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Biological noise can reduce sonar performance in parts of the Indian Ocean Region (IOR), but its effects are local, frequency-dependent and variable—not a single penalty that applies across the basin. Snapping-shrimp crackle is a concern in warm, shallow reef and hard-bottom waters, particularly for higher-frequency systems. Fish choruses can add noise at lower frequencies. Whether either source matters to a particular sonar depends on the received noise in the system’s band, propagation conditions, array and detector design, and the target signal.
What “sonar performance” means
Sonar performance is more than whether a contact appears on a display. It includes detection (whether a signal is distinguished from the background), classification (whether there is enough information to identify it), localization (estimating its direction or position), and tracking (maintaining a reliable contact over time). Biological sound can affect these stages differently: a target might remain detectable but be harder to classify, or produce an unstable bearing.
Underwater communications face a related problem. Noise overlapping a modem’s signal band can increase bit errors, reduce range or data rate, and prompt more robust coding or retransmission. These effects depend on the particular link and environment.
Biological sound in a varied ocean
Biological noise is sound generated by living organisms: snapping shrimp and other crustaceans, calling or chorusing fish, marine mammals, and reef-associated invertebrates. Feeding, spawning, territorial activity and daily movement can all contribute. It is one part of a soundscape that also includes physical sources such as wind and waves, and human sources such as ships, fishing boats, construction and sonar. NOAA’s overview of ocean noise uses this biological, environmental and human-generated framing.
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The IOR is not one acoustic environment. Deep-ocean basins differ from continental shelves, ports, estuaries, reefs and atolls. In shallow water, sound can repeatedly interact with the surface and seabed. Bathymetry, sediment, temperature, salinity and sound-speed structure affect both the target signal and the sound reaching a receiver. Monsoon conditions and busy shipping corridors add further variation. Results from deep water cannot simply be transferred to a tropical reef, or from one reef to the whole region.
Which organisms contribute, and at what frequencies?
Snapping shrimp produce brief, broadband impulses through rapid claw closure and associated cavitation. In dense colonies, many snaps combine into a nearly continuous crackling or sizzling sound. Historical reviews identify shrimp as an important source in warm, shallow water and commonly describe broad spectral energy around 2–15 kHz (National Academies review). Indian Ocean measurements near Lakshadweep identified a shrimp-dominated band extending approximately from 2 to 30 kHz.
Fish choruses can be important lower down the spectrum. The Lakshadweep study identified a dusk chorus around 200–600 Hz and another biological chorus around 1,000–1,200 Hz. Marine-mammal calls and other reef-associated sounds are also species- and site-dependent, and may overlap different receiver bands.
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| Source | Indicative frequencies in cited observations | Potential relevance |
|---|---|---|
| Fish choruses | Hundreds of hertz to around 1–2 kHz | May mask signals received by low-frequency passive systems. |
| Snapping shrimp | About 2–30 kHz in Lakshadweep observations; historical summaries often cite 2–15 kHz | May affect higher-frequency sonar, broadband receivers and acoustic telemetry. |
| Marine mammals and other reef organisms | Species-, call- and site-dependent | May overlap particular monitoring or sonar bands; requires local measurement. |
These are indicative bands, not universal boundaries. Species, habitat, depth, distance, hydrophone response, season and analysis method all influence what is recorded. A sound’s measured peak, average level and spectrum also answer different questions. A peak measured at a hydrophone is not automatically the source level of an individual animal or an entire colony.
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What Indian Ocean measurements show
A soundscape study around Lakshadweep provides a geographically relevant example, not a basin-wide survey. Recordings were collected from January through October 2019 at two hydrophone sites, at approximately 11 m and 18 m depth, with recorded bandwidth from about 20 Hz to 48 kHz. The study reported biological bands around 200–600 Hz, 1,000–1,200 Hz and 2,000–30,000 Hz, and found variation associated with time of day, season, moon phase and environmental conditions including salinity, chlorophyll and wind (study; deployment context).
At Lakshadweep, low-frequency biological choruses peaked in inter-monsoon months, while low-frequency geophysical noise increased during the southwest monsoon. Shrimp-chorus levels were associated with lower wind speeds and, at one site, higher sea-surface salinity. Two sites cannot characterize every reef, shelf, port or open-ocean route in the IOR. Indian port recordings have also reported fish and shrimp signals alongside other sources, underscoring that a high noise level should not automatically be attributed to biology (port study).
How biological sound affects sonar
Passive sonar: masking and signal-to-noise ratio
For passive sonar, a useful conceptual measure is detection margin: target signal received at the sensor, after propagation loss, relative to noise, with array and processing gains taken into account. Biological noise matters when it raises noise in the receiver’s relevant band and at the array. A lower signal-to-noise ratio can reduce detection range or make a contact intermittent, but the relationship is not a simple conversion from a certain number of decibels to a fixed range loss. Propagation, bandwidth, array geometry, detector type and the target spectrum all matter.
Active sonar: echoes, reverberation and false alarms
With active sonar, biological noise can mask weak echoes, complicate noise-floor and detection-threshold estimates, or contribute to false alarms. It may also interfere with broadband transmissions. But active-sonar performance depends on more than ambient biology: reverberation, surface and bottom scattering, multipath, platform self-noise, shipping and errors in the assumed sound-speed profile may be more important in a particular case. Biological sound is one part of that problem, not a universal explanation for poor results.
