Listening to a Kelp Forest: What Reef Soundscapes Tell Us About Health

If you snorkel into a kelp forest and hold still — really still, drifting with your fins tucked — you will begin to notice that the forest is not quiet. There is a faint, continuous hiss, like distance-worn rain. Closer to the bottom, rhythmic scraping rises and falls. If you stay past dusk near the right coast, a low hum gathers from everywhere at once, produced by fish you cannot see. The kelp forest is a talking place, and its conversation is a record of who lives there, who is eating, and who is missing.

This is a field guide to that conversation: the voices of a temperate reef, how the soundtrack changes across a single day and a single year, how scientists translate it into pictures of reef health, and what the silences mean. It is a companion to our piece on how sound travels underwater — this time, we listen to the forest itself.

A word about what makes the sound

Kelp itself is nearly mute. Blades rustle softly against one another in the surge and gas vesicles occasionally creak, but the kelp is architecture, not orchestra. The sound of a kelp forest is made by its inhabitants — animals advertising, feeding, fighting, courting — and by the physics around them: surf breaking on the point, rain dimpling the surface, swell passing through thick enough to sift out into a ground tone. Biologists divide any underwater soundscape into three contributions:

• Biophony — sounds made by living things: snaps, rasps, grunts, whistles, clicks.
• Geophony — the non-biological natural backdrop: waves, wind, rain, distant earthquakes.
• Anthropophony — human noise, mostly vessel engines, which in loud shipping lanes can blanket everything under a few hundred hertz.

A healthy kelp forest scores high on biophony and geophony and low on anthropophony. A degraded one inverts that ratio.

Twenty-four hours of sound

The kelp forest keeps a schedule, and you can read a day's recording like a face. Hour by hour, the soundscape foregrounds different voices.

The night watch

After dark, grazing invertebrates come out of their shelters. Sea urchins — especially dense aggregations of species like the purple sea urchin — scrape their five-toothed lanterns across encrusted rock in unison, and the summed rasp of tens of thousands of animals becomes one of the loudest biological sounds in temperate seas. New Zealand research has shown this night-time rasping can set the volume floor of coastal sound. Lobsters stridulate against predators. In shallows and estuarine pockets near kelp coasts, fish like the plainfin midshipman drone for hours through summer nights — a sustained hum that underwater mystified houseboat owners in California for years before its fishy source was identified.

The dawn and dusk choruses

Twice a day the forest tends to swell in volume. Many reef fish — drums, croakers, damselfish, ictalids — call most intensely around dawn and dusk, and when dozens call together the individual voices blur into a chorus, a raised band of sound at characteristic frequencies. These choruses are among the most reliable signatures in temperate recordings: they mark spawning seasons, map spawning aggregations, and — because they swell with the number of callers — quietly report fish abundance. Divers who know benthic kelp communities sometimes remark that you can hear a place light up at sunset, and the spectrograms agree.

The midday hiss

Through bright hours, the broadband bed of many shallow reefs is held up by snapping shrimp — all a few centimetres long, each armed with a single oversized claw whose closure collapses a cavitation bubble with a crack sharp enough to stun prey. Their combined output is a ceaseless, staticky fizz, energetic enough in some areas that navies and ocean engineers must account for it acoustically. Shrimp activity rises with water temperature, so this layer has its own seasonal and daily arc.

The passing traffic

Layered across all of it: sea lion barks, dolphin click trains sweeping through like a creaking door harried down a corridor, the minutes-long passes of small vessels whose propeller noise peaks and recedes. On recorded kelp forests near working harbours, the anthropophony may claim most of the low-frequency daylight hours. The soundscape, in other words, is also a census of what we are doing.

Reading a spectrogram

The standard tool for making sense of all this is the spectrogram — a plot of frequency against time, colour-coded by loudness. Reading one for a kelp forest is a learnable skill:

• The snapping-shrimp band appears as a bright horizontal wash in the mid-to-high kilohertz range, brightest at night and in summer.
• Urchin rasping shows as crackles of broadband clicks with rhythmic clumps, pulsing stronger after dark.
• Fish choruses appear as dense vertical striations near 100–1,000 hertz, concentrated at dawn and dusk and often far more intense in particular months.
• Boats are unmistakable: sharp diagonal sweeps of harmonic lines that rise and fall as the vessel approaches and departs.
• Wind and rain inflate energy across broad bands evenly — weather is the white-noise background that ecologists usually try to subtract.

Analysts compress spectrograms into acoustic indices — single numbers summarising complexity, diversity and biological share of sound — that can be compared across sites, seasons and years. The indices are coarse, but run over hundreds of stations and seasons they map the choruses of a coastline.

