Bias introduced by the non-random movement of fish in visual transect surveys

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 simulation program Reefex was used to examine the relationship between the speed and approach angle of fish, and the degree of bias introduced 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 introduced a positive bias which increased linearly with fish speed. Fish moving in the direction of the diver created a negative bias, fish counts decreased linearly until fish speed matched that of the diver. This minimum value reflected the immediately visible portion of the entire transect that could be surveyed instantaneously by the diver when the 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.