Competing models of community structure in assemblages of coral reef fishes have suggested that (1) these assemblages are structured by deterministic interactions between species, or between species and resources, or (2) the composition of these assemblages are determined by highly variable settlement from planktonic larvae. We examined interactions among newly recruited juvenile fishes and between juvenile fishes and transplanted resident damslfish on artificial reefs in St. Croix, United States Virgin Island. Two kinds of priority effects occurred: (1) recruitment of three species of settling juveniles significantly decreased in the presence of the territorial damselfish, and (2) prior settlement of a juvenile predator lowered successful recruitment of two juvenile prey species. The first effect increases determinism in the structure of coral reef fish assemblages, while the second decreases their predictability.

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The behavior of dwarf gouramis (Colisa lalia, Pisces, Belontiidae) and their vulnerability to predation by snakeheads (Channa micropeltes, Pisces, Channidae) were observed at four dissolved oxygen concentrations. When the oxygen level was near saturation (8 ppm) or only moderately reduced (3 ppm), the gouramis rarely breathed air and spent most of their time in a patch of artificial vegetation. At extremely low oxygen concentrations (1 and 0 ppm) the gouramis increased their air-breathing frequency and spent less time in cover. Gouramis were more likely to be captured by the snakeheads when out of the vegetation, and thus were caught more quickly at lower oxygen levels, even though their air-breathing frequency declined and use of cover increased in the presence of the predators. These results indicate that dissolved oxygen concentration, which varies considerably in many fish habitats, can affect use of cover and risk of predation in fishes.

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