ROB McCAULEY was puzzled. He had checked his instruments before he set out,
so why were they playing up? It was dark and McCauley was sitting in a boat on
the edge of the Great Barrier Reef off the Queensland coast. He had lowered an
underwater microphone over the side to pick up some of the squawks and squeaks
of the animals swimming about below. What he heard was an almighty roar. 鈥淚
pulled up the hydrophones and gave them a bang,鈥� says McCauley, 鈥淚 changed the
batteries and tried again鈥攆our times.鈥� At the fifth attempt, the marine
biologist from James Cook University in Townsville, north Queensland, realised
that the roar was real. It was the sound of an underwater choir, the massed
voices of a multitude of fish.
McCauley has made a career of eavesdropping on the animals in Australia鈥檚
tropical seas. Against a background of breaking waves and falling rain, he and
acoustics expert Doug Cato, from the Defence Science and Technology Organisation
in Sydney, can pick out the echolocating clicks of dolphins, the songs of whales
and the ubiquitous crackle of snapping shrimp. Occasionally there is a
tremendous crash as a 40-tonne humpback whale breaches and then slams its tail
back into the water. The rumble of shipping, underwater earth tremors and the
explosive bangs as the oil industry fires its seismic guns all add to the
racket. But as McCauley, now at Curtin University of Technology in Perth, and
Cato have discovered, fish can outdo them all. A fish chorus in full swing can
raise the noise level by as much as 35 decibels鈥攖he difference between a
quiet suburban street and the roar of rush-hour traffic.
Fishermen often claim to hear cries from the fish they catch. Some Indian
fishermen listen for shoals of fish through long bamboo poles dipped into the
sea. And biologists have spent years trying to interpret the fighting talk of
territorial fish or the serenades of those seeking a mate. But the racket going
on around Australia鈥檚 coasts is on a different scale. For more than a decade,
Cato and McCauley have lowered their hydrophones all around the continent and
discovered that fish choruses echo through the seas all year round鈥攖hough
mainly at night. 鈥淛ust about everywhere we record we hear an evening
chorus鈥攊n shallow waters and deep waters, temperate waters and tropical
waters, on reefs and off reefs,鈥� says Cato. Some nights, they believe, the
waters along the entire length of the Great Barrier Reef鈥攁ll 2000
kilometres of it鈥攔everberate with the noise of night choruses.
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Many marine animals rely on sound to communicate, especially at night and
where the water is murky and visibility low. Sound travels long distances
through water and requires little energy to generate. Fish often have anatomical
adaptations both for making their own distinctive sounds and for receiving them.
Their main instrument is usually the gas-filled swim bladder, the organ that
helps to control buoyancy as they move up and down. A few fish, such as catfish
and triggerfish, beat a spine-like appendage against their body wall, causing
the bladder to vibrate and boom like a drum. But most fish generate sound by
contracting muscles attached to the swim bladder. The design of the swim
bladder, the arrangement of muscles and the rate of contraction all determine
the length and pitch of a call.
Calls may be short pulses that last a few milliseconds, or long drawn-out
tones lasting several seconds. The pitch can range from a deep 50-hertz rumble
at the low end of the audio spectrum all the way to a higher-frequency squeak
around 5 kilohertz. There are almost as many different sounds as there are
calling fish. Some grunt and others give long, slow hoots, while at least one
makes a noise like a foghorn. Some fish seem to be knocking on wood鈥攆ast
like a woodpecker or more slowly like a fist pounding on the door. One
unidentified species makes a noise like a bottle filling with water while
another drones like a Second World War bomber. And a more musical species plays
what sounds like a short snatch of Beethoven on a bass guitar.
Most of this music is made by soloists, repeating the same calls over and
over. They are loud, but never as loud as the multitude of overlapping calls
that make up a chorus. To find out more about this synchronised singing,
McCauley and Cato set up a listening system just inside the central stretch of
the Great Barrier Reef. Hydrophones on the seafloor were linked by cable to a
small island two kilometres away, and from there the recordings were transmitted
by radio link to a 鈥渓istening hut鈥� on shore near Innisfail in north Queensland.
This allowed McCauley to record the local fish 24 hours a day, all year round.
