Q-1: What is an NDE? What is the process for requesting one?

A: A National Defense Exemption (NDE) is a provision within Section 101(f) of the Marine Mammal Protection Act (MMPA) that allows the Secretary of Defense, after conferring with the Secretary of Commerce, Secretary of Interior, or both, as appropriate, to exempt any action or category of actions undertaken by the Department of Defense or its components from compliance with any requirement of the MMPA, if the Secretary of Defense determines that it is necessary for national defense.

Q-2: How long does the NDE last?

A: The Secretary has invoked the exemption for up to two years, or until the National Marine Fisheries Service issues Regulations and Letters of Authorization for SURTASS LFA sonar military readiness activities, whichever is earlier.

Q-3: When will a new MMPA permit be issued by NMFS?

A: NMFS authorized a new MMPA Rule and 7-year Letter of Authorization for SURTASS LFA sonar on August 12, 2019.

Q-4: Will the Navy continue to use SURTASS LFA sonar during the exemption period?

A: Yes. During the period of the NDE, the Navy continued to employ SURTASS LFA sonar due to its critical importance to the security of the United States and will continue to comply with all marine mammal protection requirements of the previous permit as well as additional restrictions agreed to during the ongoing rule making process.

Q-5: Will the NDE result in diminished protections for marine mammals?

A: No. During the period of the NDE, the Navy committed to complying with all of the previous mitigation and monitoring requirements under the previous MMPA permit and LOAs. The Navy also implemented the additional mitigations that were agreed to during the initiation of the 2017 rulemaking process.


Q-1: Why does the Navy need the SURTASS LFA sonar system?

A: The Surveillance Towed Array Sensor System (SURTASS) Low Frequency Active (LFA) system is essential to the Navy’s anti-submarine warfare efforts. The passive part of the system (SURTASS) uses an array of underwater hydrophones that are towed behind a Navy surveillance vessel to listen for sounds in the ocean produced by submarines. The active acoustic part of the system transmits low frequency active (LFA) sonar sounds that reflect off objects such as submarines that are too quiet to hear with a passive acoustic system alone. By using specialized signals and echo detection, the LFA sonar system increases the distance at which submarines can be detected and tracked. Only low frequency active sonar can provide this kind of long-range submarine detection.

Q-2: What is the Navy doing to protect marine life?

A: The Navy has made a tremendous effort to ensure that the low frequency active (LFA) system does not pose a risk to marine life. Some of the steps that have been taken to minimize or prevent risk to marine mammals, sea turtles, and marine fishes include:

  • A $10 million, three-phase Scientific Research Program (SRP) studied behavioral responses to SURTASS LFA sonar signals by baleen whales in California and Hawaii waters to determine if exposure to LFA sonar caused the whales to change their behavior when they were conducting biologically important behaviors. Baleen whales are low-frequency hearing specialists, so scientists believed they would be most affected by the proposed SURTASS LFA sonar signals. This research was independently conducted by recognized experts in the fields of marine bioacoustics, marine mammal biology, and underwater acoustics.
  • An independent Scientific Working Group was established in 1997 to recommend research priorities and to assess the research for environmental implications.
  • Quantitative 4D (3D space plus time) modeling of animal movements and acoustical exposure is conducted to estimate the potential impacts on marine mammals from exposure to SURTASS LFA sonar. Scientific research and publications are continuously monitored to ensure that the best available information is used in all acoustic impact analyses the Navy conducts.
  • Monitoring and mitigation procedures are implemented onboard the LFA sonar vessels any time LFA sonar is transmitting during training and testing activities. These procedures minimize or prevent harm to marine mammals and sea turtles from exposure to LFA sonar transmissions.
  • Research was conducted on the potential impact of SURTASS LFA sonar on human divers.
  • A $3 million set of controlled exposure experiments was conducted to determine the effects of SURTASS LFA sonar signals on fishes. This independent research was conducted by eminent scientists in the fields of fish biology and fish bioacoustics.
  • The Navy recognizes 39 offshore biologically important areas (OBIAs) globally in which marine mammals conduct vital activities such as calving, feeding, mating, or migrating during specified season(s); 14 OBIAs have been designated in the western and central North Pacific and eastern Indian oceans where SURTASS LFA sonar is currently used for training and testing activities. To protect marine mammals during these important times of the year, SURTASS LFA sonar is currently limited to transmitting such that the received level of sound will not exceed 180 dB re 1 µPa (rms) within 0.54 nautical miles / 1 kilometer of any OBIA boundary.

