Twilight Activities and Temporal Structure in a Freshwater Fish Community

Underwater observations of temperate lake fishes suggest that their twilight activities are basically similar to those described by other researchers for coral reef fishes. At dusk, diurnally active lake fishes progressively increased swimming, ceased feeding, disbanded groups, slowed swimming, and finally stopped. At dawn, this sequence was reversed, although at lower light levels than corresponding dusk behaviors. Aggressive, cleaning, and comfort behavior increased at dusk, and fishes changed from group to individual behaviors. By day, nocturnally active fishes rested singly or in aggregations. Predators were apparently more active during twilight. Older individuals of several species were active later in the evening or were increasingly nocturnal in their activities. Characteristic diel and twilight patterns broke down during the breeding season. Temperate lake species differed from coral reef fishes in that they were less attracted to physical structure at night, twilight migrations were less common, and no apparent "interim period" occurred between cessation of activity by diurnal fishes and initiation of activity by nocturnal ones. In general, twilight changeover in the temperate system was more variable and prolonged. These differences may be due to lake fishes having evolved under variable conditions of community diversity, water clarity, predator abundance, and twilight length. Such factors are more constant or predictable in coral reef communities, where twilight activity patterns appear more precise.