The mortality threshold for juvenile herring exposed to sonar signals is 180-190 dB (re 1μPa), and for other species even higher. This report analyses the potential impact of sonar transmissions on recruitment to important economical and ecological fish stocks in Norwegian waters, based on a worst-case scenario approach and a real Anti Submarine Warfare (ASW) exercise. The analysis has shown that sonar intense exercises in a herring spawning area will affect less than 0.3‰ of the total stock of juvenile fish. This constitutes less than 1% of the natural daily mortality. The effect of the sonar on juvenile herring, is thus considered to be insignificant. For other species of fish, the effect is expected to be even less. Consequently, there is no need for drastic limitations on the use of active sonar in Norwegian waters, based on the direct physiological impact on juvenile fish. However, for herring, CW-transmissions (Continuous Wave) at frequencies within the frequency band corresponding to the swim bladder resonance escalate the impact significantly. Although the impact is still small (1‰) compared to daily natural mortality (5%), moderate restrictions on the use of CW-signals at specific frequencies, areas and time periods related to the presence of high densities of juvenile herring should be considered. Further studies are necessary to investigate possible behavioural effects of sonar signals on herring.

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