Behaviour of herring (Clupea harengus L.) towards an approaching autonomous underwater vehicle

The reaction of schooling wintering herring (Clupea harengus L.) in Ofotfjord in northern Norway is studied when approached by an autonomous underwater vehicle (AUV) with electrical propulsion. The reaction of herring is recorded running the AUV in the beam of the mother vessel's 38-kHz echosounder and in more detail with an onboard 120-kHz echosounder. The results indicate an insignificant reaction of herring to the approaching AUV, although some variations were observed depending on the experimental set-up. Technical uncertainty in the recordings close to the AUV transducer creates some ambiguity in the results. No reaction could be identified from the ship's sounder when the AUV passed under the vessel. Processing of the onboard echosounder data suggests a mean avoidance distance of 8.0 m in these experiments. In a realistic autonomous survey situation it is assumed that the AUV can approach as closely as 5–10 m to herring schools without affecting the acoustic observation, which makes it a potentially useful platform for hydroacoustic research and survey. More systematic studies are needed to precisely define the threshold reaction distance to the AUV, and the work should be conducted with transducers on a more silent platform than RV “Johan Hjort”, which was used in this study.