Three-dimensional swimming behavior of individual zooplankters: observations using the acoustical imaging system FishTV

There is increasing recognition that three-dimensional tracks of individual zoo-plankters are needed for studies in biological oceanography, including, for example, the role of individual behavior in patch formation and maintenance. The three-dimensional acoustical imaging system FishTV provides a means of non-invasively examining zooplankton swimming behavior. The system forms a set of 64 acoustic beams in an 8 by 8 pattern, each beam 2° by 2°, for a total coverage of 16° by 16°. The 8 by 8 beams form two dimensions of the image; range provides the third dimension. The system has 20 kHz of bandwidth and operates at a center frequency of 445 kHz. Observations of zooplankton swimming at 37 m depth were made from the research platform RP “FLIP” at a site in the San Diego Trough, 28 km south-west of San Diego, California, USA. In a 1-min-long sequence of 60 images, acquired at 2101 h, 24 March 1993, 314 plankters were tracked for lengths of time ranging from 2 to 13 s. The animals ranged from −83.0 dB re 1 m 2 to −57.7 dB target strength. Movement of each animal was divided into directed and random components. The directed component was attributed to currents, waves and (possibly) a slight vertical migration. The random component was attributed to random behavior. Analysis of random velocities and turning behavior showed an inverse relationship between swimming speed and path curvature. Turning rates were approximately 1.2 rad s −1 regardless of the size or speed of the animals. Mean swimming speeds were positively correlated with distance from a pair of bright lights (part of a video system), while mean path curvatures were negatively correlated. These observations are consistent with a model for swimming behavior that leads to aggregation.