Interdecadal Change in Reef Fish Populations at French Frigate Shoals and Midway Atoll, Northwestern Hawaiian Islands: Statistical Power in Retrospect

Reef fish faunas were compared between surveys conducted more than a decade apart at each of two isolated and nearly pristine oceanic atolls (French Frigate Shoals [FFS] and Midway Atoll) in the northwestern Hawaiian Islands. Species composition, assemblage structure (density rankings), and population densities were compared to test the hypothesis that reef fishes had declined during a period of lower oceanic productivity in the central North Pacific. Within each of two principal habitats (barrier and patch reefs) at each atoll, species composition and rank densities differed little between sampling periods. Densities, however, generally declined by about one-third for many numerically dominant species and for taxa pooled into functional categories (trophic levels, feeding guilds). Patterns of temporal change were partly confounded by distributional shifts between barrier and patch reef habitats at FFS, perhaps reflecting indirect effects of storm disturbance on benthic algal habitat. Such effects, together with related changes in climate and oceanic productivity on an interdecadal time scale, might have been responsible for the apparent declines in reef fish densities at both atolls. Temporal patterns were clearer at Midway Atoll, where changes in benthic algae were not evident. Our observations indicate that the statistical power necessary to detect changes in the population densities of reef fish species that exhibit large temporal fluctuations in numbers, particularly on oceanic islands, is generally low. Power to detect twofold changes may be adequate, however, if taxa are pooled into functional categories.