Non-random movement has been observed in a number of reef fish species
but its effect on visual counts has not been previously examined. A s
imulation program Reefex was used to examine the relationship between
the speed and approach angle of fish, and the degree of bias introduce
d in estimates of fish numbers from visual transects. Fish approaching
at right-angles to the direction of the transect did not introduce a
bias regardless of their speed. Fish approaching against the diver int
roduced a positive bias which increased linearly with fish speed. Fish
moving in the direction of the diver created a negative bias, fish co
unts decreased linearly until fish speed matched that of the diver. Th
is minimum value reflected the immediately visible portion of the enti
re transect that could be surveyed instantaneously by the diver when t
he survey began. Changes in the effective area surveyed determine bias
. An equation is presented which relates bias to fish speed, angle of
approach, diver speed, transect length and visibility.