Comparison of density estimates derived from strip transect and distance sampling for underwater visual censuses: a case study of Chaetodontidae and Pomacanthidae

Despite its wide use in terrestrial ecology, distance sampling is as yet ra rely used in underwater visual censuses. The present study attempts to comp are density estimators based on distance sampling and on strip transects. T hree stations with increasing densities of Chaetodontidae and Pomacanthidae were sampled twice by two divers of unequal experience, using two differen t transect types. A total of 96 transects and 2970 records of Chaetodontida e and Pomacanthidae were analysed. Nine estimators based on distance sampli ng were calculated and only the best fit (DT estimator) was kept for compar ison with other estimators. These were either based on the average distance of the fish to the transect (AD estimator), or a 3-m- or 5 m-wide strip tr ansect estimator (FW3 and FW5, respectively). There were no significant dif ferences between the means found by DT, AD and FW3. Lower density estimates were given by FW5 in all cases. FW3 and FW5 did not detect several signifi cant differences between stations which were otherwise detected by DT or AD . The number of transects needed to detect a significant difference between stations was four to ten times higher with FW3 or FW5 than with DT or AD. Diver experience was found to be a significant factor in density estimates. However, this factor was less important than the choice of the density est imator Transect type or the day of sampling had no consequence for the esti mates. The distance distributions of fish were divided into three different patterns which may be explained by a combination of detectability function and a behavioural component. (C) 1999 Ifremer/Cnrs/Inra/Ird/Cemagref/Editi ons scientifiques et medicales Elsevier SAS.