Three-dimensional acoustic measurements of zooplankton swimming behavior in the Red Sea

Abstract

Three-dimensional swimming trajectories of several hundred thousand individual zooplankters were measured using the 445-kHz acoustical imaging system FishTV during a three night moored deployment in the Gulf of Eilat, Israel. The sonar examined a 5-m3 volume at 27 m depth, in water 300-m deep. The image rate was 4 images/s. Targets were tracked from image to image using an automatic three-dimensional tracking algorithm. Over 14 000 targets remained in view for over 5 s, and their trajectories were used in subsequent analysis. Data from net tows indicated that most targets were euphausiids. The horizontal speeds of targets −75 dB and below were highly correlated with flow measurements from an S4 current meter. These targets also exhibited strong vertical motions, apparently due to internal waves. Estimated mean flow was subtracted from each trajectory to compute the swimming speeds of the animals themselves. These were generally much lower than the mean flow. During the night, variance in the vertical component of the flow-removed tracks was much greater than variance in the horizontal. However, the variance became more isotropic as dawn approached. A hop-and-sink foraging behavior offers one possible explanation for this.