Rhythms of Underwater Life

All life on earth is governed by rhythms, such as the day-night cycle or the changing of the seasons, and fish are no exception. These rhythms predict when fish are most active, when they eat and sleep and when they migrate and breed.

Fish have three main aims over the course of their lives: to eat, to avoid being eaten, and to reproduce themselves. The way they approach these functions varies from species to species, but it is all governed by rhythms. Three main rhythms affect underwater life: the day-night cycle caused by the earth spinning on its axis and the annual cycle of the earth orbiting the sun are particularly important to freshwater fish, while the cycle of the tides affects only marine fish and estuarine species.

How do fish keep track of time?

In each case, the behavior of the fish is governed by an internal biological clock that is synchronized to cyclic changes in the environment by so-called "zeitgebers," or "time givers." These are signals that act on the animal's biological clock, such as the arrival of dawn or the rise in temperatures that herald spring, and they re-set it and keep it accurate.

The biological clock starts to develop early in a fish's life; in fact it is usually almost fully developed by the time the fish hatches from the egg.

How accurate are these internal clocks?

If an animal is taken from the wild and kept in a constant environment for a time, say, in constant light or at
a constant temperature, its rhythmic behavior will gradually decrease in the absence of those vital zeitgebers to update its biological clock.

However, the internal clock is quite robust; depending on the species, it can take anything up to six weeks for a fish to lose its connection to the rhythms of the natural world completely.To see fish behaving naturally in the home aquarium, it is therefore necessary to replicate the natural day-night fluctuations and annual temperature cycles that the fish would experience in the wild.

What other rhythms do fish respond to?

Fish must also respond to other cycles, including the need to forage when hungry and the need to synchronize their activities with those of their prey and the activities of rivals or mates from within their own species. While the environment imposes its own rhythms on fish, the animals also undergo a life cycle from birth to death as they hatch, grow, reproduce, and die. In species with a distinct spawning season, the population is seen to be structured into year groups of different ages and sizes. In those species that spawn all year round, the cycles are less clear and the community will consist of fish from a range of different ages and sizes.

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