INTERDECADAL CHANGE IN REEF FISH POPULATIONS AT FRENCH FRIGATE SHOALSAND MIDWAY ATOLL, NORTHWESTERN HAWAIIAN-ISLANDS - STATISTICAL POWER IN RETROSPECT

Reef fish faunas were compared between surveys conducted more than a d ecade apart at each of two isolated and nearly pristine oceanic atolls (French Frigate Shoals [FFS] and Midway Atoll) in the northwestern Ha waiian Islands. Species composition, assemblage structure (density ran kings), and population densities were compared to test the hypothesis that reef fishes had declined during a period of lower oceanic product ivity in the central North Pacific. Within each of two principal habit ats (barrier and patch reefs) at each atoll, species composition and r ank densities differed little between sampling periods. Densities, how ever, generally declined by about one-third for many numerically domin ant species and for tars pooled into functional categories (trophic le vels, feeding guilds). Patterns of temporal change were partly confoun ded by distributional shifts between barrier and patch reef habitats a t FFS, perhaps reflecting indirect effects of storm disturbance on ben thic algal habitat. Such effects, together with related changes in cli mate and oceanic productivity on an interdecadal time scale, might hav e been responsible for the apparent declines in reef fish densities al both atolls. Temporal patterns were clearer at Midway Atoll, where ch anges in benthic algae were not evident. Our observations indicate tha t the statistical power necessary to detect changes in the population densities of reef fish species that exhibit large temporal fluctuation s in numbers, particularly on oceanic islands, is generally low. Power to detect twofold changes may be adequate, however, if taxa are poole d into functional categories.