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Orcas Avoid Mid-frequency Sonar Signals, but not LFAS; Herring Unaffected

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Kvadsheim, Benders, Miller, Doksaeter, Knudsen, Tyack, Nordlund, Lam, Samarra, Kleivane, Godo. Herring (slid), killer whales (spekknogger) and sonar – the 3S-2006 cruise report with preliminary results. Norwegian Defence Research Establishment (FFI). 30 April 2007 
This paper reports preliminary results from an ambitious multi-national Controlled Exposure Experiment using acoustic D-tags, which allow researchers to record received sound levels while charting dive patterns. Six orcas were tagged, with 17 hours of data collected, with one animal exposed to LFAS signals, two to mid-frequency active sonar signals, and one used as a control, fewer samples than hoped. The whale exposed to LFAS signal did not change its behavior, nor did its companions. However, the group exposed to mid-frequency sonar signals ceased feeding and moved rapidly away; in addition, they exhibited an unusual dive pattern, diving roughly twice as deep (over 60m) as normal (20-45m), including a highly unusual reversal of their ascent (coming up to 15m from the surface, then retreating again to 60m). [Ed. note, not part of the report narrative, based on observation of included diagrams: Four other dives during the 35-minute sonar transmission were within the normal range of depth (the deep dive was the third of five dives during sonar transmission), with three longer than normal rest periods at the surface during the hour after sonar transmission ceased.]  Received levels did not exceed 150dB re 1uPa. Looking at longer-term responses, while the experimental use of sonar in the herring trials did not seem to deter orcas from the general area, by contrast, no orcas were seen during three days of a planned Navy exercise in the area that included mid-frequency sonar use. The herring trials used bottom-mounted sensors to chart the mean depth of a school of herring as a sonar vessels passed nearby. Herring at shallow depths (25-50m) tended to show a minor downward reaction as the sonar source ship passed overhead, while herring located deeper, or in less dense schools, showed no detectable reaction; no horizontal avoidance was seen. While herring hearing curves suggest they should be able to clearly hear LFAS signals, but not mid-frequency signals, the responses were the same for all signals (including ship-only, no sonar), suggesting that the herring may have been responding to the sound of the ship, rather than the sonar signal; such reactions to ships have been seen for many fish species. A related trial using orca echolocation signals on the herring showed more of a response, suggesting that the herring could distinguish between the mid-frequency sonar and the orca sounds, but the three trials all produced slightly different response patterns, suggesting the need for further study.

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