Nighttime Fish Behavior

For fish, dawn and dusk are times of great activity, comparable to the rush hour familiar to human commuters the world over. As the sun rises, diurnal fish move from deeper water where they often rest, to begin foraging in the first light. Nocturnal fish make the opposite journey, returning to deeper water and to concealment to wait out the coming day. Dawn also triggers spawning in a huge variety of fish, possibly because it offers a window of opportunity for parents, lowering the risk of the eggs being eaten by either nocturnal fish or their diurnal counterparts. Later, as the sun starts to set, the process seen at dawn is reversed: diurnal fishes migrate to deeper water or into vegetation and the nocturnal fishes emerge.

At night, the underwater world is a very different place. Much like a human city, the hustle and bustle of the day is replaced by a quiet, slightly eerie scene. Also, just as in a city, the night can be a dangerous time: although visual predators, such as birds, are not active, a variety of nocturnal predators are out hunting, hoping to take advantage of unwary fish. The amount of light that penetrates the water depends on the phase of the moon, but even so, beyond the uppermost layers, pitch darkness descends. At the same time, temperatures begin to drop after the heat of the day, and plants, which are no longer photosynthesizing, use up oxygen in the water.

Do fish sleep?

As day gives way to night fish seek out places to rest. Fish have no eyelids and so cannot close their eyes. Nonetheless, they do exhibit two other hallmarks of sleep — they stop moving and their breathing slows. Although many of the fishe's major predators take the opportunity to rest during the evening, new threats emerge.

In the streams of Trinidad, home to the guppy, large prawns leave their daytime hiding places to scour the bottom for scraps of food and even unwary sleeping fish. They do not need light to hunt, using their powerful sense of smell instead. As a result, the fish cannot afford to relax completely and they do not; their lateral lines are able to detect movement in the water nearby. If a predator approaches, they dart away with a sudden flick of their tails. Many diurnal fishes change color during the night, adopting less showy colors. The pencilfish (Nonnostomus beckfordi) changes its pattern from a horizontal band along the length of its body during the day, to two or three vertical bands. This helps it blend in with the aquatic plants in which it shelters overnight.

Do parental fish get a chance to sleep?

Not all diurnal fishes get the chance to sleep once night falls. Parental fish guarding vulnerable eggs and young maintain their vigils throughout the night. Using infrared lighting, which is undetectable to the fish, researchers have been able to watch how convict cichlids keep fanning the brood to maintain the flow of precious oxygen to the developing offspring. As oxygen levels drop during the night, diligent parents cannot afford to take time off at this critical stage. As well as keeping up with the housekeeping, the threat of predators means that the parents must keep up their defensive efforts all night.

How do nocturnal fish find their way around?

The pressures of life during the day have led some fish species to switch to being nocturnal — being active at night. In this way, they can avoid both visual predators such as birds and the mass of competitors. The one disadvantage, of course, is that there is little light at night. Many of the species that opt for the nocturnal way of life normally live in light-deprived areas anyway, such as catfish. These fish are often characterized by having small eyes and using senses other than vision to guide them on their hunting forays, especially smell and taste. Seeking the chemical cues produced by their prey means that many nocturnal predators swim slowly and more deliberately than diurnal fishes that hunt by sight.

How do levels of light in the habitat relate to fishes activity?

Light levels are directly related to fishe's activity patterns. The decrease in light levels that happens each evening as the sun sets prompts changes in fishe's behavior. For example, schools start to break up as visual contact becomes more difficult and fish start to swim more slowly. Remarkable changes occur in the fishe's eyes at this time. Cells are repositioned in the retina, enabling the fish to see in very low light levels. But the changes in fish behavior that accompany dawn and dusk are not controlled solely by light levels. The fishe's internal clocks dictate that they begin the transition to a new phase of activity well in advance of sunrise or sunset. If they are kept on a consistent night and day regime, fish in an aquarium will switch from night to day behavior and vice versa at the correct time even if no light reaches them for a few days. After this, the absence of the zeitgeber will cause the timings to go awry.

Why are dawn and dusk critical times for fish?

One reason why dawn and dusk are so hectic is because this is when visual predators, such as pike cichlids, are at their most active. A recent study showed that predatory fish catch 60-70% of their food at these times. The critical thing for a predator is to get as close as possible to its prey without being seen before launching an attack so predators use the half light to close the gap between themselves and their prey. As a consequence, a visual “arms race” exists between predators and their prey: the most successful predators are those with the best eyesight, because they catch the most food. These individuals are more likely to have offspring and to pass on their excellent eyesight to them. Set against this, only those prey that are able to see well enough to avoid the predators in the gloom survive to reproduce. As a result both predator populations and their prey gradually evolve better and better eyesight in response to one another

How do parental fishes respond to the approaching night?

As the afternoon moves into evening, parental cichlids prepare by corralling their offspring into tightly packed groups. They dig pits in the substrate and transfer the young into them for the night. In fact the task of constructing the pits is undertaken well before dusk as the fishe's internal clocks inform their activity and allow them to do the housekeeping in good time. The parents have little option but to maintain their vigilance throughout the night keeping the young safe is a 24-hour job.