Low frequency acoustics from hundreds of Hz to 10 kHz includes the range for Navy sonars and have recently been tested assessing the distribution, density, and behaviour of pelagic fish. These frequencies are within the hearing range of fish and marine ma mmals. Operation at these frequencies may affect the behaviour, if not harm, marine organisms such as marine mammals and fish.For the Norwegian navy, who are soon putting their new sonar into operation, it is important to establish safe operation guideli nes. Institute of Marine Research look at this as an opportunity to investigate the potential of using low frequency acoustics in studies of marine life at the Norwegian continental shelf. The project goals will be solved in three steps. In the preparato ry step a theoretical study of the new navy sonar will be done with respect to performance in relation to the expected experimental conditions. Further, we will establish the field instrumentation needed to detect behavioural effects on fish and mammals a s well as validation of biomass measurements. In the controlled experiments step we will carry out measurements on herring. Detailed knowledge of the semi natural behaviour of the fish in captivity gives an excellent basis for studying reaction patterns and detection ranges.The third step, field experiments, will mainly be carried out on herring in the wintering situation off Northern Norway. This area is also excellent for effect studies on killer whale as hundreds of them normally feed on the overwint ering herring schools. The low frequency sonar will be operated from a navy vessel. Stationary acoustic platforms equipped with echo sounders and microphone will record vocalisation of whale and fish and a vessel with scientific sonar and echo sounders wi ll monitor behaviour of herring and validate density estimates.A group of scientists from FFI, WHOI, MIT and IMR will secure the theoretical and operational capabilities of the project.