**Improving the statistical power of visual length estimates of reef fish: A comparison of divers and stereo-video
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*Euan Harvey, Departmen of Marine Scince, University of Otago
David Fletcher, Department of Mathematics and Statistics, University of Otago
Mark Shortis, Department of Geomatics, University of Melbourne
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The abundance, size frequency and biomass of reef fish are often used as environmental indicators of anthropogenic and natural impacts. Traditionally, programmes which monitor reef fish populations have depended on data collected by SCUBA divers. We question whether the data collected by these divers is sufficiently accurate and precise to detect changes with an adequate level of statistical power.

We calculate the statistical power of visual length estimates made by novice and experienced scientific SCUBA divers to detect changes in the mean length or biomass of three common species of fish from New Zealand coastal waters. We show that by using length estimates from a stereo-video system much greater statistical power can be obtained to detect changes in the mean length or biomass of two of the three species. Stereo-video is particularly effective where a small effect size is selected (5-10cm), or where low numbers of fish are recorded per sample.