Quantifying relative diver effects in underwater visual censuses

Abstract

Diver-based Underwater Visual Censuses (UVCs), particularly transect-based surveys, are key tools in the study of coral reef fish ecology. These techniques, however, have inherent problems that make it difficult to collect accurate numerical data. One of these problems is the diver effect (defined as the reaction of fish to a diver). Although widely recognised, its effects have yet to be quantified and the extent of taxonomic variation remains to be determined. We therefore examined relative diver effects on a reef fish assemblage on the Great Barrier Reef. Using common UVC methods, the recorded abundance of seven reef fish groups were significantly affected by the ongoing presence of SCUBA divers. Overall, the diver effect resulted in a 52% decrease in the mean number of individuals recorded, with declines of up to 70% in individual families. Although the diver effect appears to be a significant problem, UVCs remain a useful approach for quantifying spatial and temporal variation in relative fish abundances, especially if using methods that minimise the exposure of fishes to divers. Fixed distance transects using tapes or lines deployed by a second diver (or GPS-calibrated timed swims) would appear to maximise fish counts and minimise diver effects.

Figure 1. Relative diver effects – fixed distance and immediate return.

Relative diver effects on estimated reef fish densities comparing counts over a fixed distance 50 m transect and counts along the tape immediately after deployment. Values indicate the proportional decrease in abundance; asterisks represent significant differences using a Bonferroni corrected alpha-value (p<0.006). The parrotfishes formerly in the family Scaridae are now a distinct lineage within in Labridae .

Figure 2. Relative diver effects – fixed distance and 5 minute waiting period.

Relative diver effects on estimated reef fish densities comparing counts over a fixed distance 50 m transect and counts after a 5 minute waiting period. Values indicate the proportional decrease in abundance, asterisks represent significant differences using a Bonferroni corrected alpha-value (p<0.006).

Figure 3. Principal components analysis showing species level differences in mean fish abundance between the three UVC techniques.

(A) All 3 transects are clearly separated along PC1. (B) Vector plot showing the fish species driving patterns in relation to the three transects shown in (A).