How the acquired behaviour of commercial reef fishes may influence the results obtained from visual censuses

Distances between fishes and a transect line, as observed by underwater visual censuses, were analyzed according to fish behaviour, school size and fish size. The data were separated into four levels of human disturbance: very low, low, medium and high. These levels were a function of the people/km(2) of lagoon in different places. A total of 110 000 fish occurrences, representing 430 000 fishes belonging to 293 Indo-Pacific reef fish species were examined. The detectability of fishes varied with behaviour and disturbance levels. In particular, most species avoided the observer as disturbance rate increased, generating detectability functions that varied with both species and disturbance level. School size also played a role in the distance at which fishes were detected. For some species, detection increased as school size increased, for other species the opposite was observed. It is hypothesized that these different patterns could be due to relationships between fish size and school size, some species form large schools as juveniles but increasingly smaller schools as they grow. The detectability of fishes increased with size for most species. However, for a number of species the largest fishes displayed the opposite pattern. These results, when applied to underwater visual census methods, suggest that many studies using these methods might have drawn erroneous conclusions, especially when comparing reserves with fished areas or when comparing different species or different size classes. (C) 1998 Elsevier Science B.V.