Correcting for avoidance in acoustic abundance estimates for herring using a generalized linear model

When a research vessel passes over a herring school or layer, the herring may avoid the vessel by swimming downwards and horizontally. The fish may also change its orientation, which may alter its mean target strength. Consequently, the echo abundance measured by the relatively narrow echo sounder beam does not always reflect the true density of the school. The fish reaction is strongest in the upper parts of the water column. This avoidance behaviour has been quantified in several experiments where a stationary, submerged transducer has been used to measure the changes in echo abundance during the passage of a survey vessel. In this paper two approaches for correcting the echo abundance for avoidance are investigated. The first approach is to correct the echo abundance in each depth layer separately; the second is to correct the total echo abundance, letting the correction depend on the mean depth of the fish at passing. In both approaches generalized linear models are fitted to the experimental data. Since the parameters are estimated with uncertainty, this uncertainty can be taken into account when the fitted models are used for correcting standard survey data