Kelp forests hiss, rasp and hum. Meet the shrimp, urchins and fish behind the noise — and the hydrophones turning the din into reef science.
Slip below the surface of a kelp forest and your ears get there before your eyes do. Long before a fish crosses your mask, you hear the habitat: a sizzling backdrop like fat in a pan, a slow scraping from the rocks below, and — at the right hour, on the right coast — a low, gathered humming with no visible source. None of it is the kelp. All of it is the kelp forest. This guide introduces the animals and physics that build that sound, mechanism by mechanism, and then walks through the microphones and machine listening that scientists use to eavesdrop at scale. For the bigger picture of what these sounds reveal about reef health, our companion piece on the kelp forest soundscape takes the health-monitoring view; here, we meet the performers.
The staticky crackle that underlies so many shallow reefs comes from an animal you could sit on your thumbnail. Snapping shrimp (family Alpheidae) carry one grotesquely oversized claw that works like a living water hammer: a plunger fires into a socket so fast that it ejects a jet of water, and the jet's speed drops the local pressure below vapour pressure, tearing a cavitation bubble into existence. The bang you hear is not the claw parts hitting each other — it is the bubble collapsing a split-second later.
That collapse is violent out of all proportion to the animal. The pressure spike stuns the small prey the shrimp hunts, makes one of the loudest biological noises in the sea, and even produces a faint flash of light — a sonoluminescence quirk that physicists have measured and romantics enjoy mentioning at parties. A reef holding thousands of snapping shrimp fuses all those pops into a continuous, rain-on-a-tin-roof hiss.
Two practical details matter. First, snapping shrimp are warmth-lovers: their choruses dominate tropical and warm-temperate reefs, and thin out toward the cold ends of kelp country — a kelp forest in southern Australia crackles far harder than one in the subarctic. Second, their firing rate climbs with water temperature, so the crackle swells on warm afternoons and in warm years. The background sizzle of a reef is, almost literally, a thermometer you can hear.
After sunset, another band takes over. Sea urchins feed by dragging Aristotle's lantern — the five-toothed jaw on their underside — across encrusted rock, and each scrape is a tiny broadband click. One urchin is nothing. Tens of thousands of urchins, grazing in loose synchrony as they leave their daytime shelters, add up to a measurable rise in the noise floor. New Zealand experiments with the local sea urchin showed the effect clearly: evening feeding pushed ambient sound levels up within a characteristic band, with the animal's hard test acting like a resonating chamber that amplifies the scraping.
This gives the acoustician a strange gift: you can hear grazing pressure. Quiet, kelp-covered reefs and loud, urchin-worked rocks sound different, and the nightly rasp is part of why — a sonic shadow of the grazer balance that decides whether a reef carries kelp or has flipped into an urchin barren. Limpets and abalone contribute their own fainter radula rasps to the same frequency neighbourhood, a brush section behind the urchins' drum kit.
Fish make sound with several different instruments, and kelp forests host players of each:
Because fish calls carry courtship, spawning and territorial messages, they cluster at dawn and dusk and in particular months. When many callers overlap, the voices blur into a chorus — the raised hum on the recording that tells an analyst: it is that season, and there are this many of them. Sound, here, is abundance translated into air pressure.
Mammals write in a bolder hand. Sea lions bark underwater in blunt, drum-like reports that carry across a forest. Dolphins sweep through in trains of echolocation clicks — to a hydrophone, a creaking hinge accelerating into a buzz as the animal locks onto prey — with whistles threading underneath. In otter country, sea otters that open shellfish against a chest-stone anvil add an irregular, workmanlike tapping. And in deep channels beside kelp coasts, the low notes of passing whales fold into the bottom of the mix, crossing whole basins in the way described in our primer on how sound travels underwater.
Each of these is identifiable by ear or algorithm, which means a single anchored microphone keeps a long-running attendance sheet for species that are otherwise expensive to count.
Kelp blades are nearly silent — a soft leather rustle in the surge, a creak from a strained stipe. But the forest shapes sound even without making it. The gas-filled floats (pneumatocysts) of species like giant kelp and bull kelp scatter and absorb acoustic energy at frequencies tied to the bubble sizes, rather the way a foam panel deadens a room. Dense canopy can measurably filter the sound that passes through it. Layered over the biological noise are the geophonies — surf thundering on the point, rain prickling the surface, gravel rolling in the shorebreak, wind hissing at the sea's skin — and, on working coasts, the anthropophony of engines, which swamps the low frequencies and is increasingly managed as a pollutant in its own right.
All of these voices become usable science through one deceptively simple instrument: the moored hydrophone recorder. The modern craft of passive acoustic monitoring (PAM) runs on a few engineering decisions:
The payoff is uniquely long memory. Hydrophones are cheap enough to deploy as networks, patient enough to archive a decade of dawns, and they work in the dark, in storms and in water too murky for any camera. As a restoration instrument, acoustics asks the forest not what it looks like but who is showing up — and it keeps asking all night, every night, for years.
At Tidewright — a fictional company created as a design and content demonstration — our illustrative monitoring stack treats hydrophones as the night shift of Canopy Ledger: moored recorders at project reefs log chorus seasons, grazer-rasp balance and vessel-noise burden alongside the satellite and diver data, because a forest that is filling back up with life should, quite literally, get noisier in the right ways. Whether our fictional fish choruses swell on cue is a story our field notes will keep telling.
Yes. The shrimp crackle is audible as a faint static through a wetsuit hood on many coasts, and dusk fish choruses in season can be heard by a patient, motionless diver. Directional hearing, however, largely fails underwater — humans locate sound by the timing difference between our two ears, which collapses in water — so everything seems to arrive from everywhere at once.
Not in any meaningful way. The cavitation blast is microscopic and spent within millimetres — enough to stun a copepod, not a swimmer. Working aquaculture divers and boat hulls are untroubled by them; it is sonar engineers, not people, who must plan around shrimp beds.
Where deep water brushes kelp coasts — certain headlands, fjords and island channels — whales pass regularly and their calls sit inside kelp-forest recordings. Baleen-whale notes occupy the lowest frequencies; toothed whales contribute clicks and whistles much higher up. They are visitors to the forest's acoustic map even when they never enter the canopy.
Duty-cycled autonomous recorders commonly run for months between servicing, limited by battery and memory rather than the sensor. Long programs rotate units, keep calibration references, and treat the accumulating archive as an asset — because the real signal of change lives in year-on-year comparison, not in any single season.
Usually, on warm-temperate coasts: shrimp fire faster in warm water, many fishes chorus in spawning season, and calm weather lowers the wave-noise floor so biological sounds stand out. High-latitude forests run quieter year-round simply because their cast is colder-adapted and smaller — which is why acoustic health is always judged against a local baseline, never a global one.
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