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The available evidence supports careful, species-aware risk assessment rather than a single “safe” or “harmful” cutoff. Sea lions and fur seals are grouped by NOAA as otariid pinnipeds, but that category does not mean every species hears the same frequencies or responds identically.
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What underwater noise does to pinnipeds
Noise is unwanted sound from the animal’s perspective. A vessel, construction project, seismic survey, sonar system, or other activity can add energy to the underwater soundscape. The biological effect depends on how that sound overlaps the animal’s hearing and communication, how strong it is when it reaches the animal, and how long exposure lasts.
Four endpoints should be kept separate because they describe different kinds of harm or disturbance:
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| Endpoint | What it means | What it does not establish by itself |
|---|---|---|
| Masking | Noise makes another sound harder to detect, such as a call, approaching predator, prey signal, or environmental cue. | It does not prove permanent hearing damage. |
| Temporary threshold shift | After exposure, an animal may need a louder sound to detect the same signal; sensitivity can recover. | It is not automatically permanent injury or a population-level effect. |
| Auditory injury | Exposure is intense enough to damage hearing structures or produce a lasting impairment. | A hearing-injury criterion does not predict every behavioral response. |
| Behavioral disturbance | The animal changes activity, movement, spacing, or time allocation—for example, avoiding an area or interrupting feeding. | Behavioral change does not necessarily mean the ear was injured. |
NOAA’s behavioral-disturbance descriptions include increased energy expenditure, avoidance of an action area, and disruption of feeding, resting, or nursing. These are possible responses, not guaranteed outcomes for every animal or project.
What sea lions and fur seals can hear
NOAA Fisheries’ 2023 summary gives otariid pinnipeds—a group that includes sea lions and fur seals—a generalized underwater hearing range of 60 Hz to 39 kHz. This is a composite group range, not a measurement showing that every species hears across the entire span. Individual species generally have narrower ranges, and sensitivity varies within a range rather than switching on at one exact lower or upper boundary.
A 2025 controlled study, “Low-frequency hearing and masking parameters in representative seals and sea lions,” adds evidence at the low-frequency end. Researchers used behavioral tests with two trained bearded seals and one California sea lion, measuring detection in quiet and in controlled background noise. All three subjects detected sounds down to 40 Hz. Between 40 and 100 Hz, the bearded seals had thresholds up to 24 dB lower than the tested California sea lion, meaning the seals were more sensitive in that interval under the study conditions.
That experiment is informative but narrow. It included one sea lion and no fur seal; bearded seals are not fur seals. Its measurements therefore show why low-frequency masking deserves attention, not that every fur seal or sea lion shares those exact thresholds.
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| Evidence | Finding | How to use it |
|---|---|---|
| NOAA/NMFS summary, 2023 | Generalized otariid underwater hearing range: 60 Hz–39 kHz | Use as a group-level screening range, not a species-specific audiogram. |
| Low-frequency hearing study, 2025 | Two bearded seals and one California sea lion detected 40 Hz | Shows that frequencies below the generalized 60 Hz lower bound can be detectable in some tested pinnipeds. |
| Same 2025 study | Bearded-seal thresholds were up to 24 dB lower than the tested California sea lion from 40–100 Hz | Demonstrates interspecific sensitivity differences; it cannot be extrapolated directly to fur seals. |
How noise can interfere with hearing
Masking of calls and environmental sounds
Masking occurs when noise overlaps a signal the animal would otherwise detect. A continuous low-frequency rumble can make a call or other cue less conspicuous even if the sound is not strong enough to injure the ear. The amount of masking depends on the relative frequency content, received level, duration, and the listener’s hearing sensitivity. Background noise matters too: an added source may have a larger effect in an otherwise quiet habitat than in an already noisy one, while a signal that overlaps a particularly sensitive part of hearing can remain difficult to detect.
Masking can change the effective range over which animals detect one another or perceive their surroundings. The available evidence does not establish a universal distance, percentage of calls lost, or number of animals affected for sea lions or fur seals.
Temporary changes in sensitivity
Acute exposure to sufficiently strong sound can produce a temporary threshold shift. The animal’s hearing threshold rises, so a sound must be louder to be detected immediately after exposure. NOAA’s current guidance treats temporary threshold shift as an auditory endpoint to be assessed from received exposure and hearing sensitivity. Recovery, if it occurs, depends on the exposure and the animal; a temporary shift should not be casually treated as harmless.
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Very intense exposure can injure hearing structures. NOAA’s 2024 technical guidance provides updated criteria for predicting the onset of auditory injury and temporary threshold shifts from acute anthropogenic sound underwater and in air. The criteria are assessment tools: they help evaluate an exposure scenario but do not describe every possible behavioral effect or prove a population-wide consequence.
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The 2024 guidance replaces the 2018 NMFS technical guidance. Anyone evaluating a current project should use the 2024 version rather than quoting the superseded 2018 criteria as current.
Why the same sound can produce different responses
Risk is determined by the received exposure and the animal’s circumstances. A useful assessment considers these variables together:
| Factor | Why it matters |
|---|---|
| Frequency content | Effects are more plausible when energy overlaps hearing sensitivity or biologically important signals. Low-frequency energy deserves particular attention because some tested pinnipeds detected frequencies near 40 Hz. |
| Received level | The level at the animal, rather than the source level alone, determines exposure. Distance, depth, directionality, and propagation can change the received level substantially. |
| Duration and repetition | A brief impulse and a prolonged or repeatedly occurring sound can produce different exposure profiles. Cumulative exposure may matter even when individual events are separated. |
| Impulsive versus continuous character | Sharp transient sounds have a different time pattern from steady machinery or vessel noise. The distinction affects how exposure is calculated and how animals may respond. |
| Ambient noise and propagation | Wind, sea state, seabed conditions, water depth, and other sources influence how far sound travels and how easily it is detected. |
| Animal context | A feeding animal, a resting haul-out group, a nursing mother, and a traveling animal may have different costs when they interrupt an activity or leave an area. |
| Endpoint being measured | Masking, temporary threshold shift, auditory injury, and behavioral disturbance require different evidence and should not be substituted for one another. |
How common noise sources differ—and what cannot be ranked
Human activities produce different acoustic signatures, but the available evidence does not support declaring one source universally most harmful to all sea lions and fur seals. A 2019 review of Antarctic conditions discusses ship traffic, seismic surveying, and construction noise without establishing a universal ranking among them.
