Figure 1.
Effect size plots representing differences in the density (A) and biomass (C) of nocturnal, diurnal, and cathemeral prey fish on reefs at predator-depleted Tabuaeran Atoll relative to predator-heavy Palmyra Atoll.
Positive values represent increases at Tabuaeran. Percentage of surveys (B) in which nocturnal prey fish were sighted at Palmyra and Tabuaeran. Comparisons of the biomass of all large (≥10 kg) diurnal and cathemeral piscivorous predators (D) at both atolls. All values are mean, ±1 SE. Surveys marked with the same letters in each species grouping are not significantly different (after post-hoc correction).
Figure 2.
Effect sizes of reef fish density regressed against the ratio of fish eye diameter to standard length (SL).
Large effect size values represent strong increases in density at predator impoverished Tabuaeran Atoll. Nocturnal, diurnal and cathemeral are segregated in the plot. Fish with large eye diameter: SL are thought to be increasingly well adapted for functioning at night. The most dark adapted fish showed the strongest responses to predator depletions. There was no significant relationship between effect size response and eye diameter: SL for diurnal or cathemeral fishes.
Figure 3.
Examples of two species of nocturnal reef fish surveyed in this study: Myripristis berndti (A) and Priacanthus hamrur (B).
The large eye to body length ratios of these two species (M. berndti: 0.15; P. hamrur: 0.11) give evidence of their nocturnal lifestyle. Like many nocturnal fish, M. berndti and P. hamrur both dramatically increased in abundance at Tabuaeran where large daytime active predators were less abundant. Example diurnal fish Chlorurus sordidus (C) and Epibulus insidiator (D) exhibit the smaller eye to body length ratios (C. sordidus: 0.05; E. insidiator: 0.05) that are more characteristic of diurnal reef fish. These species were among the diurnal prey fish taxa whose abundance showed negative or weak responses to predator depletion.