His vast collection of recordings reveals complex patterns of calling, with
daily cycles and seasonal cycles鈥攊n some cases even lunar cycles.
The tapes record an assortment of pops, squeaks, chirps and grunts at all
times of day and night, but five sounds are regular and persistent. Two of them,
a repetitive knocking and a drumming sound, never quite coalesce into a full
chorus but are frequent enough and loud enough to raise the background noise by
30 decibels. Three others, a trumpet blast, a squawk and a short, sharp popping
noise, merge into choruses as countless fish call together. But this music is
not just a meaningless cacophony as fish compete to make themselves heard.
Instead, it is carefully orchestrated, with the night divided into 鈥渟ound
sectors鈥�, as different fish stagger the time they call.
From around 10 am to 1 pm, the most obvious sound is a knocking, rather like
a steady hammering on wood. The main suspect is a catfish. Occasionally, a
drumming interrupts. Here the chief suspect is a species of Protonibea,
a local member of the croaker family. By late afternoon, the knocking stops and
the drumming increases, reaching a peak around dusk鈥攚hen it stops
abruptly. 鈥淎t dusk everything changes dramatically,鈥� says McCauley.
Dusk is a critical time for fish around the reef. As the Sun sinks, the day
shift heads home to take shelter in the reef, and the night shift takes over.
Daytime fish are brightly coloured and highly patterned. Many of them call, but
sound is less important to them than vision. 鈥淭he daytime fish are pipsqueaks.
Their world is visually dominated,鈥� says McCauley. As they retreat for the
night, an assortment of nocturnal fish emerges from the nooks and crannies where
they have been holed up during the day. These fish are usually larger and duller
coloured: sound rather than vision dominates their world.
The first change at dusk is an increase in the noise from the snapping
shrimp. All day and night these shrimp click their claws, creating a constant
background crackle. But at dusk they turn up the volume, increasing their output
by 10 decibels. Soon they are joined by the popping voices. To begin with it鈥檚
possible to pick out the calls of individual fish but the calls quickly merge
into one loud song. 鈥淵ou suddenly have a massive chorus,鈥� says McCauley. These
calls belong to big plankton-eating fish, most probably soldierfish and bigeyes,
perhaps with a few sweepers and bullseyes calling alongside them. All these fish
have the right sort of anatomy to make the popping calls. They have specialised
muscles at the front end of the swim bladder that could pinch the sac quickly to
make a 鈥減op鈥�. They also have hearing systems that seem well adapted to pick out
short, high-pitched notes.
By 10 pm, the poppers have moved further out from the reef in search of dense
patches of plankton. If it鈥檚 summer, this is the time when the terapons tune up.
Terapons are responsible for two of the chorus sounds. They have a repertoire of
calls鈥攁 squawk, a trumpet and an alarm call that sounds something like a
muted trumpet. Alarm calls are sporadic, but both the squawks and trumpets can
merge into choruses. Terapons have a two-chambered swim bladder and the sound
comes from squeezing air through the tight constriction between these. They
change their tune by altering the speed at which they contract the muscles at
the front of the bladder and tightening or loosening the central sphincter. A
tight sphincter produces a squawk; an open one gives a blast of the trumpet. 鈥淲e
don鈥檛 know the significance of the two types of call,鈥� says McCauley. 鈥淏ut they
might simply broaden the frequency band of the chorus, which might be an
补诲惫补苍迟补驳别.鈥�
Unlike the plankton-eaters from the reef, terapons inhabit the shallow waters
closer to shore and eat almost anything that lives on the seabed. They call only
during the summer and the chorus reaches a peak around midnight. 鈥淪ometimes it鈥檚
so loud you can hear it above the surface. You can certainly hear it through the
hull of a boat,鈥� says McCauley. 鈥淵achtsmen complain about the fish keeping them
awake at night.鈥� Terapons continue chorusing until the small hours of the
morning when the knocking starts again, interrupted by occasional drumming
rhythms.
