Improving the statistical power of length estimates of reef fish: a comparison of estimates determined visually by divers with estimates produced by a stereo-video system

We calculated the power of visual length estimates by novice and experience d scientific SCUBA divers and estimates generated by a stereo-video system to detect changes in the mean length of three common species of reef fish f rom New Zealand. Length estimates from a stereo-video system had much great er power for blue cod (mean length=33.1 cm., range 19.5-50.1 cm.) and snapp er (mean length=31.7 cm., range 23-71 cm.). For a third species, red cod (m ean length=42.5 cm., range 13-74 cm.), the statistical power of diver and s tereo-video estimates was much less for an equivalent number of samples owi ng to the greater variation in the true mean length of red cod recorded at different sites. At 90% power, a stereo-video system detected a 15% (simila r to5-cm) change in the mean length of blue cod with 63% less samples (10) than those required by the experienced scientific divers (27). Novice scien tific divers required 28 samples.