3-DIMENSIONAL SWIMMING BEHAVIOR OF INDIVIDUAL ZOOPLANKTERS - OBSERVATIONS USING THE ACOUSTICAL IMAGING-SYSTEM FISHTV

There is increasing recognition that three-dimensional tracks of indiv idual zooplankters are needed for studies in biological oceanography, including, for example, the role of individual behavior in patch forma tion and maintenance. The three-dimensional acoustical imaging system FishTV provides a means of non-invasively examining zooplankton swimmi ng behavior. The system forms a set of 64 acoustic beams in an 8 by 8 pattern, each beam 2 degrees by 2 degrees, for a total coverage of 16 degrees by 16 degrees. The 8 by 8 beams form two dimensions of the ima ge; range provides the third dimension. The system has 20 kHz of bandw idth and operates at a center frequency of 445 kHz. Observations of zo oplankton 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, acquir ed al 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 l m( 2) to -57.7 dB target strength. Movement of each animal was divided in to directed and random components. The directed component was attribut ed 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 betwee n swimming speed and path curvature. Turning rates were approximately 1.2 rad s(-1) regardless of the size or speed of the animals. Mean swi mming speeds were positively correlated with distance from a pair of b right 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. (C) 1996 Internation al Council for the Exploration of the Sea