Aj. Cheal et Aa. Thompson, COMPARING VISUAL COUNTS OF CORAL-REEF FISH - IMPLICATIONS OF TRANSECTWIDTH AND SPECIES SELECTION, Marine ecology. Progress series, 158, 1997, pp. 241-248
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.