In situ target strength and behaviour of northern krill (Meganyctiphanes norvegica)

Target strengths (TS) of individual krill were measured directly in situ. The study focused on scattering layers consisting mainly of krill, using a submerged echosounder to resolve individual krill at their daylight depths. Individuals were tracked for extended periods, and the method was capable of discerning differences in TS and behaviour within seemingly homogenous scattering layers. Krill size increased with depth. This was sometimes, but not always, reflected in TS. The relationship between TS and size broke down when larger krill deeper in the water column were prone to more upward swimming than shallow-living specimens, which reduced TS, but there were also unexplained variations in TS by depth. The variability of TS within the krill tracks was generally high, with deviations routinely exceeding 10 dB, further emphasizing the dependence of krill TS on behaviour. Therefore, although measured values of TS were within the ranges previously reported for krill, it was difficult to apply fixed TS-size relationships. The study underscores the need to determine the impact of behaviour on TS, and shows that appropriate methods to do so are now available.