FISHERY BULLETIN: VOL. 80. NO. 2 



SHIP 



Figure 1.— Path of helicopter in front of ship during search 

 phase of study. 



549 m). Maximum altitude was determined by 

 the cloud ceiling. During each flight, two 

 scientific observers aboard the ship searched 

 independently with 20 X 120 mm binoculars for 

 dolphins. The observers were not in com- 

 munication with the helicopter and were gener- 

 ally unaware of its position because of its range 

 and because of their visual concentration on the 

 sea surface. The ship's speed was between 11 and 

 13 kn. 



Once a school was located, the helicopter re- 

 mained near the school to serve as a radar target 

 to fix the position of the dolphins relative to the 

 vessel. Each time the helicopter passed over the 

 school, we signaled the deck officer aboard ship 

 via radio to record our radar range and bearing. 

 These measurements from the ship were taken at 

 successive time intervals to enable tracking the 

 movement of the school. There was no indication 

 to us that the helicopter affected school behavior. 

 Indeed, the schools usually appeared to be 

 swimming calmly throughout the tracking, until 

 the ship approached to within a mile of the 

 dolphins. During this tracking phase, ship 

 course changes were minimized in order to 

 determine how closely the school would pass the 

 approaching vessel if not pursued. In some cases 

 the ship was turned so its projected track would 

 pass near the school, but course changes were 

 minimal thereafter. 



The shipboard radar used was a Decca-RM 

 1630. Its rated accuracy is to within 300 yd 

 (274 m) of range at a distance of 10 nmi (18.5 km) 

 and to within 1° of angular bearing. The radar 

 measurements were made by a trained deck 

 officer. 



At the end of the tracking phase the ship 

 approached closely or followed each school until 

 the observers aboard had completed their esti- 

 mates of school size and species composition. 

 Meanwhile, we continued to take aerial photo- 

 graphs (35 and 70 mm still and 16 mm movie) 

 and notes on school size and behavior that had 

 begun when the school was first sighted. The 

 movements and speeds of the schools as de- 

 scribed below do not refer to this last phase of the 

 operation. 



School movement and speed were calculated 

 whenever possible from relative motion plots 

 since such plots portray the situation as seen 

 from a ship. Required information for each plot 

 includes the time interval between radar fixes, 

 the course and speed vector of the ship, and the 

 relative motion vector of the school, as deter- 

 mined by the radar ranges and bearings (the 

 method is described by Bowditch 1966). These 

 data were then used to construct vector triangles 

 which were solved to get school speed vectors. 

 Distance (range) was measured in nautical miles 

 (nmi) and speed in knots (kn). The results were 

 checked by plotting the sequential, absolute 

 positions of the vessel and school from the data on 

 vessel speed and data on range and bearing of 

 ship to helicopter (school). School movement was 

 measured from this absolute plot, and speed 

 determined from the time interval between fixes 

 to give results that should be the same as those 

 obtained from the relative motion plots. When 

 the ship made a course change, disrupting the 

 relative position analysis for that time interval, 

 the absolute position plot was the only solution. 



A hypothetical example of a relative motion 

 plot is presented in Figure 2. The ship is at the 

 center (0) of the polar plot, proceeding straight 

 ahead (000° or top of plot). Sequential radar 

 ranges and bearings, from the moving ship to a 

 dolphin school, are obtained at 0800, 0815, ..., 

 and 0900 h. These fixes are plotted, and the line 

 connecting them shows the relative motion of the 

 school that is passing around to the right of the 

 ship. The actual swimming vectors of the school, 

 which produce this relative motion, can be ob- 

 tained by solving vector triangles such as that 

 shown at the center of the plot. For example, the 



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