Behavior of captive herring exposed to naval sonar transmissions (1.0-1.6 kHz) throughout a yearly cycle.

Atlantic herring, Clupea harengus, is a hearing specialist, and several studies have demonstrated strong responses to man-made noise, for example, from an approaching vessel. To avoid negative impacts from naval sonar operations, a set of studies of reaction patters of herring to low-frequency (1.0-1.5 kHz) naval sonar signals has been undertaken. This paper presents herring reactions to sonar signals and other stimuli when kept in captivity under detailed acoustic and video monitoring. Throughout the experiment, spanning three seasons of a year, the fish did not react significantly to sonar signals from a passing frigate, at received root-mean-square sound-pressure level (SPL) up to 168 dB re 1 µPa. In contrast, the fish did exhibit a significant diving reaction when exposed to other sounds, with a much lower SPL, e.g., from a two-stroke engine. This shows that the experimental setup is sensitive to herring reactions when occurring. The lack of herring reaction to sonar signals is consistent with earlier in situ behavioral studies. The complexity of the behavioral reactions in captivity underline the need for better understanding of the causal relationship between stimuli and reaction patterns of fish.

Behavioral responses of herring (Clupea harengus) to 1-2 and 6-7 kHz sonar signals and killer whale feeding sounds.

Military antisubmarine sonars produce intense sounds within the hearing range of most clupeid fish. The behavioral reactions of overwintering herring (Clupea harengus) to sonar signals of two different frequency ranges (1-2 and 6-7 kHz), and to playback of killer whale feeding sounds, were tested in controlled exposure experiments in Vestfjorden, Norway, November 2006. The behavior of free ranging herring was monitored by two upward-looking echosounders. A vessel towing an operational naval sonar source approached and passed over one of them in a block design setup. No significant escape reactions, either vertically or horizontally, were detected in response to sonar transmissions. Killer whale feeding sounds induced vertical and horizontal movements of herring. The results indicate that neither transmission of 1-2 kHz nor 6-7 kHz have significant negative influence on herring on the received sound pressure level tested (127-197 and 139-209 dB(rms) re 1 microPa, respectively). Military sonars of such frequencies and source levels may thus be operated in areas of overwintering herring without substantially affecting herring behavior or herring fishery. The avoidance during playback of killer whale sounds demonstrates the nature of an avoidance reaction and the ability of the experimental design to reveal it.