Differences in fish-assemblage structure between fished and unfished atolls in the northern Line Islands, central Pacific

We describe the abundance, biomass, size composition, and trophic structure of fish assemblages of shallow (10 m depth) fore reef habitats at 2 US Pacific atolls (Kingman, Palmyra) and 2 Kiribati-owned atolls (Tabuaeran, Kiritimati) in the northern Line Islands. Our characterization spans several coincident gradients (in human habitation and exploitation, latitude, and nutrient upwelling) from uninhabited, presently unfished, and oligotrophic Kingman to relatively densely populated, fished, and equatorially upwelled Kiritimati. Major findings are most consistent with direct effects of extraction on large-bodied predators and indirect effects on lower-level assemblage structure. Fish assemblages at Palmyra and especially Kingman atolls were characterized by high total standing biomass, large average body sizes, a preponderance of apex predators and other piscivorous fishes in an inverted biomass pyramid, few and small planktivorous fishes, and herbivores dominated by non-territorial species. Median body sizes at color change from initial to terminal phase (an index of sex change in parrotfishes) were also small for 4 species of parrotfish at Kingman and Palmyra. Fish assemblages at Tabuaeran and especially Kiritimati atolls had starkly contrasting characteristics: piscivorous and other fisheries-targeted fishes were depauperate, lower-trophic levels dominated fish biomass, planktivorous fishes were larger-bodied and more numerous, territorial herbivores were better represented, and size at maturation in parrotfishes was proportionately larger. Our results show the effects that even modest fishing effort can have on assemblage structure and indicate the importance of reefs like Kingman as increasingly rare relicts of natural coral reefs, providing insights into the natural structure and function of these ecosystems.