What the soundscape says about health

Two generalisations have held up as temperate-reef acoustic studies accumulated over the last decade:

1. Structure-rich habitats are louder. Complex, kelp-covered, fish-dense reefs produce more biologically generated sound than flattened or denuded seafloor — a pattern that mirrors the uptick of complexity seen when sea otters reopen the grazer cascade. The connection is mechanical: more animals calling, grazing, jamming into the acoustic band.
2. The daytime and night-time balances invert under stress. An urchin barren (see How Urchin Barrens Form) is often dominated by grazing rasp at night and thin elsewise, while the standing crop of fish choruses at dawn shrinks away. The daytime-nighttime character of the soundscape shifts before the reef re-arranges itself visually.

Neither claim is a law. Healthy soundscapes vary enormously by species pool, salinity, substrate and basin. The way forward is relative comparison: the same recorder, the same reef, year on year, treated as a long trend rather than a one-off reading. In that framing, acoustics becomes one of the cheapest time-series instruments in marine monitoring — recorders run for weeks untended, and archives accumulate.

The restoration question: can sound call life back?

One of the field's most striking experimental results came from tropical work: broadcasting recordings of healthy-reef sound over degraded coral rubble attracted more juvenile fish, in more species, than silent controls. The principle is old — larval fish and crustaceans use sound as a settlement beacon — but the application is young, and evidence in kelp systems is still thin. Tidewright's field teams have logged arrival rates of settlement-stage fish onto Holdfast-seeded reefs while testing whether broadcast healthy-forest sound speeds the early weeks of community assembly; those trials are ongoing and, like all our numbers, illustrative. The cautious reading: sound may help call the neighbourhood's children home, but only if the food and shelter are real when they arrive.

Why noise is a conservation issue

Because the soundscape is information, it is also a resource that can be destroyed. Chronic engine noise masks fish choruses, interferes with the settlement cues larvae use, and raises stress indicators in several tested species — compounding the direct, kelp-specific pressures our heatwave explainers describe. The pushback in favour of quiet — slower vessel speeds, rerouted shipping, protected quiet hours — is increasingly written into marine spatial planning, and kelp forests, with their concentration of sound-sensitive residents, have a direct stake.

Key takeaways

• A kelp forest's sound is made by its inhabitants and environment, not by the kelp; biologists split any underwater recording into biophony, geophony and anthropophony.
• The forest runs on a timetable: rasping grazers own the night, fish choruses peak at dawn and dusk, and snapping shrimp hold the broadband bed through the day.
• Spectrograms let analysts assign sounds to sources and compress them into indices that track reef health over years.
• Structurally complex, fish-rich reefs are generally louder than degraded ones; tracking the same site over time is more reliable than one-off readings.
• Reef noise has real ecological functions — settlement cues for larvae, for instance — which is why chronic vessel noise is now treated as a conservation pressure in its own right.

FAQ

Can a diver actually hear these sounds in person?

Often, yes. Snapping shrimp are audible through a thin wetsuit hood as a constant faint static; boat hum and dolphin clicks are obvious. Fish choruses take patience — they are loudest at dusk in season and at depths divers can only visit briefly. A sealed-off latitude of hearing, however, shifts underwater: human directional hearing largely fails, and many people initially interpret underwater sound as coming from directly overhead.

Do kelp forests sound different around the world?

Very. The same instruments off Norway, California, South Africa and Tasmania return distinct voices: different snapping shrimp faunas, different chorusing fishes, different pinnipeds. This is why site-specific baselining matters — a "quiet" Antarctic Peninsula kelp edge is not degraded, it is simply a colder, different ensemble.

Does recording sound disturb the animals?

Not appreciably — passive hydrophones emit nothing, which is a principal attraction of the method compared with active sonar (see Counting Fish With Sound for the active alternative). The main disturbance risk is installing and retrieving moorings, and even that is modest compared to the value of multi-month continuous data.

How is Tidewright using soundscape data?

Within Canopy Ledger, hydrophones work alongside satellite and drone canopy mapping and eDNA sampling as a continuous layer — flagging dawn-chorus energy, grazer rasping balance and vessel-noise burden at our project reefs. All such figures in our dashboard are clearly marked illustrative; the value is demonstrating what integrated, multi-sensor reef monitoring looks like.

Is ocean noise pollution getting worse?

Probably, in aggregate: global commercial vessel numbers and tonnage have grown for decades, adding low-frequency energy to most coastal basins. Local trends sometimes run the other way — rerouted lanes, quiet-ship technologies and seasonal restrictions demonstrably lower noise floors. As with coral-vs-kelp geography, the honest statement is spatially uneven, and your best guide is the trend of your own coastline's monitoring network.