Twilight transitions in coral reef fish: the input of light-induced changes in foraging behaviour

The timing of twilight transition behaviour of diurnal reef fish is likely to be determined by the trade-off between food intake and risk of predation. However, the correspondence between fish behaviour and light-dependent feeding is poorly documented. We used flume measurements of prey detection and consumption by small reef-dwelling zooplanktivorous fish to examine the effects of light and prey flux on the fish's in situ behaviour during twilight. Feeding rates increased from nearly nil to saturation as light level increased, corresponding to about 45 min of the morning twilight. Changes in feeding rates were mostly due to light-dependent changes in reactive distance. In the reef, fish emerged from their nocturnal shelter at lower light intensities than those at which they retreated at dusk. These light intensities at which the fish emerged and retreated in the field greatly exceeded the level required to detect prey. We examined whether the approach of measuring relative ratio of mortality risk to food gained can be applied to predict the observed patterns in the twilight transition behaviour of the fish studied. We suggest that both the times of emergence and retreat and the dawn-dusk asymmetry are the result of a direct response to temporal and spatial variations in predation risk; no evidence for fine-tuned adjustments to diel fluctuations in food availability by changes in the time of emergence and retreat was found.