Somerton et al.: Quantifying the behavior of fish in response to a moving camera vehicle 
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Deployment - mwfsc test07 Current track Number - 27 set current track 
Frame number - 3 Frame time - 
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Figure 2 
Screen shot of a session in Stereo Tracker (Williams et al. 5 ), the software used for tracking a school of vermilion 
snapper ( Rhomboplites aurorubens ) in 2014 in the northeast Gulf of Mexico. The upper 2 frames show the paired 
images from the previous time interval; the lower frames show the paired images for the current time interval. All 
previously measured fish are indicated with red diamonds, and the fish being tracked is indicated with a green circle 
to ease identification of the same fish across time intervals. 
the vehicle frame (0.6 m) to the y coordinates. The dis- 
tal and proximal edges of the vehicle sampling path, 
spanning the width of the area for counting fish, were 
then calculated as the center line path (± the approxi- 
mate half width of the estimated field of view of the 
vehicle [~3.7 m]). The sampling path was used, in con- 
junction with the measured fish positions on the hori- 
zontal plane, to determine whether vermilion snapper 
occupied the sampling path prior to the arrival of the 
camera vehicle and, if so, when they left the path. 
Individual velocity, mean speed, and direction of swim- 
ming Individual swimming velocity (Vj) in each coor- 
dinate direction (V x j, Vyj, V z j) was calculated for each 
fish (indexed by j) during each second as the difference 
in the smoothed beginning and ending positions divid- 
ed by the elapsed time (0.4 s). Mean individual speed 
and direction were calculated from the velocity vectors 
both in 3 dimensions, as well as from projections on the 
x-y plane and the z-axis separately to better visual- 
ize the lateral and vertical movements associated with 
avoidance behavior. Mean swimming speed (S) during 
each second was calculated first by determining the in- 
dividual speeds from the estimated velocity vectors and 
then by determining the means of these values; that is, 
S = sqrt(V?j + V* j + V z 2 j , (1) 
where V z was omitted in this equation for horizontal 
speed, and V x and V y were omitted for vertical speed. 
Mean swimming heading, on the x-y plane, was calcu- 
lated as the mean of the individual swimming head- 
ings, expressed in degrees counter clockwise from the 
direction of vehicle travel, after correcting for the slight 
misalignment between the direction of travel and the 
x-axis of the camera coordinate system. 
Group velocity, speed, and swimming polarity Group 
velocity (U) on each coordinate axis ( U „ U y , U z ) was 
calculated each second from the difference in the mean 
positions of the 25 fish between the second and fourth 
frames, divided by the elapsed time. Group speed (S g ) 
