HOW THE ACQUIRED BEHAVIOR OF COMMERCIAL REEF FISHES MAY INFLUENCE THERESULTS OBTAINED FROM VISUAL CENSUSES

Distances between fishes and a transect line, as observed by underwate r visual censuses, were analyzed according to fish behaviour, school s ize and fish size. The data were separated into four levels of human d isturbance: very low, low, medium and high. These levels were a functi on of the people/km(2) of lagoon in different places. A total of 110 0 00 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 spec ies avoided the observer as disturbance rate increased, generating det ectability functions that varied with both species and disturbance lev el. School size also played a role in the distance at which fishes wer e detected. For some species, detection increased as school size incre ased, for other species the opposite was observed. It is hypothesized that these different patterns could be due to relationships between fi sh size and school size, some species form large schools as juveniles but increasingly smaller schools as they grow. The detectability of fi shes increased with size for most species. However, for a number of sp ecies 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, es pecially when comparing reserves with fished areas or when comparing d ifferent species or different size classes. (C) 1998 Elsevier Science B.V.