Orcas, commonly known as killer whales, are found in every ocean across the globe, establishing themselves at the top of the marine food web. As the largest members of the oceanic dolphin family, they possess the intelligence and cooperative hunting strategies necessary to target a vast array of prey. The diversity in their diet is immense, ranging from small schooling fish to large baleen whales, but individual killer whale populations exhibit extreme specialization. This dietary specialization forms the basis of distinct, non-interbreeding cultures within the species, dictating their social structure and hunting vocalizations.
Defining the Major Feeding Groups
The global killer whale population is divided into distinct groups known as ecotypes, which are genetically and behaviorally separate, even when they occupy the same waters. In the Northeast Pacific, three primary ecotypes have been identified: Residents, Transients (also called Bigg’s Killer Whales), and Offshores. These groups do not interbreed or share feeding habits, a separation that has led to unique physical and cultural adaptations.
Resident Orcas and Their Fish-Exclusive Diet
The diet of Resident killer whales is almost entirely composed of fish, representing a complete specialization away from marine mammals. In the Pacific Northwest, these orcas exhibit a strong preference for Chinook salmon, which can constitute up to 96% of the salmon portion of their diet. This preference is driven by the fact that Chinook are the largest and fattiest of the Pacific salmon species, providing the highest caloric reward for the effort expended in the chase. An adult Resident orca must consume between 18 and 25 adult salmon daily to meet its high energy requirements.
The reliance on fish dictates a unique hunting culture, characterized by the extensive use of echolocation to find their prey. The whales emit high-frequency clicks that bounce off fish, allowing them to create a detailed auditory picture of the underwater environment, discerning the size and species of a salmon. This acoustic hunting strategy is effective against fish, which generally lack the sophisticated hearing to evade the sound pulses. Resident whales live in large, stable pods where offspring remain with their mothers for their entire lives, a social structure supported by the predictability of fish runs. When Chinook salmon are less available, the whales will diversify their diet to include other fish species like coho and chum salmon, or even bottom-dwelling fish such as halibut and lingcod.
Transient Orcas and Their Marine Mammal Diet
Transient killer whales, or Bigg’s Killer Whales, are the mammal-hunting specialists, subsisting on a diet that includes seals, sea lions, porpoises, dolphins, and various species of whales. Unlike their fish-eating counterparts, Transient orcas rely on stealth and surprise to successfully ambush their highly alert, sound-sensitive prey. They often hunt in smaller, more fluid groups and travel in near-total silence, suppressing the use of echolocation until the moment of attack.
The hunting techniques of Transient orcas are highly coordinated. To take down large or dangerous prey, they employ forceful tactics such as ramming the animal with their bodies or using powerful tail-slaps to stun or disorient it. Against seals resting on ice floes, groups of orcas will synchronize their movements to create large, powerful waves that wash the prey into the water. In the case of Gray whale calves, the orcas work as a team to separate the calf from its mother, preventing it from reaching the surface to breathe.
Offshore Orcas and Their Deep-Sea Prey
The Offshore ecotype is the least understood of the three, as they primarily inhabit deep waters over the continental shelf and slope, far from coastal observation. Current scientific understanding suggests their diet is composed of large, schooling fish and, notably, sharks. Observations have confirmed that they actively hunt species such as the Pacific Sleeper Shark, a large, deep-dwelling predator. The inclusion of sharks in their diet explains a distinguishing physical feature of the Offshore population: their severely worn teeth.
The skin of sharks is covered in tiny, hard, tooth-like structures called dermal denticles, which give it a rough, sandpaper-like texture. The constant friction of tearing and consuming this abrasive skin causes the teeth of Offshore orcas to wear down significantly over their lifetime, sometimes to the gum line. Offshore orcas tend to congregate in large pods, sometimes numbering up to 200 individuals, suggesting a cooperative hunting strategy is necessary to subdue their large, deep-sea prey.