A monitoring and research provision was included in the 2012 MMPA Final Rule and subsequent annual LOAs during that 5-year rule period for SURTASS LFA sonar that required the Navy to consider research or monitoring strategies that would increase the understanding of the potential effects of SURTASS LFA sonar transmissions on beaked whales and/or harbor porpoises. The Navy’s first step was to convene a group of recognized scientific subject matter experts in the fields of marine bio-acoustics, marine mammal biology and hearing, and marine animal behavioral sciences to evaluate and recommend possible research or monitoring efforts. The goal of this Scientific Advisory Group (SAG) was to specifically evaluate and recommend a practical strategy for the Navy on the possible monitoring and research efforts it could undertake to provide information on how exposure to SURTASS LFA sonar might affect the behavior of beaked whales and/or harbor porpoises.

The SAG included these scientists and acousticians:

  • Dr. Christopher W. Clark
  • Dr. Daniel P. Costa
  • Dr. William T. Ellison
  • Dr. Jason Gedamke
  • Dr. Ronald A. Kastelein
  • Dr. Brandon L. Southall (chair)

The SAG convened twice during 2012 to 2013 and developed a strategic, iterative, parallel research approach for beaked whales (primarily involving field work with existing methods) and harbor porpoises (primarily involving laboratory studies with existing methods) that could be implemented to address specific information gaps, if deemed necessary. The report of the SAG’s recommended research approach was submitted to the National Marine Fisheries Service in August 2013.

In their report, the SAG concluded that the available data suggest that the potential for adverse effects to beaked whales and harbor porpoises from exposure to SURTASS LFA sonar appears limited. They also concluded that the potential effects of SURTASS LFA sonar on marine mammal species within these two odontocete taxa would depend largely on the frequencies of the transmissions relative to the species’ hearing sensitivity, the species’ responsiveness to underwater sounds, and the spatial overlap between SURTASS LFA sonar mission areas and the species’ distributions. However, the SAG concluded that a number of research questions should be addressed to verify this conclusion and provided several specific recommendations to address the research questions.


The Navy sponsored a $3-million series of controlled exposure experiments conducted to determine the effects of exposure to SURTASS LFA sonar transmissions on fishes. LFA sonar transmits at frequencies from 100 to 500 Hz, the range in which most fish can detect sound and the range of best hearing for many fish species. The independent research, led by Dr. Arthur Popper, was conducted by prominent scientists in the fields of fish biology and fish bioacoustics. The controlled exposure experiments were conducted at the Navy’s Lake Seneca, NY sonar test facility using fish-holding test tanks that were lowered to a water depth of 459 ft / 140 m.  Five species of fishes (rainbow trout, channel catfish, hybrid sunfish, largemouth bass, and yellow perch) were used in the experiments and were exposed to relatively high sound SURTASS LFA sonar transmissions from a single LFA sonar element. Source levels of ~215 dB re 1 µPa (rms) at 1 m from the source were transmitted during the experiments, which equated to maximum received levels of ~195 dB re 1 µPa (rms), as measured by six hydrophones in the test tank. In an operational deployment of LFA sonar array, these sound levels would occur only within about 656 ft / 200 m of the transmitting LFA sonar array. Feeds from video cameras on the test cages allowed for observation of the fish’s behavioral responses.

The experiments examined the impacts from exposure to LFA sonar transmissions on fish hearing, physiology (sensory hair cells in the fish’s inner ears and non-auditory tissues), and to a lesser extent, behavior. Up to 96 hours post-exposure, no indications of any physiological or pathological damage to sensory cells or to any other tissues were found in any of the fish species. None of the fish experienced damage to their ear’s sensory hair cells or any damage to their tissues and no fish died as the result of the sound exposure. Impacts to hearing were observed in two fish species, the rainbow trout and channel catfish. Small hearing threshold shifts in the channel catfish were observed after exposure to LFA sonar transmissions, but the hearing loss was temporary, with recovery within 72 hours after exposure. Following exposure, two hearing threshold shifts were observed in two stocks of rainbow trout: a 25 dB threshold shift at 400 Hz in one test group and 5 dB threshold shift at 200 Hz; not all exposed rainbow trout showed hearing loss. Although the fish species reacted immediately to the LFA sound, their behavior was no different after exposure than prior to the experimental exposures. Since the fish were in confined experimental space, their behavioral reactions are not representative of free-ranging fishes.

