Effects of observer swimming speed on sample counts of temperate rocky reef fish assemblages

Effect on estimates of fish abundance of speed at which an observer swlms an underwater transect was studied for a temperate rocky reef fish community in eastern Australia. Observer speed had a cntical though differential effect upon sample counts. Small or cryptic fishes were severely underestimated at relatively fast observer speeds because not enough time was available to search thoroughly for them. Some highly mobile species, however, were overestimated at slow speeds, due to their movement across the transect during counting penods, or to inadvertent double counting. Rotenone samples confirmed that, regardless of observer speed, highly cryptic species are largely overlooked by visual census It is suggested that observer speed should be standardzed in survey designs. Other suggestions for improving precision and accuracy of visual surveys are discussed.