The effects of behaviour on the acoustic target strength of capelin (Mallotus villosus) and implications for acoustic abundance estimation

Abstract

The general objective of this thesis was to collect improved knowledge about the target strength (TS) of capelin (Mallotus villosus) through experimental studies, as well as to undertake adequate observations of behaviour of capelin during surveying situations, including the effect of different depth distributions and of vessel avoidance. Pre- and post-spawning capelin from the Balsfjord and the Barents Sea stock were observed in a net pen and in a submersible rig with a calibrated 38-kHz scientific splitbeam echo sounder. The net pen experiment showed that the acoustic TS of capelin depended on swimbladder length. In females, TS tended to be negatively influenced by condition. The experiments in the submersible rig demonstrated that the TS of capelin depends on the tilt-angle distribution and ambient pressure. The avoidance reactions of Barents Sea capelin to Norwegian research vessels were studied by means of the Bergen Acoustic Buoy (BAB), which was equipped with a 38 kHz echo sounder. BAB experiments were carried out in the capelin’s feeding area in the Barents Sea in the autumn, and on its spawning grounds in Varangerfjord in North Norway in early spring. There was no significant influence of the avoidance reactions of capelin to survey vessels on the volume scattering coefficient, either in the feeding area or on the spawning grounds. However, there were indications of changes in the centre of depth distribution (diving) before the pass, especially in dense concentrations on the spawning grounds. The findings of this thesis indicate that differences in vertical distribution of capelin in different areas and seasons, or between years with differences in oceanographic conditions or predation, may bias absolute estimates of stock abundance of capelin if the depth distribution of the fish is not taken into consideration. It is therefore recommended that the effects of ambient pressure are included in the TS equation used in surveys in order to make the acoustic stock size estimate of capelin in the Barents Sea a better absolute estimate of the stock abundance.