COMPARING VISUAL COUNTS OF CORAL-REEF FISH - IMPLICATIONS OF TRANSECTWIDTH AND SPECIES SELECTION

We compared fish count data derived simultaneously from visual censuse s of transects of different width for a wide range of species on 3 cor al reefs within the Great Barrier Reef, Australia. The relationships o f count data from non-pomacentrid families between transects 10 and 5 m wide and pomacentrid genera between transects 2 and 1 m wide were fo und to be strongly Linear, with correlation coefficients of 0.94 and 0 .98 respectively. The strength of both relationships was not significa ntly compromised by differences among selected taxa or benthic habitat s. As such, comparison of data collected from different transect width s was considered feasible after application of conversion factors deri ved from the Linear relationships. We suggest that the use of experime ntally derived conversion factors may be applicable for other research ers faced with problems of data comparison across studies where differ ent transect dimensions have been used. Density estimates from wider t ransects were 22 to 26 % less than estimates from narrower transects f or the majority of taxa. The precision (coefficient of variation) of d ensity estimates did not vary significantly between transect dimension s, however precision varied greatly between species, genera and famili es, with values ranging from 0.18 to 0.85. Transect width and the beha viour, habitat specificity and number of target fish taxon are implica ted as important contributors to variation in the accuracy and precisi on of visual census data. We re-stress the need for methodological pil ot studies as precursors to visual census studies of reef fish.