Low Frequency Sound Hurts Fish and Fisheries

A recent study by Dr. Arthur Popper presented at the Acoustical Society of America showed that the effects of loud sounds on fish are much greater than anticipated. Dr. Popper exposed pink snappers to seismic air-gun sounds and found that the hearing parts of fish ears were ripped away and didn't regenerate as expected, even after 58 days (Popper and McCauley, et al 2002.)

Three decades of controlled scientific studies leave no doubt that intense low frequency sound hurts fish and damages fisheries. Many of the studies relate to the use of air-guns for geological exploration, but the threat is amplified with the use of Low Frequency Active sonar (LFA) that the National Marine Fisheries Service has given permission to use in eighty percent of the world's oceans.

The source level of LFA is about 240 decibels, close to the level of the more intense air-gun arrays, but the duration of each pulse is up to 100 seconds, far longer than the very quick air-gun bursts. Even though the Navy argues that it drops off to 180 decibels after a kilometer, in practice LFA has been measured at 140 decibels over a thousand miles away from the source, raising fears that it could impact fisheries over a vast area, and do great damage before the problem is even detected.

Explanatory notes:

Sound is measured in frequency (hz) and intensity (decibels). Low frequency sound ranges from 1 to 1000 hz. It is employed in LFA and air-gun arrays because of its ability to travel long distances underwater with little diminution. LFA employs frequencies from 100-500 Hz.

The decibel scale is logarithmic: 110 decibels is ten times greater than 100 decibels, 120 decibels is one hundred times greater. In air, sound at 130 decibels will cause permanent hearing loss in human beings in one minute. An F-16 jet fighter 3' away from your ear with afterburners blasting emits about 160 decibels.

Because of different properties of air and water, to compare sound pressures from the same source, 26 decibels are subtracted from the level of sound in air to give you the equivalent level of sound in water. Therefore, the source level of LFA underwater, at 240 decibels, is equivalent to 214 decibels of sound in air. This is a level of sound over 100 million times louder than the 130 decibels level at which humans lose the ability to hear forever. Sound travels much farther underwater, however, than in air. (All references to decibels are presented in standard units of sound pressure called micropascals.)

 

What do the studies show about the effect of intense low frequency sound on fish and other marine life?

  1. Air-gun pulses of 223 decibels at the source caused a catch-per-unit-effort decline in the rockfish hook and line fishery off the coast of California by about 50%. A similar decline was noted in catches of chilipepper, bocaccio, and greenspotted rockfish. Pearson& Skalski, 1992.

  2. Air-gun pulses of about 200 decibels severely affected fish distribution, local abundance and catch rates across the entire investigation area of 40 X 40 miles. Trawl catches of cod and haddock and longline catches of haddock declined on average 50%. Abundance and catch rates did not return to pre-shooting levels during the 5 day period after the seismic shooting stopped. Engas et al: 1995.

  3. Analysis of catch data from long-lines and trawlers before, during and after seismic shooting in Norwegian waters showed that catches of cod were reduced 55-85% during seismic shooting. Engas, Lokkeborg and Soldal, 1993.

  4. The general threshold for alarm responses in captive rockfish is about 180 decibels. More subtle behavioral responses might become evident at 161 decibels of received sound. Pearson and Skalski, et al (1992).

  5. Under controlled testing conditions, the viability of the eggs of one species of estuarine fishes (Cyprinodon variegates) was reduced in aquaria when a 40-1,000 Hz noise source at 105- 120 decibels was maintained over a number of consecutive days. Banner and Hyatt (1973).

  6. Researcher suggests keeping sound levels under 150 decibels where fish are concentrated. Croll et al (1999).

  7. Cod has a body resonance frequency of 400 to 560 Hz at depths of 10-30 meters. McCarthy and Stubbs (1971). Note: this frequency level is right within the range of LFA. Body resonance is a factor of fish size and depth and describes the sympathetic vibration of internal organs to certain frequencies. This study corroborates the concern that the damage to marine life may come not only from destroying hearing organs but by vibrating air bladders and lungs violently enough to sheer tissue.

