Accuracy and Precision of Visual Estimates and Photogrammetric Measurements of the Length of a Small-bodied Fish

We assessed the accuracy and precision of visual estimates from two divers and photogrammetric measurements from a diver-operated stereo-video camera system for determining the length of Saratoga Springs pupfish Cyprinodon nevadensis nevadensis (12-36 mm total length) under controlled conditions. Visual estimates by two divers differed significantly from true fish length (P < 0.001) but were not significantly different from each other (P = 0.42). Levels of accuracy and precision were similar to those previously reported for visual estimates by divers. On average, the two divers underestimated fish length by 2.74 mm (11%) and 2.93 mm (12%). The magnitude of underestimation error increased with fish length. Photogrammetric measurements from a stereo-video camera system were more accurate and precise than diver estimates of fish length. Little to no bias was evident (mean error = 0.05 mm), and the level of precision (coefficient of variation of the difference between observed length and true length) was 4.5% for the photogrammetric measurements compared with 10% and 11% for the two divers' estimates. In comparison with underwater visual surveys, surveys that use a stereo-video camera system may increase the consistency of long-term data sets and improve resolution to detect important length differences in small-bodied fishes. Managers must remain careful to avoid or correct sampling biases, which can affect underwater visual surveys and stereo-video surveys alike.