Q-3: Is the Navy proposing to waive environmental laws to deploy the SURTASS LFA sonar system?

A: Absolutely not. The Navy considers and complies with all environmental laws relevant to SURTASS LFA sonar, with no exceptions. A record of the Navy’s environmental compliance for SURTASS LFA sonar can be found on the Environmental Impact pages of this website.

Q-4: What about the deaths of whales in the Bahamas in March 2000?

A: SURTASS LFA sonar was not used, nor were any other low-frequency active acoustic sources, in the Bahamas in March 2000. A Joint Interim Report was published regarding the Bahamas Marine Mammal Stranding Event of 15-16 March 2000.

Q-5: Is it true that the SURTASS LFA sonar system can generate sound in the range of 235 dB, which could cause hearing loss, serious injury, or even death to whales and other marine mammals?

A: The LFA sonar system has 18 projectors, or sound sources, each capable of transmitting sound at a level of approximately 215 dB between 100 and 500 Hz. Because it is an array, if one looks at LFA from a distance, it will appear to have an “effective source level” on the order of 230 to 240 dB. However, in the actual water column, no portion of the ocean will experience sound levels greater than 215 dB.

Q-6: What are the potential effects that low frequency sound may have on marine mammals?

A: The most likely effect on marine mammals from exposure to SURTASS LFA sonar transmissions may be a change in behavior, which is typical reaction whenever a marine mammal perceives any type of unusual underwater sound. Injury could occur at very high exposure levels, but this risk is confined to a relatively small area very close to the transmitting LFA sonar array and vessel, and mitigation monitoring measures are undertaken to ensure that no marine mammals or sea turtles could be exposed to injurious sound levels. The majority of impacts to marine mammals are behavioral responses that do not lead to population-level effects.

Q-7: What mitigation measures are implemented to protect marine animals?

A: The Navy has developed and implements comprehensive measures to minimize or prevent risks to marine mammals and sea turtles. Three types of monitoring (visual, active acoustics, and passive acoustics) ensure that marine mammals and sea turtles within a 2,000 yard radius around the SURTASS LFA sonar vessel / LFA sonar array are detected before they can be exposed to high sound levels. These mitigation monitoring actions effectively reduce to near-zero the chance of any marine mammal or sea turtle being exposed to SURTASS LFA sonar at high enough sound levels to potentially cause injury. The three mitigation monitoring measures used when SURTASS LFA sonar is transmitting during training and testing activities are:

  • Visual monitoring – daytime observations for marine mammals or sea turtles from the bridge of SURTASS LFA sonar vessel by personnel trained in detecting and identifying these species.
  • Passive acoustic monitoring – using the SURTASS horizontal line array (the passive or “listening” part of the system) to receive sounds of vocalizing marine mammals as an indicator of their presence.
  • Active acoustic monitoring – the high frequency (HF) active sonar is effective in detecting marine mammals (and possibly sea turtles) within 2,000 yards of the vessel

If marine mammals or sea turtles are detected within 2,000 yards of the LFA sonar vessel, transmissions from the LFA sonar system will be stopped or delayed immediately until the marine animals are no longer detected to ensure that no marine mammals or sea turtles are exposed to potentially injurious sound levels.

Additionally, geographic restrictions to the power level of LFA sonar transmissions apply to areas within 12 nautical miles / 22 kilometers of any emergent land, including islands; designated offshore biologically important areas (OBIAs) for marine mammals; and known recreational and commercial dive sites.