  8. "Only the larger pelagic fish species (such as tuna) have swim bladders large enough to possibly be resonated by low frequency sound." Final Environmental Impact Statement on LFA issued by the Navy (volume 1, section 4. 1-4.).

  9. The sensory hair cells (the lagena and utricle) of oscar fishes (Astronotus ocellatus) are damaged from  a continuous exposure to sound at 300 Hz. of frequency and 180 decibels of volume. Hastings et al. (1996).

  10. Goldfish exposed to pure tones of 250 and 500 Hz at 204 and 197 decibels respectively showed indications of sensory hair cell damage. Cox et al (1986a, b; 1987).

  11. Some ciliary bundles on sensory cells of the inner ear of the cod were destroyed when exposed to sounds at several frequencies from 50 to 400 Hz for 1-5 hours. Enger (1981).

  12. Low frequency sound produced by fishing boats and gear is strongly associated with fish avoiding the vessel. Maniwa (1971), Suzuki et al (1979), Konigaya, (1980).

  13. Ambient noise at higher sea states in the ocean have masking effects in cod, haddock and pollock, raising the question of the potential for low frequency sound to obstruct communication between fish, detecting prey, and avoiding capture. Chapman and Hawkins (1973).

  14. Lemon sharks (Nagaprion brevirostris) exhibited withdrawal responses to low to mid frequency sounds (500-4,000 Hz) from 98 decibels to 123 decibels of received level. Klimley and Myberg (1979).

  15. "In assessing the potential for SURTASS LFA sonar signals to affect shark migrations, it is noted that the SURTASS LFA sonar source frequency is between 100 and 500 Hz, a region of the acoustic spectrum where these species are best able to hear sound. Furthermore, the signal usually has no ramp-up, an acoustic property that has been shown to provoke withdrawal in an inshore species and two pelagic species (Carcharhinus falciformis and C. longimanus) Myberg et al., (1978). These studies suggest that sharks can detect sounds with intensities below 180 decibels." US Navy Final EIS on LFA (2001).

  16. At 166 decibels of air-gun emissions a green and a loggerhead turtle exhibit a noticeable increase in swimming behavior. At 175 decibels, their behavior became increasingly erratic with probable avoidance. From 156- 161 decibels various captive fin-fishes showed a common alarm behavior of forming a huddle on the cage bottom center, but a noticeable increase in alarm behaviors begins at a lower level of sound. Squid in cage trials showed a startle response (ink sac fire) and avoidance to the startup of an air-gun array from .9 to 1.5 kilometers away. They showed a noticeable increase in alarm behaviors at from 156-161 decibels. At 166 decibels, they showed a significant alternation in swimming speed patterns, perhaps using the sound shadow near the water surface. McCauley et al (2000).

  17. The acoustic impedance of fish and other animals nearly matches that of water, so much of the sound energy will enter their bodies if they are in the vicinity of the source. Results of previous studies reviewed by Norris and Mohl (1983) and experiments reported by Enger (1981) and the author (1986 and 1987)indicate that fish suffer damage to their auditory system as well as other parts of their bodies, and may even die, when exposed to sufficient sound pressure levels underwater for relatively short periods of time. I recommend a maximum sound pressure level of 150 decibels for bony fishes. Mardi Hastings (1991).

  18. The auditory thresholds for seven non-ostariophysine species of fish range from 40 to 1000 hz. Popper (1972). Note: The area of most acute hearing falls within the LFA range of 500-1000 Hz.

  19. Low frequency sound waves (30Hz) was successful in inhibiting the settling of immature barnacles. Less than 1% of very young cyprids settled in the presence of sound waves. Low frequency sound reduces the percentage of metamorphosis for cyprids up to 13 days old. Branscomb et al (1984).

  20. Zebra mussels exposed to very low frequency sound (60 Hz) die within 40 days. The irradiation appeared to cause zebra mussels to lose large amounts of calcium- essential for shell health and muscle control- as well as sodium and potassium. Ryan et al, Purdue University (2001).

  21. Eggs and larvae can be damaged 1.5 m below air-guns and up to 5.5 m below large sub-array. Brunswig et al (2000).