A VIDEO SONAR AS A NEW TOOL TO STUDY MARINE MAMMALS IN THE WILD - MEASUREMENTS OF DOLPHIN SWIMMING SPEED

The use of a multibeam sonar for fine-scale studies of the foraging ac tivity of bottlenose dolphins (Tursiops truncatus) was tested in a coa stal environment where a group of 14 individuals is known to be reside nt. The sonar was set on a rubber boat and the signals were received c ontinuously as the dolphins were foraging in a pass swept by strong ti dal currents. These signals were recorded in HI 8 video standard. The system produced real-time undistorted images of the dolphins and of th eir movements within the limits of the sonar beam (15 degrees vertical width, 90 degrees horizontal width, range set at 50 m). The average s wimming speed relative to the bottom was 1.8 m.sec(-1) but swimming sp eed relative to the water was 2.2 m sec-l when allowance was made for the average 1.2 m.sec(-1) tidal current. Maximum swimming speed relati ve to the water was as high as 4.8 m.sec(-1). It is estimated from stu dies of swimming energetics that continuously foraging in a current ve in Rowing at 1.2 m.sec(-1) would represent only small additional costs compared to foraging our of these currents. However, previous observa tions of the same pod.