Study: Killer Whales Tell One
Salmon From Another With sound
Southern Resident killer whales can tell a
lot about salmon using only sound.
This is especially interesting to Marla
Holt, who studies how integral sound
is to the lives of Southern Resident
killer whales.
Indeed, it’s critical to their hunting
abilities. It is so precise that the
whales can tell one species of fish
from another. Their hunting is also
easily interrupted by noise. Along
with disturbance from boat traffic,
noise is considered one of the key
threats to these endangered whales.
The Southern Residents are the
southernmost population of fish-eating
orca in the eastern Pacific. They
prefer chinook salmon to all other
prey. They seek out the largest, oldest
individuals. Recent studies even
suggest that killer whale predation of
larger chinook salmon is contributing
to a decrease in the average size
of chinook salmon throughout the
Pacific Northwest.
How does a Southern Resident
killer whale find and hunt down these
larger, older chinook salmon while
ranging across more than 15,000
square miles? This vast expanse is
occupied by hundreds of other fish
species, including five other species
of salmon whose populations are often
far more abundant than chinook
salmon. Southern Resident killer
whales may need to eat 10 or more
chinook salmon each day depending
on the size of the whale and the
salmon. They have been observed
eating mostly chinook salmon. The
whales hunt with echolocation,
which is the use of sound waves and
echoes to locate objects.
When hunting, a killer whale
sends out a series of clicks, called a
click train, that spread through the
water like a flashlight beam of sound.
If the sound waves hit an object,
echoes bounce back to the whale.
Echolocation allows killer whales to
detect fish at distances of up to 500
feet, much farther than they could see
in the dark water.
“The foraging behavior of the
Southern Residents indicates that the
whales are using click trains to detect
individual fish,” says Holt. “The vast
majority of the sound energy in a
click train is above what humans can
hear.”
How Does a Whale Tell a chinook
salmon from a coho?
Answer: the salmon’s swim bladder.
A swim bladder is an organ filled
with air that a fish uses to maintain
its depth in the water column. The
pocket of gas in the swim bladder
strongly reflects the whales’ sound
waves, making it the perfect target for
echolocation.
“The whales use echolocation
clicks to recognize the size and orientation
of a chinook’s swim bladder,”
says Holt. Researchers feel confident
that the whales can identify the
acoustic signature of the swim bladders
of different species of salmon.
One piece of evidence, Holt explains,
is a laboratory experiment that Whitlow
Au and peers at the University of
Washington conducted over a decade
ago. Holt studies how integral sound
is to the lives of Southern Resident
killer whales.
The researchers placed similarly-sized
chinook, coho, and sockeye
salmon in a tank, anesthetizing the
fish to keep them still. They then
simulated killer whale echolocation
clicks, aiming the sound at the fish,
and measured the returning echoes.
The echo structures of each species
were different and probably recognizable
by foraging killer whales.
Echolocation helps the whales
hunt in other ways.
“Are the whales looking for fish
or for good fish habitat when they
hunt?” asks Holt. “Experienced
fishermen will tell you that chinook
salmon like to hide in rocky crevices.”
So the whales may use echolocation
to search out those habitats,
bouncing their sound waves off the
underwater topography.
Southern Residents eat other species
of fish besides their preferred chinook
salmon. The whales’ winter diet
can include steelhead, lingcod, and
halibut. Halibut is a bottom-dweller
with no swim bladder. So we know
that the whales can detect a fish without
needing to detect a swim bladder,
explains Holt. NOAA
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