Much of our knowledge of coral reefs is based on surveys by scuba divers. But what if those divers are scaring away the fish they’re trying to count?

A new study in PLoS One suggests that the so-called ‘diver effect’ drives away an average of half the fish in a given area, with 70% of individuals from some species fleeing.

A popular method for assessing reef populations is an underwater visual census or UVC. In this, one diver lays a tape measure, and then another follows along behind for a set distance and records everything he or she sees.

David Bellwood, a marine biologist at James Cook University in Townsville, Australia, and his team ran a series of UVCs on the Great Barrier Reef. First, one diver swam in advance of a second laying a 50 m tape measure behind them. Then the divers immediately re-censused the same transect. After this they waited for five minutes and counted fish along the tape again.

On average there was a 52% decrease in the mean number of fish recorded between the first and the second surveys. Waiting for a bit led to some recovery, but there was still a 27% drop from the first survey. Some species were more affected than others, with 70% of parrotfish bolting after the first pass.

“Our results suggest that the censusing of fishes after laying a measuring tape can have a profound effect on fish counts,” write Bellwood et al. “While this may not be detrimental for analyses of relative abundances within a single study, it may severely limit our ability to combine visual census data in meta-analyses, or to compare values among studies.”

Being aware of these effects and taking precautions such as having one diver record while the other lays a tape should assist conservationists in dealing with these problems, they note.

Image: Steve Prutz / Wikimedia Commons via Flickr