Several scientific papers documenting the research results and conclusions of these controlled exposure experiments on fishes were published in peer-reviewed scientific journals: Popper et al., 2007; Kane et al., 2010; and Halvorsen et al., 2013 (click on the author names to access the papers).


When the use of SURTASS LFA sonar was first proposed in the mid 1990’s, very little data existed upon which an assessment of the potential effects could be based. Therefore, the Navy convened the Low Frequency Sound Scientific Research Program (LFS SRP) to investigate the reaction of species engaged in critical, biologically-important behaviors to the low-frequency transmissions produced by the LFA system. The selected marine mammal species are believed to be sensitive to low-frequency sound. If the prevailing hypothesis was confirmed that received sound levels of approximately 140 dB would cause avoidance behaviors by marine mammals, then LFA sonar transmissions would not have been suspended or its use would have been severely restricted.

The LFS SRP was conducted in three phases. The following map figure shows the location of each SRP phase.

  • Phase I (Sept-Oct 97) – Species: Blue and Fin Whales
  • Phase II (Jan 98) – Species: Gray Whales
  • Phase III (Feb-Mar 98) – Species: Humpback Whales



The primary objective of SRP Phase I was to determine whether exposure to low-frequency sounds elicited disturbance reactions from feeding blue or fin whales. The goal was to characterize how whale reactions vary to the sounds depending on: (1) the received level of the sound, (2) changes in the received level, and (3) whether the system was operating at a relatively constant distance or approaching a whale.

This map shows the test area, located west of San Nicolas Island, off the coast of southern California.

Bottom Bounce Acoustic Field

  • Sound is refracted (bent) downward and reflects upward off the seafloor
  • Simulated the type of sound field that whales could experience from a distant source

Direct Path Acoustic Field

  • Omni-directional sound field (sound levels are same throughout the water column)
  • Simulated the type of sound field that whales could experience from an approaching source

Phase I Research Assets

Research Vessel Cory Chouest

  • Source Ship (played LFA acoustic transmissions)
  • Collected acoustic recordings with SURTASS towed array, especially of whale vocalizations

Research Vessel Dariabar

  • Independent observation vessel
  • Marine mammal experts observed fin and blue whale behavior
  • Collected acoustic recordings, especially of whale vocalizations

Research Vessel John Martin

  • Surveyed prey fields (whale food)
  • Tagged blue and fin whales with time/depth recorders to assess dive behavior

Aerial Surveys

  • Documented the distribution and density of marine mammals

Pop-Up Units

  • Autonomous seafloor acoustic recording units

SOSUS (Navy Underwater Acoustic Systems)

  • Acoustic recordings from Navy seafloor passive hydrophone arrays

Phase I Results:

  • Full and reduced power of LFA sonar transmissions were used.
  • Highest received levels at whales was estimated to be 148 to 155 dB rms.
  • In 19 focal whale observations (4 blue whales and 15 fin whales), no overt behavioral responses were observed. Note: A “focal whale” is an individual whale selected for intensive observation during the experiment.
  • No changes in whale distribution could be related to LFA sonar transmissions; whale distributions closely tracked the distribution of their food / prey.
  • A preliminary analysis of sound generated by the whales during the experiment indicated a slight decrease in whale calling activity during LFA sonar transmissions, but secondary analyses did not confirm that observation.

SRP Phase II


  • Quantify responses of migrating gray whales to low-frequency sound signals.
  • Compare gray whale responses to different received levels (RL).
  • Determine whether gray whales respond more strongly to RL, sound gradient, or range to the source.
  • Compare gray whale avoidance responses to the low-frequency source when it is located in the center of the migration corridor versus offshore (outside) the migration corridor.

For this second phase of the SRP, an LFA sonar source was positioned offshore of the central California coast, near Point Buchon. Shore-based observers tracked gray whales using methods that provided highly sensitive measures of avoidance responses. Observers on the playback vessel (100-ft work boat) also carefully monitored marine mammals so that LFA sonar transmissions could be stopped if the behavioral reactions were significant or if any marine mammals were sighted at close range to the sound source, which might lead to exposures that could exceed the maximum planned exposure level (155 dB).

