Pre-spawning herring in a protected area showed only moderate reaction to a surveying vessel

We studied the influence of a surveying vessel on the behaviour of shallowly distributed local pre-spawning herring ( Clupea harengus L.) in a protected area in the Trondheimsfjord (West Norway). The vessel passed an acoustic buoy at standard survey speed at a distance of 10–40 m four times at night and seven times by day. Bottom depths ranged from 40 to 60 m and a herring layer consisting mostly of pre-spawners was recorded from transducer depth (2.5 m) down to 15–30 m. The herring mass centre was deeper during vessel passage than during undisturbed reference periods, indicating a diving reaction. Backscattered echo energy also decreased during most passages, but the trend was less consistent than increased centre depth, especially during night-time passages. In summary, indications of vessel avoidance were weaker than those previously reported for herring in the same period. We interpret vessel avoidance as the reaction to a perceived threat, and the weak response was unexpected. Pre-spawning herring should act in order to maximise their chances of survival until reproduction and a strong reaction to perceived threats as documented from earlier studies was therefore expected. The shallowness of the location in combination with many gadoids close to the bottom, as observed acoustically, may have been important factors that attenuated the reaction of the fish to the vessel in our study.