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

Abstract

AbstractWe 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.Received August 5, 2010; accepted December 6, 2010