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|---|---|---|
| Vessel traffic | Often adds recurring, broadband or low-frequency engine and propeller noise; exposure can continue as a vessel passes or remains nearby. | What level and frequency content reach animals along the route, and how often does traffic recur? |
| Seismic surveying | Uses repeated, high-energy impulses, creating a distinctive time pattern and potentially repeated exposure. | How strong are individual pulses at the animal, and what is the duty cycle over the survey period? |
| Construction | Activity can include impulsive events, such as piling, or continuous machinery, depending on the method. | Which equipment operates, for how long, and what propagation conditions apply at the site? |
| Sonar and other systems | Signals vary widely in frequency, level, pulse shape, and operating schedule. | Does the signal overlap the relevant species’ hearing and communication, and what is the received exposure? |
Calling a source “loud” is not enough to predict its effect. Two projects with different equipment can produce similar exposure at an animal, while the same source can produce different exposure in shallow and deep water or under different sea states.
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How NOAA’s 2024 guidance is used
NOAA’s 2024 technical guidance is designed for acute exposure assessments. It predicts the onset of temporary threshold shifts and auditory injury using received sound exposure and hearing-related criteria. It is not a behavioral-disturbance rule and is not a universal line separating harmless from harmful sound.
- Define the activity. Identify every sound-producing operation, its schedule, whether it is impulsive or continuous, and whether multiple sources operate at once.
- Characterize the source. Document frequency content, source level, pulse duration or duty cycle, and directionality using measurements or a defensible acoustic model.
- Model propagation. Account for distance, depth, bathymetry, seabed, water conditions, and other variables that determine the level reaching an animal.
- Estimate received exposure. Calculate the level and duration at the animal, including repeated events and cumulative exposure where appropriate. Source level alone cannot answer this step.
- Select the appropriate hearing group. Use the relevant NOAA pinniped category while recognizing that a group composite is not a complete species audiogram. Where species-specific data are available, state their limits.
- Evaluate auditory endpoints separately. Compare the exposure with the 2024 criteria for temporary threshold shift and auditory injury; do not treat a result as a prediction of all behavior.
- Assess behavior and context independently. Consider feeding, resting, nursing, travel, haul-out access, avoidance, and extra energy expenditure. Describe these as possible responses unless evidence demonstrates otherwise.
- Monitor and update. Measurements during operations can test whether modeled exposure matches conditions. If animals respond differently from expectations, management should be revisited rather than assuming the original model captured every effect.
What disturbance might look like
A behavioral response can occur without measurable hearing injury. Animals may move away from a sound source, change dive or travel patterns, stop feeding temporarily, spend more energy relocating, or alter how they use a resting or nursing area. The significance depends on the duration and timing of the interruption, the availability of alternative habitat, and the animal’s energetic state.
Short avoidance by a traveling animal is not equivalent to repeated displacement from a critical nursing site. Conversely, a lack of visible avoidance does not prove that masking or a subtle hearing change is absent. Observations should therefore record both behavior and exposure conditions, rather than using behavior as a proxy for auditory injury.
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No single measure can be recommended for every sea lion and fur seal situation from the available species-specific evidence. Measures should be selected for the source, location, season, and endpoint being managed. A project plan commonly evaluates the following options:
- Avoidance: schedule or place noisy work away from areas and periods in which animals feed, rest, nurse, or congregate, when ecological information supports that choice.
- Source reduction: use quieter equipment, lower operating intensity, or engineering controls where they reduce the received exposure.
- Exposure management: limit operating duration, avoid unnecessary repetition, and coordinate simultaneous sources so cumulative exposure is understood.
- Spatial controls: establish observation or management zones based on modeled and measured received levels, not an arbitrary radius detached from propagation conditions.
- Monitoring: combine acoustic measurements with species-appropriate observations of presence, movement, and activity. Monitoring should test stated decision points rather than merely collect observations.
- Adaptive response: define in advance what change in operations follows a verified threshold exceedance or a meaningful disturbance pattern.
These are planning principles, not a guarantee that any one control prevents masking, threshold shifts, injury, or disturbance. The 2024 NOAA hearing guidance should anchor auditory-effect calculations, while behavioral assessment remains a separate task.
What is—and is not—known about population effects
The evidence establishes plausible mechanisms and provides hearing measurements for particular subjects, but it does not provide a verified percentage of sea lions or fur seals affected by underwater noise. It also does not establish a general population-level outcome or a universal ranking of shipping, sonar, seismic work, and construction.
Three limits are especially important when interpreting claims:
- The 60 Hz–39 kHz NOAA range is a generalized otariid composite, not the measured range of every species.
- The 2025 low-frequency experiment involved two bearded seals and one California sea lion, with no fur seal; its 40 Hz and 24 dB findings cannot be assigned wholesale to fur seals.
- Auditory criteria and behavioral observations answer different questions. Passing one does not demonstrate that the other is unaffected.
A scientifically responsible conclusion is therefore conditional: underwater noise can interfere with hearing and behavior, but the magnitude and consequences must be determined for the species, sound, exposure, and activity being evaluated.
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