Perryman and Lynn: Stock and school structure of Stenella coeruleoalba 



123 



30' 



20' 



10' 



10' 



20' 



160° W 



150° 



120° 



1 10" 



90» 



80° W 



Figure 1 



Distribution of schools of striped dolphin, Stenella coeruleoalba, (dark circles), from which data were taken for 

 this report. Boundaries for northern and southern stocks were taken from Perrin et al. 1985. 



camera mounted below the fuselage of a Hughes 

 500D helicopter that was launched from the NOAA 

 Ship David Starr Jordan. This photographic sam- 

 pling was part of a long-term research effort con- 

 ducted by NMFS to monitor trends in abundance of 

 dolphin populations in the ETP (Holt and Sexton, 

 1989; Wade and Gerrodette, in press). 



The reconnaissance camera was equipped with a 

 very fast, medium focal length lens (152 mm) and a 

 forward image motion compensation system that 

 eliminated the blur normally found in images taken 

 from a low altitude, high-speed platform. We used 

 Kodak Plus-X Aerecon II (thin-base) film, exposed 

 through a medium yellow filter, throughout the experi- 

 ment. This filter significantly reduced the amount of 

 blue light reaching the film, thus enhancing both the 

 contrast and resolution of our photographs. 



The observer sitting in the right front seat of the 

 helicopter triggered the camera, controlled cycle rate 

 and shutter speed, and adjusted the forward motion 

 compensation system. As each firing pulse was sent 

 to the camera, a data acquisition system recorded 



the time that the image was captured and an alti- 

 tude reading from the helicopter's radar altimeter. 

 To check for accuracy in our recorded altitude data 

 (A J, we photographed calibration target arrays and 

 compared altitude calculated from measurements of 

 these known distances with recorded altitude (see 

 Perryman and Lynn, 1993). 



We found a consistent bias in A r and used the lin- 

 ear regression equation shown below to calculate a 

 corrected altitude (AJ for each photograph used in 

 this report. 



A c = (A r ) 1.013 - 33.755 (r 2 = 0.993) . 



Length determination 



We reviewed the images of 88 schools of striped 

 dolphins photographed from 1987 through 1990 and 

 selected the images of 28 schools that provided the 

 best combination of image clarity and water pen- 

 etration. From this sample, we selected the photo- 

 graphic pass over each school that captured the larg- 

 est number of dolphins swimming parallel to and 



