Effects of Simulated Mid-Frequency Sonar Signals on Juvenile Alewife

This study investigated the effects of simulated mid-frequency (1-10 kilohertz) sonar on juvenile alewife (Alosa psuedoharengus). Alewife was selected because it is a Species of Concern and has an expanded hearing range. Trials exposed fish to frequency modulated signals and continuous wave tones with a received peak level target of 195 decibels referenced to one micro-Pascal (dB re 1 μPa). Fish were also exposed to incrementally reduced source levels as low as 135 dB re 1 μPa. The goal of the study was to assess behavior and potential mortality of early stage juvenile fish. Clear behavioral reactions to sound signals were observed at received peak level targets of 195 and 185 dB re 1 μPa. No mortality occurred in 2013 (n=90), however, in 2014 (n=104), mortality was observed in ten percent of fish exposed to the highest source levels. This disparity was likely due to the smaller size of the early stage juvenile fish. Combining data for both years, mortality occurred in five percent of juveniles tested at full power. All mortality was limited to fish less than 38 millimeters. This study helps to fill a data gap addressing the potential effects of high intensity sound sources on fish.