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Fleeing fish could skew scuba surveys

diver survey.jpgMuch 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

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