Unlike the salmon migration for residents, transient prey is abundant year-round in coastal waters. Thus, there is no seasonal abundance of transient killer whale populations as there is for residents. (Ford, 2009; Ford and Ellis, 2013)
Steller Sea Lion (Eumetopias jubatus) (NOAA. 2022. https://www.fisheries.noaa.gov/species/steller-sea-lion)
Hunting
Unlike residents, transient killer whales hunt co-operatively in small groups and in silence. Stealth is important because their prey can detect echolocation and communication calls and swim away. Transient killer whales have two foraging strategies: one for nearshore hunting and one for open water hunting. Nearshore hunting is often focused on the seals and sea lions that like to haul out along coastlines. Transients hunting nearshore circle around good hauling out sites such as islets and reefs. In open water foraging, individuals spread out over a much wider area and hunt for mainly porpoises and dolphins. In both foraging strategies, transients use cooperative hunting strategies and share prey more often than residents. Groups of transients have been seen to circle prey in open water, with individuals taking turns ramming or hitting the prey until it can be easily consumed. Transients also have a "tag-team" strategy, where individuals take turns chasing the prey until they are too exhausted to put up a fight. (Ford, 2009; Ford and Ellis, 2013).
Offshore Killer Whales
The Shark Eaters
Offshore killer whales are the least understood of the three ecotypes. They have been found to feed primarily on small sharks and large teleost prey. Analysis of stomach contents suggest that blue sharks and pacific sleeper sharks are some of the most common prey. Offshore killer whales often have abrasive skin and high levels of tooth wear. Likely a consequence of their diet, as taking down a shark is no easy feat. (Ford and Ellis, 2013)
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At the current moment, there is little to no knowledge about foraging strategies of offshore killer whales. They have been observed in large groups of up to 100+ individuals, although it is uncertain if these groups are part of their foraging ecology. (Ford and Ellis, 2013)
Food Toxicity & Prey Abundance
Being the apex predators in an ecosystem as large as the ocean comes with a lot of responsibility. The loss of a keystone species can have major impacts on the structure of an ecosystem. There are many consequences that killer whales face while enjoying their place at the top of the food chain. In this section, I describe how environmental contamination and prey abundance can impact the diet of killer whales.
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Toxic Food
As human activity continues to grow, environments across the world are becoming increasingly polluted with chemicals, and the ocean is no exception. Prey species become contaminated with toxins, passing them on to whichever animal consumes them. These toxins work their way up the food chain, ultimately accumulating in top predators such as killer whales. As more toxins accumulate, the negative effects become more severe.
One family of chemicals, polychlorinated biphenyls (PCBs), is found to accumulate in high concentrations in orcas. PCBs have been banned for nearly 30 years, although their high persistence means they are still accumulating in killer whales. PCBs alone can greatly affect the long-term viability of a killer whale population by negatively impacting reproductive success. The issue here is not with PCBs specifically. The concern is that if a single chemical family can have such major effects on killer whale populations, what happens when a variety of different toxins accumulate? (Reeves et al., 2017; Desforges et al., 2018)
Prey Abundance
It should be no surprise that killer whales need to eat a lot to maintain their lifestyle. The amount of food required by a group of orcas can actually be enough to significantly alter their prey populations. Coupled with the fact that salmonid abundance is seasonal, reduced prey populations can have large impacts on the health of killer whales.
Considering resident killer whales, their prey is seasonally abundant. A study measured stress hormones of resident killer whales and compared them to prey abundance. They found that stress levels were highest when prey abundance was lowest. Further, they recorded the highest number of deaths and lowest number of births following the year with the lowest prey abundance. (Ayres et al., 2012)
Transients are a little different because their prey is not seasonal. A study that investigated the prey requirements of transients found that a population of killer whales requires enough prey to cause population declines of sea lions and otters. When we consider human impacts on these prey populations as well, the risk of inadequate food for killer whales becomes a more concerning possibility. (Williams et al., 2004)