McCauley and Cato now have a detailed picture of the patterns of song around
Australia. But making sense of the songs is another matter. 鈥淚t鈥檚 a bit like
being in a rainforest. You can hear all these weird sounds but you can鈥檛 see
what鈥檚 making them. It makes you wonder what鈥檚 going on,鈥� says Cato. Almost
nothing is known about the behaviour of the night fish. Even fixing the identity
of the singers is fraught with difficulty. Divers have observed nocturnal fish
in their daytime lairs on the reef, but no one swims there at night. It鈥檚 too
dark to see what the fish are doing and sharks are all too active, particularly
around dusk. McCauley and Cato base their identifications on fish captured
during choruses, the nature of their sound-making apparatus and their presence
in the right place at the right time. In some cases, such as the terapons, they
are confident they have the right fish. In others, there are sometimes several
candidates.
Despite the uncertainties, one thing is clear. Fish songs are intended to
communicate something to other fish of the same species. 鈥淧eople think of fish
as slimy things. But they have complicated social systems and yell at each other
like mad,鈥� says McCauley. He and Cato are convinced that the plankton-eaters鈥�
pop music is associated with their feeding activities. They chorus all year,
although they sing loudest in summer and at the time of the new Moon鈥攚hen
it is darkest and their eyes are even less useful.
By tracking individual fish, the researchers have found that the choruses
come from depths where plankton form dense patches. And the singing moves
around, turning up at spots where aggregations of plankton collect鈥攚here
eddies trap larvae or fish eggs, for instance. McCauley and Cato believe
chorusing holds the loose schools of fish together and allows them to share
information on where the best patches of food are.
The terapons are clearly not singing about their supper: if they were they
would chorus all year round. And there wouldn鈥檛 be much point: their food is all
over the seabed rather than in moving patches. The terapons sing only in
summer鈥攄uring the breeding season. Fish caught during the choruses are
ripe and ready to spawn, so their choruses are probably linked to mating.
McCauley calculates that in ideal conditions individual calls could travel as
far as two kilometres. Singing together, he has detected them from as far away
as eight kilometres. This increases the catchment area for potential mates
16-fold.
Chorusing could have another function, perhaps triggering production of the
hormones that control spawning. Or it might prompt the fish to alter their
behaviour鈥攖o search for a mate rather than food, for instance. The loud
roar of the chorus could even sensitise the fish鈥檚 hearing systems so that they
can pick out individual calls from nearby fish more easily. This would help them
find a potential mate in the dark.
The knockers and drummers are also signalling for reproductive reasons, says
McCauley. They call all day and sometimes at night, but only during the summer.
The knocking calls peak around midday, suggesting that the fish spawn then. The
drumming reaches a peak at dusk, which coincides with the appearance of a mass
of croaker eggs in the water. 鈥淭he late night calling is maybe overoptimistic on
the fish鈥檚 part although some may spawn outside the main period,鈥� says
McCauley.
There are lots of advantages in using sound to communicate. But there is also
a serious drawback. It is not a very private way of sending messages. The
drumming calls of the croakers reach a peak as the fish release their eggs, then
stop abruptly. By then it鈥檚 too late: the plankton-eaters are out in search of
anything small enough to swallow. Fish eggs are ideal. 鈥淗alf an hour after the
drumming, you hear the schools of planktivores coming through,鈥� says McCauley.
鈥淭hey do this every night during the season. Whether they home in when they hear
the calls of spawning fish and say `food!鈥� we don鈥檛 know.鈥�
The terapons鈥� chorus also provides a clear signal to predators such as sharks
and dolphins. One of McCauley鈥檚 tapes begins innocently enough, with the
background crackle of snapping shrimp overlaid by the louder chorus of terapons.
Then there鈥檚 a new sound: the distinctive echolocating clicks of a dolphin. As
the dolphin app-roaches, the clicks slow down. 鈥淚t鈥檚 as if it鈥檚 trying to hide
its own sound so it doesn鈥檛 warn off the fish,鈥� says McCauley. At this point
imagination takes over from science. The fish chorus suddenly grows quieter,
stops, starts again and then remains very tentative for the rest of the tape.
Have the fish realised the danger and stopped calling? Or has the dolphin
swallowed one section of the choir? Until someone finds a way to watch the fish
as well as eavesdrop on them, it is impossible to know.