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Impulsive noise is not ordinary steady hiss
Shrimp colonies produce many short transients. A noise field with strong impulses can depart from the stationary, Gaussian assumptions used by some conventional detection methods. The result may be unstable thresholds, biased noise-floor estimates, excess false alarms or dynamic-range stress. Strong transients can also be mistaken for contacts. A Cochin University thesis reviews earlier work on detector performance in impulsive shrimp noise and non-Gaussian approaches; it is useful technical context, not evidence of an operational IOR sonar trial (thesis).
Effects depend on the receiver. A high-frequency telemetry link may be affected by shrimp crackle while a low-frequency passive array is not; a fish chorus may pose the reverse problem. A signal’s overlap with the receiver band, received level, persistence and spatial character matter more than the broad label “biological noise.”
Why effects vary in space and time
Noise can change over short distances with reef proximity, colony density, bottom type, depth, island geometry, local circulation, shipping and hydrophone position. Even the two Lakshadweep sites showed site-specific biological patterns. At another time, dawn or dusk choruses, seasonal activity, lunar phase, tides, monsoon weather, salinity or temperature may change the local soundscape. A short daytime recording is therefore not a dependable proxy for another watch period or season.
Nor does a source travel unchanged from its origin to a sonar. Spreading loss, refraction, absorption, surface and seabed interaction, multipath, scattering and bathymetry shape the received field. A shrimp bed may be prominent close to a receiver but much less important at another depth or range. Source intensity alone cannot establish operational impact.
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How to measure the problem responsibly
A useful assessment needs calibrated, broadband recordings over enough time to capture relevant biological and environmental cycles. Researchers should document sensor response and deployment details—including hydrophone depth, bandwidth, site, timing and measurement distance—and report the analysis method. Distinguish peak levels from RMS levels, sound exposure levels and power spectral density; do not present one as another.
Long-term spectral averages are helpful but should be paired with time-resolved and percentile statistics that preserve information about impulsive events and changing noise floors. Snap-rate or transient analyses can help characterize shrimp activity, while spectrogram review and event analysis can identify choruses. Synchronize recordings with environmental observations such as wind, tide, temperature, salinity, chlorophyll, bathymetry, seabed type and vessel activity. ISO 7605:2025 addresses measurement of underwater ambient sound; its methodology is relevant to making surveys more comparable (ISO standard).
To estimate sonar impact rather than simply describe a soundscape, combine source characterization with local propagation modelling and the relevant receiver and signal characteristics. Where feasible, controlled or injected test signals can help evaluate detection and classification under measured noise conditions. Report uncertainty and compare like with like across sites. A biological-noise map without propagation and receiver context is incomplete.
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- Map before deployment. Build site- and season-specific baselines rather than relying on a regional average. Include biological, shipping and physical sound sources.
- Use processing suited to the noise. Robust noise-floor estimation, percentile or median spectral methods, transient-aware detection, time-frequency analysis, beamforming and non-Gaussian models may help. Which works best depends on the array and mission.
- Test for target damage. Impulse blanking or denoising can make a spectrogram look cleaner while erasing weak target transients. Validate with controlled signals and measure detection, classification and localization—not visual neatness alone.
- Consider frequency and waveform choices. If requirements permit, select bands with less measured biological overlap or use waveform diversity. A frequency shift trades against absorption, propagation, resolution and target scattering; it is not a universal fix.
- Plan around known cycles where practical. Long-term data may reveal quieter periods or biological hotspots. Operational timing may not be flexible, and weather, shipping or target behaviour can matter more.
- Integrate the environment. Combine soundscape measurements with sound-speed profiles, bathymetry, seabed classification, habitat, shipping, wind and seasonal data.
Machine-learning approaches have been tested for extracting marine-mammal vocalizations from shrimp-dominated noise, showing potential for specialized signal processing (2024 study). That is not proof that a general-purpose denoiser will improve naval sonar. Processing cannot recover information already lost to propagation or receiver saturation, and it can distort the target signal.
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Biological sound is also environmental information
A reef soundscape is not only interference to be suppressed. Biological activity can help researchers characterize habitat and seasonal ecological patterns. The Lakshadweep study treats choruses as useful evidence about soundscape and environmental variation. Passive acoustic monitoring can therefore serve both operational assessment and environmental research—provided investigators distinguish biological signals from ships, fishing activity, construction and other sources.
What the evidence does—and does not—establish
The public evidence supports a real, conditional problem: biological sound can degrade signal detection and communication in shallow-water settings, and Indian Ocean observations confirm multiple biological bands and substantial temporal and spatial variation. It does not establish a reproducible, universal percentage loss in sonar range across the IOR. Public operational trial data are limited; geographic sampling is uneven; biological and anthropogenic sources can overlap; and soundscape intensity alone is not the same as measured target-detection loss.
A 2015 article on the subject reports shrimp pulses of roughly 3–8 ms and a peak amplitude around 150 dB re 1 μPa at 1 m, and recounts an operational difficulty involving INS Chakra (article). Those figures and the anecdote should be treated as claims reported in that historical account, not universal IOR measurements or independently verified public operational evidence. They do not justify saying shrimp noise dominates every sonar or overwhelms the entire region.
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