Fish behaviour and acoustic sampling

Abstract

Fish behaviour may lead in several ways to uncertainties in acoustic abundance estimation. Practical echo-surveying work faces a number of situations where aspects of fish behaviour may be of importance. Unfavourable vertical fish distributions, such as when fish stay too close to the bottom or too close to the surface, have long been recognized. Considerably less is known about the impact on acoustic sampling of other behavioural patterns, including schooling as opposed to non-schooling, avoidance behaviour, or vertical migration. Current knowledge in this field is reviewed, as are influences of varying fish behaviour on target strength, effects of sound absorption on fish concentrations, and avoidance reactions in survey situations. Some new experimental results, indicating a significant effect on target strength when fish change depth, are also presented.