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. Consequently, visual estimates of reef-fish length are often an important component of reef-fish monitoring programs, many of which increasingly use volunteers. We compared estimates of the length of plastic fish silhouettes determined 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 improvements in accuracy and precision were provided by a stereo-video system (mean error: 0.6 cm). After minimal training in the use of hardware and software, volunteers can obtain a high degree of measurement accuracy and precision with a stereo-video system, allowing them to assist with monitoring reef-fish lengths.