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

Abstract

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.

Received August 5, 2010; accepted December 6, 2010

ACKNOWLEDGMENTS

We thank Jim Seager for assistance with stereo-video camera system design and operation. We thank the Devils Hole Dive Team for their support and assistance in conducting trials with the stereo-video camera system, and we are grateful to Stan Hillyard and Zane Marshall for providing visual estimates of fish length. We also thank Paul Barrett, Sean Harris, and Stephanie Kyriazis for help in the field. Steve Parmenter (California Department of Fish and Game), Rob Fulton (Desert Studies Center), and Debra Hughson (Mohave National Preserve) provided important assistance with permitting and site logistics. Funding was provided by a donation from the Offield Family Foundation to the Death Valley Fund and by Death Valley National Park. Dive Rite provided SCUBA equipment support through the Death Valley Natural History Association. We are grateful to Henry Golas, David Blacker, and Candace Lieber for their help in enabling this research. The use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.