Estimation of reef fish length by divers and by stereo-video - A first comparison of the accuracy and precision in the field on living fish under operational conditions

Using simultaneous measurements from an underwater stereo-video system the accuracy and precision of length estimates of reef fish made by three experienced diver scientists under field conditions is determined. The trial showed that under optimal conditions the divers' estimates were very accurate (mean error = 0.87 cm) but lacked precision (mean S.D. = 5.29 cm). The effects of the low precision are then demonstrated by using these field estimates to model the theoretical statistical power of the scientific divers to detect changes in the mean length of three species of fish from New Zealand coastal waters. The results suggest that the experienced diver scientists would have a much lower statistical power than stereo-video measurements to detect changes in the mean length, especially where low numbers of fish are recorded or where a research program aims to detect a change in the mean length of a fish population of 30% or less. (C) 2002 Elsevier Science B.V. All rights reserved.