Fish in the wild synchronize their activity with the availability of food each day, often visiting a particular location at a specific time. These foraging cycles exist when the availability of food is predictable in time and space and the fish can rapidly satisfy their feeding requirements. As well as reaping rich food rewards by learning when and where food may be available, fish also benefit by reducing their risk of predation -- fish are extremely vulnerable while feeding because their levels of vigilance drop.
Do fish know when it's dinner time?
As we have seen, fish have an internal clock. The first real evidence of this came during the 20th century when experimenters showed that if goldfish were fed every day at the same time, they would start to behave in an anticipatory way -- for example, increasing their activity and spending more time at the feeding site -- shortly before their mealtime. More recently, researchers in Canada have shown that fish can become accustomed to feeding at different times of the day in different parts of their aquarium, learning to associate feeding at certain times at specific places.
When do fish forage in the wild?
The simple answer is whenever there is food. Wild fish tend to show typical daily foraging patterns, especially adults who, unlike juveniles, can sometimes satisfy their daily feeding requirements in one intense feeding session. As we have seen, many smaller lake fishes tend to move from deep waters early in the morning to feed in the rich littoral zones along the lake shore before returning to the deeper water in the afternoon.
Similarly, river fish may move into the main channel to feed on drifting prey, returning to slacker water once they have completed feeding. In each case, the pattern of foraging is dictated by the food, so predators often step up their activity at twilight to take advantage of low light levels. Blind cave fish show typical daily rhythms, even though there is no difference between night and day in their permanently dark world. In fact, the reason they show these rhythms is that the cave fish feed on the guano and dislodged cave insects that are provided by the bats that share their subterranean world and which leave en masse at dusk each day. The bats peak activity cycles are reflected in those of the cave fish.
Do fish match their behavior to changing food levels?
Many Rift Lake cichlids feed by browsing on the algal film that covers the rocks close to shore. As with all plants, algae photosynthesize, converting the sunlight they receive into plant sugars, which they store and use as energy. The plants produce most sugars, and are therefore very valuable as a food source, in the early afternoon when the sun is at its most powerful. A study on the cichlid Tropheus mooni in Lake Tanganyika showed that the fish concentrate their foraging activity primarily between 12 noon and 4 pm, even to the extent of aggressively defending small feeding territories. At all other times, levels of aggression and territoriality are much lower -- there being less at stake -- and the fish gather into large groups.
Feeding Fish in the Aquarium
In the wild, fish can browse throughout the day on a variety of different foods. Aquarium fish, however, are dependent upon having their food provided, Modem flake or pellet foods for domestic fish are rich in nutrients and provide an excellent diet for fish. Supplementing the diet with fresh and frozen foods can help to keep fish in peak condition by replicating the dietary variation experienced by their wild counterparts.