A comparison of the precision and accuracy of estimates of reef-fish lengths determined visually by divers with estimates produced by a stereo-video system

Visual estimates of reef-fish length are a nondestructive and useful way of determining the biomass, mean length, or length frequency of reef fish. Co nsequently, visual estimates of reef-fish length are often an important com ponent of reef-fish monitoring programs, many of which increasingly use vol unteers. We compared estimates of the length of plastic fish silhouettes de termined visually by experienced scientific and novice SCUBA divers. Novice divers showed a similar level of accuracy (mean error: 2.3 cm) to that of experienced scientific divers (mean error: 2.1 cm). Significant improvement s in accuracy and precision were provided by a stereo-video system (mean er ror: 0.6 cm). After minimal training in the use of hardware and software, v olunteers can obtain a high degree of measurement accuracy and precision wi th a stereo-video system, allowing them to assist with monitoring reef-fish lengths.