Q-8: Is it true that the Scientific Research Program (SRP) was not a representative scientific study of the impact on marine mammals because the sound levels used during the sea tests were below those of actual planned system use?

A: A guiding principle for the SRP was that no experimental animal would be exposed to sounds in excess of 155 dB. This was agreed upon after consultation with the National Marine Fisheries Service, which issued the permits for the experiments.

In practice, it proved difficult to realize a received level of 155 dB for the experimental animals, even when the full source power of the LFA sonar system was used. Subsequent modeling has proven that the overwhelming majority of marine mammals would never be exposed to LFA sonar received levels in excess of 155 dB even during full-power sonar operations.

Q-9: Is it true that the SRP did not study impacts from received levels from 155 to 180 dB?

A: None of the experiments with free-ranging animals during the SRP generated exposure levels in excess of 155 dB. In order to account for this data gap, the Navy has assumed that risk increases quite rapidly at exposed levels above 155 dB, reaching a value near 100% at 180 dB.

Q-10: Why did the SRP only study four species of whales?

A: It is impossible to conduct studies on all marine animal species. Four mysticete (baleen) whale species (blue, fin, gray, and humpback whales) were selected for study because:

  • They are considered to have the best hearing in the low frequency band of all marine mammals
  • These species have protected status under the law
  • There is prior evidence that these species react to low frequency sounds.

The SRP was designed such that responses of these representative species to low frequency sound signals would serve as indicators for responses of other potentially low frequency-sensitive species, which should be less vulnerable to low frequency active system signals.

During SRP Phases I and III, all other marine mammal species in the study area were observed from the LFA sonar vessel. During Phase I, aerial surveys were also used to monitor all marine mammal species. Protocols were in place to suspend the experiments if any dramatic responses were detected. These data have been analyzed to look for changes in the numbers and distribution of these species during the course of the experiments.

Q-11: Is it true that during Phase III of the SRP conducted off Hawaii, whale-watch boat captains reported humpback whales disappearing from the testing area as soon as the broadcast began?

A: Some whale watch captains did assert that they observed a premature decrease in the number of humpback whales. This assertion was tested by comparing the quantitative measures of abundance from the SRP shore station with identical measures made during previous years of study. The pattern of abundance observed during LFA SRP Phase III was completely consistent with the historical record. The decline in humpback numbers reflected the normal seasonal pattern of migration.

Q-12: What about theories that the LFA sonar system will affect the migration of whales?

A: SRP Phase II and prior studies of migrating gray whales indicate that whales avoid a low frequency sound source that is in their path, and resume migrating on their historical route within a few kilometers.

During SRP Phase II, a significant new result showed that this deviation from the migration pathway only occurred when the sound source was located in the middle of their migration route (source at 1.1 nautical miles / 2 kilometers from shore). Gray whales in the same migration corridor did not appear to react to the LFA source transmissions when it was located outside the corridor (2 nautical miles / 4 kilometers from shore), even though the source level was adjusted so that the migrating whales’ received sound levels were the same.

Whales tend to swim closer to the surface during migration than during other activities. This diminishes their exposure to LFA sonar transmissions, because low frequency sounds are attenuated near the ocean surface. These facts suggest that whales migrating in the open ocean would not be adversely affected by LFA sonar operations. The sound level of LFA sonar transmissions will never be higher than 180 dB within 12 nautical miles / 22 kilometers of the coast where many marine mammal migrations occur.

Q-13: Are there potential long-term impacts from the Navy’s SURTASS LFA sonar system that could affect the behavior and viability of entire populations of marine mammals and push endangered species into extinction?

A: Exposure to LFA sonar is intermittent because of the short duration of its use at sea, since it is a surveillance system, and the low duty cycle of the sonar transmissions. It is unlikely that LFA sonar would be used repeatedly in exactly the same ocean area, again, because it is a surveillance system. Therefore, the potential for cumulative impact is estimated to be low.

