Comparison of krill (Euphausia superba) density estimates using 38 and 120 kHz echosounders

A series of observations, using a dual-frequency calibrated echosounder operating at 38 and 120 kHz, of a patch of Euphausia superba close to South Georgia in 1986 is described. Sea state is shown to cause significant noise close to the surface, but to cause no significant signal attenuation. There is a consistent difference of 5 dB between the signal levels at the two frequencies which is in line with the difference noted from independent observations, theoretical models and in studies on encaged aggregations of krill.