The ability of a low frequency (20-30 kHz) and high frequency (110-122 kHz) fisheries sonar to detect killer whales has been evaluated. A fishing vessel equipped with the Simrad SP90 (low frequency) and SH80 (high frequency) sonar’s were used to survey an area with large numbers of killer whales in the northern part of Norway in November 2006. Whales appeared as distinct echoes on both sonar’s. Visual observations were used for confirmation of whale detections. Detection range on the SP90 sonar was at least 1500 m and for the SH80 reliable detections was obtained up to 400 m range. In addition to the direct echo from the whale, vocalization (whistles and clicks) and wakes from swimming whales (surfacing) were picked up on both sonar’s providing strong criteria for positive detection and classification of the target. Whales were detected during dives without any effect of water depth. No apparent behavioral reactions were observed during sonar operation. An acoustic target with echo strength similar to killer whales was suspended at 10 m water depth in the Oslo fjord in January 2007. The plan was to position the target at different water depths from the surface to the bottom and evaluate detection at different ranges. However, due to cold surface water a strong surface sound channel had developed making it impossible to complete this experiment. Detection range for the target was up to 2000 m. Ray tracing, sound propagation (transmission loss) and detection probability were simulated for both sonar’s based on actual sound speed profile (CTD) measurements from the field work using the software Lybin. It was good agreement between actual observations and simulations. Simulations were also made based on CTDs collected from an area relevant for seismic surveys. It is recommended that simulations are performed of sound propagation and detection probability based on CTD profiles, surface and bottom conditions and predicted echo strength of whales in the actual survey area, as a supplement to sonar observations.

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