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Body sizes at protogynous (sequential, female-to-male) sex change are described for 8 species of labroid fishes (4 labrids, 4 scarids) based on in situ diver observations of the sizes of sex- related color morphs for populations at atolls in the NW Hawaiian Islands. Sizes at sex change are deduced from the median overlap in size distributions of initial phase (IP: female or female and pre- cocious male) and terminal phase (TP) male individuals for each labroid species and from the median sizes of transitional morph individuals of the endemic, spectacled parrotfish Chlorurus perspicillatus. Relative (to maximum) sizes at sex change vary significantly among 4 major species and, within these species, among the 6 reef populations, a finding that contrasts with a recent model of invariant rela- tive size at sex change in protogynous fishes. Overall median size at sex change is compared to the density of a large carangid (giant trevally Caranx ignobilis, the biomass-dominant apex predator) and to that of the respective labroid species population at each atoll. Size at sex change is inversely related to the density of giant trevally but not meaningfully related to the density of the respective labroid population. The size distributions of the labroids—like those of all other prey reef fishes—are strongly skewed toward smaller and larger fish at atolls with higher and lower densities, respectively, of giant trevally. Altogether these observations strongly suggest that higher mortality from predation at greater predator densities, rather than slower growth rates under greater intraspecific competition at higher prey densities, is the cause of the observed differences among atolls in prey attributes. We suggest that prey body size distribution can potentially provide a sensitive measure, complementary to predator density, for assessing changes in the abundance of apex predators on coral reefs. Pheno- typic indices of size at sex change from adult female to male in sexually dimorphic labroids with strik- ing differences in coloration (that facilitate underwater observations) have great potential as non- destructive alternatives to gonad examination requiring specimen sacrifice. Labroid color change thus represents a potentially useful index for assessing changes in the functional structure of coral reef fish assemblages and reef ecosystems.

Size at sex change in protogynous labroids, prey body size distributions, and apex predator densities at NW Hawaiian atolls