Fish’n krill: A 38/120-kHz acoustic separation

Abstract

Volume backscattering data (Sv) at 38 and 120 kHz were collected in the Gulf of St. Lawrence, during daylight, in regions where krill scattering layers (SL) and pelagic fish (mainly capelin) were present. The SLs and fish aggregations were sampled with the Bioness zooplankton samples and with small-mesh pelagic trawls equipped with acoustic monitoring systems. Echointegration was performed on small bins of ∼5 m vertically and ∼50 m horizontally. Comparisons of Sv at the two frequencies indicated that krill SLs had stronger backscattering at 120 than at 38 kHz, as expected. The ratio was relatively constant within the same SL, but varied from one SL to the other in parallel with the size of krill. The backscattering from fish schools or fish aggregations or fish layers was stronger at 38 kHz than at 120 kHz. This permitted the separation of fish from krill on the echograms, even for complex cases where fish were embedded in krill SL or had structural patterns that mimic the signature of krill SL. The 38/120-kHz backscattering ratio appears therefore useful to separate krill from fish, or vice-versa, for better interpretation and estimation of these taxa individually.