Use of Diver Visual Estimates and Photogrammetric Measurements from a Stereo-Video Camera System for Determining 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 length of Saratoga Springs pupfish Cyprinodon nevadensis nevadensis varying from 12 to 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, as measured by the coefficient of variation of the differences among observed and true lengths, was 4.5%, compared to 10 and 11% for the two divers.   In comparison to underwater visual surveys, stereo-video camera system surveys may increase the consistency of long-term datasets and improve resolution to detect important differences in the length of small-bodied fishes.  Managers must remain careful to avoid or correct sampling biases which can affect underwater visual surveys and stereo-video camera system surveys alike.