Transect techniques for censusing reef fishes, and the sources of bias inherent in them are considered. A technique, derived from aerial survey methods, is demon- strated to correct a bias in density estimates due to the width of the transect being censused. This bias is sufficient on a transect 1 m wide to underestimate density by 11.1- 26.7% for five species or species groups examined. The bias is still greater on wider transects. Because this bias varies in degree among species, comparisons among spe- cies should not be made using uncorrected transect data. Comments are made on other probable sources of bias in transect data, and on ways of minimising bias when mak- ing visual transect censuses.
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