Phase II Results:

  • A single source was used to broadcast LFA sonar signals at source levels up to 200 dB re 1 µPa at 1 m.
  • When the source was moored 1 mile / 1.6 kilometers offshore in the middle of the migration path, gray whales showed avoidance responses similar to those reported by Malme et al. (1983, 1984).

  • Gray whales returned to their migration path within a few kilometers.
  • When the LFA sonar source was moored 2 miles / 3.2 kilometers offshore (outside the direct migration path), behavioral responses where much less, even when the source level was increased to achieve the same received level for most whales in the middle of the migration corridor.
  • Offshore gray whales did not appear to avoid the louder offshore source.



The SRP Phase III objectives to assess potential effects of LFA sonar transmissions on the behavior, vocalizations, and movements of humpback whales on their breeding / calving grounds off the Kona, Hawaii coast were accomplished by:

  • Shipboard visual and acoustic observation.
  • Shore-based visual observation.
  • Controlled exposure of humpback whales to LFA sonar transmissions from a SURTASS LFA sonar source.
  • SURTASS ship (passive only) allowed better localization and additional observations.
  • Additional R/Vs collected visual, acoustic, and sound field data.
  • Applicable mitigation measures employed during all operations.

For this third phase of the SRP, the R/V Cory Chouest operated off the west coast of the big island of Hawaii. A passive SURTASS ship also participated to listen for humpback whale songs. Shore-based observers tracked the humpback whales. Calibrated hydrophones were deployed from a small vessel to measure received levels (RL), verify the transmission loss (TL), and improve determination of the sound field to which the humpback whales were exposed. A small boat (RHIB) followed individual humpback whales and described their surface behaviors before, during, and after LFA sonar transmissions (focal whale observations). Visual and acoustic observers on the playback vessel (R/V Cory Chouest) carefully monitored marine mammals so LFA sonar transmissions could be stopped if behavioral reactions were significant or if any marine mammals were sighted at close enough range that the sound exposure level might exceed the maximum planned exposure level.



Phase III Results:

  • 33 LFA sonar playback (acoustic transmission) experiments.
  • More than 950 humpback sightings.
  • 500 hours of passive acoustic data collected.
  • Whale exposure levels of 115 dB to 152 dB.
  • Variety of behavioral responses to LFA sonar playbacks including humpback whale singers temporary stopping singing and apparent temporary avoidance responses.
  • Many humpback whales continued to sing and interact with other whales during the sonar playbacks.
  • Three aerial surveys conducted as part of separate research projects  made observations in the SRP Phase III study area during the LFA sonar playbacks.
  • Humpback whale distributions and abundances compared with similar survey data from 1993 and 1995
  • Roughly half of the humpback whales ceased singing during LFA sonar transmissions but many of these did so while joining a group of whales.
  • All singers who interrupted their songs were observed to resume singing within tens of minutes.
  • Changes in the lengths of songs during LFA sonar transmissions were complex, with the changes depending upon what portion of the song was overlapped with LFA sonar transmissions.
  • A delayed response to LFA sonar transmissions was observed, when the duration of songs increased 1 to 2 hours after the last sonar transmission.
  • Overall patterns of singer and cow-calf abundance were the same throughout the experiment as observed during several previous years of observations.


Between 1993 and 1998, the Navy sponsored two controlled studies to determine the physical and behavioral effects on human divers exposed to low frequency (LF) sound. The results of this first human diver study showed no long-term effects on major organ systems and concluded that exposure to low frequency sound levels below 160 dB would not be expected to cause physiological damage to a diver. The goal of a second diver study was the development of safe exposure limits to LF sound for recreational and commercial divers. The results of the second diver study concluded that the maximum intensity used during the tests (received level of 157 dB) did not produce physiological evidence of injury in human subjects and that only a two percent “very severe” aversion reaction by divers was observed at a received level of 148 dB.

The Navy concluded that reducing the intensity by 3 dB of the lowest sound level at which reactions were noted to 145 dB, which represented a 50 percent reduction in the strength of the sound signal, would provide a safe margin for human divers. This exposure limit of 145 dB for human recreational divers to LF underwater sound was established by the Navy in 1999. This guidance is the basis of the power level mitigation measure for SURTASS LFA sonar in the vicinity of known recreational and commercial dive sites such that received sound levels will not exceed 145 dB.

Further details on the Navy’s human diver studies can be found on the Diver Studies webpage of this website.