Herring (sild), killer whales (spekkhogger) and sonar - the 3S-2006 cruise report with preliminary results

This report summarises the outcome of an international research cruise in Norwegian waters (Vestfjorden) in November 2006. The objectives of the trial were to study impacts of military low frequency - (LFAS 1-2 kHz) and mid frequency - (MFAS 6-7 kHz) active sonars on killer whales and herring. In addition the capability of active and passive sonar systems for detection of marine mammals, in order to mitigate possible effects of sonars or seismic sources, were tested. In order to fulfil these objectives we had to achieve the following tasks: 1) Tag free ranging killer whales with sensors recording behaviour, and thereafter execute controlled sonar exposure experiments on them. 2) Expose herring over-wintering in the area to sonar signals while monitoring behavioural reactions of the herring using bottom mounted echosounders. 3) Monitor the acoustic propagation conditions in the study areas by collecting sound speed profiles and use acoustic propagation models. 4) Test the capability of the Delphinus passive acoustic array for killer whale detections. 5) Test the capability of two commercially available fisheries sonars from SIMRAD (SP90 and SH80) for active detections of killer whales. The achievements of the trial include: 1) Deployment of six tags on killer whales and execution of 2 sonar exposure experiments on three animals, as well as one control experiment. 2) Survey of occurrence of killer whales in the eastern Vestfjorden basin in relationship to military sonar activity. 3) Execution of 12 sonar exposure experiments on herring. 4) Collection of 22 sound speed profiles throughout the study area and period. 5) Collection of data from 294 hours of passive acoustic survey with more than 4000 detections or marine mammals. 6) Detections of killer whales on both a long range fisheries sonar (Simrad SP90, 20-30 kHz) and a short range sonar (Simrad SH80 110-120 kHz). Data analysis is currently in progress. Preliminary results from these analyses indicate: 1) Killer whales appear to be more sensitive to MFAS signals than LFAS signals. Avoidance reactions and changes in diving behaviour were observed when received level exceeded 150 dB (re 1µPa). Killer whale occurrence in eastern Vestfjorden did not appear to be affected by transmissions from our experimental sonar, but whale number did decline with no whales seen for several days following the start of a FLOTEX exercise which included use of sonar. 2) Herring does not appear to react by neither horizontal nor vertical escape when exposed to LFAS or MFAS signals. However, they reacted to playback of killer whale feeding sounds covering the same frequency band. 3) Active sonar detection of marine mammals using fisheries sonars looks very promising at ranges up to 1500 m depending on propagation conditions. At short ranges the SH80 and SP90 sonars had similar detection performances, but the SP90 was superior at long ranges. We recommend that these studies are proceeded to obtain sufficient basis for scientific recommendations on the use of different sonar signals. Of particular importance are additional exposure experiments on killer whales and other species of cetaceans and further validation of active acoustic detection of marine mammals as a tool for marine mammal mitigation.