Correcting for vessel avoidance in acoustic-abundance estimates for herring

Hjellvik, V., Handegard, N. O., and Ona, E. 2008. Correcting for vessel avoidance in acoustic-abundance estimates for herring. – ICES Journal of Marine Science, 65: 1036–1045. When a research vessel passes over a school or layer of herring (Clupea harengus), the fish may avoid the vessel by swimming downwards and horizontally. If the orientation of the fish is changed in this process, the mean target strength may also be altered. Consequently, the echo abundance measured by the relatively narrow echosounder beam does not always reflect the true density of the school in the undisturbed situation. 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. Two approaches for correcting the echo abundance for avoidance are presented. The first is to correct in each depth layer separately, but this does not account for diving during vessel passage. The second is to correct the total echo abundance based on the mean depth of the fish at passage. Generalized linear models are fitted to the experimental data in both approaches. The parameters are estimated with uncertainty, which is taken into account when the fitted models are used for correcting standard survey data. The models were fitted to data from various experiments conducted during the period 1996–2004. The avoidance response differed strongly between experiments, indicating that correction factors estimated from one specific experiment should not be used uncritically in a standard correction procedure.