The SRP analyses looked for cumulative effects during the three phases of experiments, which were approximately the same duration as many normal LFA sonar activities. No evidence of cumulative effects was found. Nonetheless, the Navy and NMFS have agreed on monitoring protocols that are designed to preclude long-term effects.

Q-14: What will be done to protect human divers?

A: The Navy has conducted a comprehensive, scientifically-based research program on the potential effects of low frequency sound on human divers. Medical doctors and clinical researchers have carried out extensive computer modeling and testing of human divers. All testing was done within the guidelines for the protection of human subjects.

Based on the data obtained from these studies, the Navy Bureau of Medicine and Surgery (BUMED) incorporated a safety margin and established a conservative limit of 145 dB for low frequency received sound level for recreational and commercial divers. This is the basis for the Navy’s geographic mitigation measure in the vicinity of known dive sites such that the received level of LFA sonar would be no more than 145 dB.

Q-15: Will the Navy’s SURTASS LFA sonar system create additional noise in the ocean?

A: When deployed and transmitting, SURTASS LFA sonar would add to the ambient noise levels of the ocean, but the benefit to national security that SURTASS LFA sonar provides far outweighs the transitory addition of sound to the ambient noise environment.

Commercial shipping is the dominant source of man-made, low frequency noise in the sea. Seismic survey efforts also contribute more noise to the ocean than LFA sonar, as do many natural sources of low frequency sound, including lightning strikes, earthquakes, volcanic eruptions, and baleen whales. Currently, only four SURTASS LFA sonar systems are in use compared to millions of commercial ships that operate worldwide.

Q-16: Is the Navy working on another system of submarine detection, other than the use of low frequency active sonar?

A: As the Navy has reported in its NEPA documentation for SURTASS LFA sonar, the only technology (acoustic or non-acoustic) capable of providing reliable, long-range detection of today’s quieter, harder-to-find submarines is low frequency active sonar. The Navy has developed other passive sonar technologies, but even with these upgraded, passive sonar systems are not sufficient to meet the needed long-range detection that only LFA sonar can provide.

When the Navy is attempting to locate submarines, the priority is to detect the target passively at distances far enough away so that the submarine is outside of its effective weapons range. When a ship uses active sonar, the active “ping” gives away the ship’s position, which can be lethal if an enemy submarine is lurking within its weapon’s range when it hears the ping. So, the Navy will only use active sonar when passive alone will not meet its needs. This can occur when the acoustic “signature” of the target cannot be heard over the din of the surrounding noise in the ocean. In this circumstance, the use of active sonar is the only viable alternative.

Q-17: Can air space resonance produced by SURTASS LFA sonar cause tissue damage and inflict injury on marine mammals?

A: The concept that resonance will increase stress on tissue to the point of damage is in reality two separate concepts: resonance and tissue damage. Cudahy and Ellison (2002) (Click HERE to read the paper) state that resonance does not equal damage and damage is not always linked to resonance. So the issue is not resonance in air/gas cavities, but tissue damage, whether it is caused by resonance or by other means. The potential for in vivo tissue damage to marine mammals from exposure to underwater LF sound will not occur at a level less than 180 to 190 dB (Cudahy and Ellison, 2002). This includes: (1) Transluminal (hydraulic) damage to tissues at intensities on the order of 190 dB or greater; (2) Vascular damage thresholds from cavitation at intensities in the 240-dB regime; (3) Tissue shear damage at intensities on the order of 190 dB or greater; and (4) Tissue damage in air-filled spaces at intensities above 180 dB.

Therefore, unless an animal is within the LFA mitigation zone, there should be no physical damage to body systems or tissues, either with or without resonance. Because of the mitigation measures used when LFA sonar is transmitting during training and testing activities, the probability that a marine mammal will be detected in the LFA mitigation zone is close to 100 percent. Consequently, the potential impact to any marine mammal stock from injury is considered negligible.