Quantitative Measurements of Euphausiids Using a 120-kHz Sounder and Their in situ Orientation

Abstract

Biomass and density of euphausiids were estimated using a 120-kHz sounder during 2 y in the Gulf of St. Lawrence. Simultaneously, biological samples were taken with two types of multiple opening and closing nets: MOCNESS-type and BIONESS. Correlations for biomass and density estimates between the acoustic and biological data range from 0.391 to 0.791. Acoustic data showed that a high percentage of euphausiids avoided the MOCNESS-type net during the day. A comparison of the relationship between biological and acoustic data for both years showed that the BIONESS sampler captured euphausiids more efficiently during both day and night. Target strength of the euphausiids estimated from acoustic and biological data showed that it was lower than would be expected if the animals were oriented horizontally. Photographs taken from the BIONESS sampler at depths having acoustic scattering layers showed that the average orientation of euphausiids changed with time, the average orientation being closest to horizontal during daytime. Orientation changed from 27° from the horizontal at 14:00 to 51° during 02:00. The effect of orientation on target strength is discussed.Key words: euphausiids, acoustics, orientation, biomass, underwater photography, multiple net samplers, zooplankton