When Killer Whales Hunt the King of Salmon

Two Southern Resident killer whales.
Southern Resident killer whales are critically endangered with only 80 individuals left in the wild. (NOAA Fisheries)

In the Pacific Northwest of the United States there is a population of killer whales with a particular taste for salmon, though not just any salmon will do. Chinook salmon are considered king—the largest salmon in the world, they are the preferred meal for Southern Resident killer whales. Scientists believe the whales can discern which fish is a Chinook just by using sound.

Like other dolphins and toothed whales, killer whales “see” underwater using sound. This adaptation is called echolocation, and it is also used by bats, some nocturnal birds, some shrews, and a mole-like mammal called a tenrec. A whale’s vision is limited underwater, especially at depths where light must filter down. Echolocation extends the range at which whales can detect objects, possibly up to 500 feet away. They effectively live in a very different world than our own—one dominated by sound.

As the largest of the salmon species, it would be easy to assume that the Chinook are easy to spot simply by their size. But even in the presence of similarly sized fish, Southern Resident whales can pick out the Chinook. There are hundreds of other fish species in their hunting range, including five other species of salmon. Pink, coho, and sockeye salmon, though on average smaller, often overlap in size with the smaller range of Chinook. Until recently, even scientists were perplexed as to how they achieved such an amazing feat.

a diagram of a killer whale using echolocation
A whale sends out an echolocation whistle using air sacs. It detects the reflected sound via its jaw and inner ear. (Uko Gorter via NOAA)

This remarkable ability to travel and hunt salmon using sound relies on a set of special adaptations that emit high-pitched sounds, called ultrasounds, and detect them. Whales and dolphins use nasal air sacs in their head to produce clicking sounds across a broad range of frequencies. They then focus and project those clicks using a mass of fatty tissue in their forehead called a melon. When the projected sound hits an object, like a fish, it reflects back to the whale, which then receives the sound through their lower jaw up to the middle ear. As the whale gets closer to their targeted prey, they increase the rate of the clicks until it becomes more of a buzz. This allows for a constant, live action auditory picture of the prey in front of them, despite the agile movement of the salmon. 

The key to this puzzle lies in a special organ found in fish—the swim bladder. A swim bladder is a sac of air that a fish fills and compresses to regulate their buoyancy in the water. It turns out, the shape and angle of the swim bladders differ from one salmon species to another. Chinook salmon have relatively small and skinny swim bladders when compared to coho and sockeye. A recent study found that it is the swim bladders’ shape that alters the reflecting sound. This makes sense considering sound moves differently through water and air. This unique sound signature is likely what the whales are listening for when they emit clicks throughout the water. What’s even more amazing is that as the fish moves, the signature alters slightly, meaning that the killer whale must recognize a whole suite of sounds that are specific to the Chinook. Since they rely heavily on sound for everyday life, it is likely that their brain is specially wired to be attuned to these subtle differences.

Understanding how whales perceive their world and use sound for day-to-day tasks is critical as humans continue to increase their presence and use of sound underwater. Southern resident killer whales live in the Salish Sea, a heavily trafficked area surrounding northern Washington and British Columbia. Boat noise, coastal construction, and other man-made noises can interfere with the killer whales’ echolocation. An adult whale must eat at least 10 Chinook salmon a day, and even the loss of one fish can be detrimental to their health. Not only must whales compete with human-made noise, but Chinook salmon population numbers in the region have dwindled to the point where they are considered endangered. Southern Resident killer whale numbers have also declined, with only about 75 individuals remaining in the population. 

The amount of sound pollution is something humans can change, and already measures are being taken to help the whales of the Salish Sea. Fish finders and echo sounders emit sound in the same range as killer whales, and now recreational fishing boats will often turn off the sounders when not in use. In Washington State waters, boats must also slow to below 7 knots when Southern Resident killer whales are within a half mile, and boats must not approach within 300 yards to the side or 400 yd from the front or behind, and must disengage their engines if a whale appears within 300 yards. Perhaps, with a little awareness and a little effort, we can coexist and shape a future that sees the killer whale thriving.



July 2021
Tags: Whales