Nearly 60 years ago Jacques Cousteau introduced the public to the submerged visual splendor of the sea in his book and film The Silent World. Unfortunately, the title introduced the misconception that all the ocean’s stunning and majestic beauty occurs in a world of somber silence. As it turns out, the ocean is anything but silent, a fact that has been appreciated since the earliest days of seafaring. Sailors’ fears of sirens and sea dragons were rooted in the many strange and haunting sounds of marine fauna that could be heard through the hulls of their boats.
The Marine Symphony
The sea is the origin of all animal life on the planet. Because light does not penetrate below a few hundred feet, and at any given time half the sea is obscured under the veil of night, it is also a perfect environment for the evolution of acoustical adaptations. In these dark settings marine animals have evolved a complex array of adaptations for hearing as well as sounding — most of which continue to evade our understanding.
Many of us are familiar with the evocative songs of the humpback whale, but it remains a mystery why all males in the same breeding group sing the same song throughout the season. This doesn’t quite square with our common conception that male animals sing to advertise breeding fitness, a strategy that might be less ambiguously expressed if they all sang different songs. And why do they all change their songs every year?
We have known for some time that dolphins and porpoises use biosonar to sense their surroundings. Their high-frequency chirps and buzzes help them locate food, navigate during cooperative, high-speed hunting tactics and frolic in their surroundings. But we have only recently learned their biosonar can also let them “see” deeply into body tissues and identify extremely fine details in their environment.
There is informed speculation that baleen whales’ low-frequency vocalizations are also a form of biosonar. They may be using long-wavelength sounds to navigate across large expanses of water — projecting these sounds a thousand miles over the horizon to bounce back to them off seamounts, trenches and continental shelves as long-distance echolocation.
In chorusing, whales, fish and marine invertebrates all sing in temporally coordinated vocalizations like crickets or frogs. Croakers (sciaenids) synchronize in acoustical “stadium waves” across 45 miles of coastal habitat, and minke whales chatter and pulse together across large expanses of open ocean. Even the whistling of barnacles carries some critical biological message that we may never decipher.
We know larval forms of reef animals imprint on the sounds of their “mother reef.” When they are first conceived these tiny organisms are not served by remaining in their birth habitats, in which millions of mouths are seeking food. So in the early stages of life they disperse out into the pelagic zone to gain some heft. But once they are large enough (and often in a completely different shape than their larval form) they need to return to the reef to live. They find home by following the sounds they know from the first stages of their life. We know they do this, but we don’t know how they imprint. In many cases we don’t even know how they hear.
Sound Waves Underwater
Terrestrial vertebrates (mammals, amphibians, reptiles and birds) sense pressure gradients in air using diaphragms (ear drums) attached to the middle and inner ears. This is a very sensitive system, but as any diver knows it is poorly adapted to the extreme pressure gradients found underwater. Marine animals use different kinds of acoustical sensing systems including swim bladders, acoustical lipids, various hair-cell systems and bone accelerometers that sense pressure change as well as particle motion in the water.
Sound works differently in water than in air. Water is much denser and is not compressible. This allows for a more efficient transfer of acoustical energy through water than through air. It is fairly easy to make a sound in water that can be heard a few thousand miles away; baleen whales do this all the time. Humans also do this all the time in water, but unlike whales that use these sounds for biological purposes, we do it for commercial, industrial and military purposes — often to the detriment of the sea animals.
A Growing Cacophony
The ocean is 10 times louder today than it was just 50 years ago. Transoceanic commerce is a major cause; some 95,000 merchant ships ply the seas with large cavitating propellers and throbbing engines that can be heard throughout the ocean. As 7 billion humans become ever more dependent on fossil fuel, the noise of deepwater seismic-exploration ships towing arrays of air guns can increasingly be heard blasting away every 10 to 15 seconds, hour after hour, day in and day out for months at a time. As deepwater fossil fuel is discovered and exploited, the sounds of seafloor-mounted wellheads and processing equipment screech and hum across vast tracts of the ocean bottom. Navies have their own repertoire of noises including low- and mid-frequency sonars, the noise of their ships and occasional explosions from torpedoes, mines and projectiles.
Humans are a noisy species, and this is nowhere more evident than in the ocean. The impacts are becoming more apparent in the form of catastrophic marine-mammal strandings, compromised and depleted fisheries and high stress levels in animals that can lead to biological dysfunction. Because we know so little about how sea animals hear and what they do with sound, we have a very limited understanding of the impact of our noise.
In some cases the sounds we make might not be such a problem; the ocean with its crashing waves, grinding ice, pounding rains and billowing hydrothermal vents can be a noisy place, and animals have adapted and evolved to account for these sounds over the millennia. But the sounds we bring into the sea are quite different from these naturally occurring noises. Thus, we must learn how the din of our ocean enterprises affects the sea’s inhabitants, or we risk creating an environment in which their survival is uncertain.
Postscript
There was a time, not that long ago, when whales could sing to each other across oceans. Cetaceans evolved in a world of sound. They, like us, are highly adaptable creatures — able to revel in the bow waves of the very machines that are polluting their acoustic environment. Yet I wonder if their ability to find joy in the face of adversity will be enough to save them, and I wonder how well we humans would fare if somebody suddenly turned out the lights.
— Bob Talbot
© Alert Diver — Summer 201