Observing the behavioral response of herring exposed to mid-frequency sonar signals

There is general concern on how mid-frequency military sonars might affect aquatic animals. Approaches used to investigate possible effects on Norwegian spring spawning herring (Clupea harengus) are presented. Experiments were performed in a sheltered fjord area, in the open ocean, and in a net pen. In the fjord area, the behavior of the exposed herring was monitored using an upward-looking bottom-moored echo sounder. For the open ocean experiment, schools of exposed herring were tracked using omnidirectional sonar. In the controlled net-pen experiments, the herring were towed into a fjord basin. The net-pen was equipped with a horizontal pointing and an upward-looking split-beam echo sounder to monitor the behavior, a hydrophone to measure the exposure levels, and a video camera to verify the observed behavior. In all experiments the herring was exposed to relevant naval sonar signals using operational sources mounted on a research vessel or operated from a frigate. For the net-pen experiments, the herring were also exposed to other audible sound sources to investigate the robustness of the methodology to detect reaction patterns. The pros and cons of the different observational approaches are discussed. [Work supported by the Research Council of Norway Grant No. 184705.]