Low Frequency Active Sonar Shows Less Impact on Fish than Airguns  

Popper, Halvorsen, Kane, Miller, Smith, Song, Stein, Wysocki. The effects of high-intensity, low-frequency active sonar on rainbow trout. J. Acoust. Soc. Am. 122 1 , July 2007. p. 623-635.

This study extended a previous line of research that had measured physiological impacts of seismic survey air guns on fish kept confined in a cage and exposed to the noise. This time, the research team exposed trout (which share hearing mechanisms with salmon, which are of special concern due to their endangered status) to sounds produced by low-frequency active sonar. LFA sonar uses frequencies (100-500Hz) that many fish can detect, often the range of most sensitive hearing. Fish were tested for hearing sensitivity using Auditory Brainstem Response (ABR), and some were sacrificed to check for physiological damage, including swim bladder or ear hair damage. Results indicate that fish had reduced hearing sensitivity after exposure to LFA sonar, ranging from 17-25dB at particular frequencies (i.e., sounds needed to be that much louder in order to be heard), and that the effects lasted at least 48 hours (the longest followup the study included). 

However, the researchers note that there was quite a lot of variability in results, with some study groups showing little shift in hearing thresholds even with longer exposure to the LFA sounds, and some frequencies of hearing being little affected; also, the fish, being captive in a cage, were exposed to constant high sound levels that are unlikely in the wild, making this a probable worst-case scenario (the levels these fish were exposed to would occur only within 100m of the sonar transmission, which would be on a moving vessel). Unlike some earlier pile-driving and explosives studies, the fish exposed to LFA sounds did not show any acute tissue or organ damage. And, unlike earlier airgun studies, there was no apparent damage to ear hair cells; such injury would likely cause a permanent reduction in hearing sensitivity. The fish did respond to the onset of the sound with a rapid burst of swimming; this will be examined in another paper.