Evaluation of a visual census method for reefs fishes at Tektite Reef, Virgin Islands National Park, St. John, US Virgin Islands: Determination of optimal sample size

Friedlander, Alan; Beets, Jim; Miller, Jeff

During investigations conducted since 1984, a random point visual census technique for fishes has been used in Virgin Islands National Park. An " over sampling" effort of the reef fishes at Tektite Reef was conducted from 9 to 11 July 1999 to determine the optimal sample size for point counts conducted on that reef. A total of 58 visual point counts was conducted on edge (n equals 13) and platform habitats (n equals 45) and compared to our typical haphazard sample of 18 censuses. Our typical sample size of 18 censuses at Tektite Reef appears adequate to detect changes in number of species and number of species and number of individuals for the entire assemblage and for many trophic guilds at a 20 percent level. Continued stratification by edge and platform microhabitats seems appropriate based on differences in biomass and detrended correspondence analysis results for assemblages structure. Despite high coefficients of variation for some parameters and guilds, our current sampling protocol for Tektite Reef can detect an ongoing decline of 5 percent in nine years. Increasing alpha levels to 0.20 would improve the ability to detect these changes and is consistent with the precautionary approach to management. For number of species, number of individuals, and biomass there were no significant differences between values obtained using random vs. haphazard sampling. From these results, continues haphazard sampling appears to be preferable to random sampling due to the time and cost involved in the latter sampling design. This study was conducted in order to evaluate the validity/accuracy of prior data collections at Tektite Reef and determine the optimal sampling size for future work at this site. It is also meant to serve as a framework to develop an optimal sampling protocol for the National Park Service for the island of St. John and other locations to evaluate coral reef fisheries resources. In order to address the issue of number of sites and number of samples at a broader spatial scale, a more intensive sampling effort will need to be conducted on other reefs and at a variety of reef types. Abstract provided by author.