422 
Fishery Bulletin 99(3) 
the levels of dissolved oxygen within the transport tank 
to ease the physiological stress associated with strenuous 
activity. Fresh seawater was continuously pumped into 
the tank to eliminate metabolic waste and maintain wa- 
ter quality. The tank had a padded top that minimized 
the potential for injury to fish during transport and re- 
duced water loss resulting from boat movement. Viewing 
ports in the padded top allowed observation of the spec- 
imens while in transport. Physical contact with individu- 
al tuna was minimized, and when contact was necessary, 
only well-padded devices were employed. Transport time 
of individual fish was variable, but generally less than 3 
hours. The number of fish simultaneously transported in 
the tank was controlled to avoid crowding and the deple- 
tion of dissolved oxygen. The tuna were recovered from the 
transport tank by using a cloth stretcher and released in- 
to the net-pen enclosure. Divers in the enclosure released 
the tuna individually, ensuring that they recovered prop- 
er spatial orientation upon release. Released specimens 
were assimilated quickly into the existing shoals of cap- 
tive fish. 
Video cameras (Hi8) in waterproof housings were used 
to record school structure of the captive tuna. A shutter 
speed of 1/2000 second was used to optimize the resolution 
of still frames within the constraint of available subsur- 
face (<15.3 m depth) light. The automatic focus feature of 
the camera was disabled to avoid rapid fluctuations in fo- 
cal depth from the intended subject to particulate matter 
suspended in the water column. Observations of schooling 
tuna were recorded with stand-alone cameras mounted 
inside the enclosure and with hand-held cameras during 
observation dives. Cameras mounted to the floating net 
pen on specialized polyvinyl chloride pipe structures were 
stabilized with elastic compensators to lessen movement 
caused by wave energy. Mounted cameras were positioned 
to provide bead-on and subsequent perpendicular views 
in relation to the axis of motion of a school. This filming 
strategy allowed a more accurate observation of the char- 
acteristics of the school in three dimensions. Schooling 
was recorded more efficiently by using hand-held camer- 
as during dives than through use of mounted cameras. 
During a dive, the entire internal volume of the enclosure 
was often visible from a given point, allowing a diver to 
anticipate the path of travel of a tuna school and to re- 
position the camera to attain the best possible images. 
Regardless of the apparent ease of tuna schools, divers po- 
sitioned themselves against the enclosure’s external wall 
to ensure minimal behavioral modification in the filmed 
schools. Video recordings from both mounted cameras and 
from held cameras were used in analysis. 
Laboratory video analysis 
Video recordings of ABT schools were reviewed, and 74 
incidences of schooling in which an entire target school 
was visible were used for further analyses. All observa- 
tions took place during daylight hours (0900 and 1600 h) 
and none took place during or within one hour of feeding 
events. An image analysis system that allowed digitiza- 
tion of points directly from a (paused) video source was 
employed for more precise quantification of school charac- 
teristics (Fig. 2). The system employed a video scan con- 
verter that overlaid the image output from a computer 
video source (640x480 pixel resolution) upon the video 
source image. The video scan converter allowed the user 
to select a color in the computer video overlay to be made 
transparent, revealing the underlying video signal (Fig.2). 
When employed in conjunction with an image analysis 
software package (SigmaScan Pro) on a personal com- 
puter, all the features of the image analysis software could 
be used on any still (paused) video source image without 
the use of a video frame-grabber. The position of each indi- 
vidual fish and school depth intervals were delineated by 
using the graphic capabilities and the Cartesian coordi- 
nate system of the image analysis software. The positions 
of individual fish in the school could be marked while 
the recording was advanced or reversed frame by frame, 
allowing the identification of poorly illuminated fish or 
fish that may have been hidden by individuals in the fore- 
ground of the school. 
