Like all animals, fish move around their habitat. Some, such as barbs or tetras, do so more than predominantly sedentary species, such as catfish, but all need to travel at some time or other to find food or mates. Being able to orientate within the environment is therefore a crucial skill for fishes. It is vital to be familiar with the environment to know where food or danger may be found. And also to know where might be a good refuge, or where your territory is.
How do they do it?
Although fishes move around a great deal, they tend to demonstrate what is known as site fidelity, so even though they have an entire lake or a huge river to explore if necessary, they usually limit themselves to a particular area of perhaps a few square feet. In order to do this, they use a wide variety of different cues to navigate. For example, they can detect the smell of their own habitat such as where two rivers converge. On a smaller scale, they can use familiar objects as "landmarks."
Fish from fairly stable environments, such as lakes and ponds, are most likely to use landmarks, whereas in rivers, where landmarks may be washed away, fish tend to rely on the direction of the current in conjunction with larger landmarks. However, fish navigation is all about having a back-up plan -- what happens if this landmark disappears, or you are chased into unfamiliar territory? As a result, fish are also known to navigate using the earth's magnetic field and, if the sky is clear enough, shallow water fishes, such as mosquitofish, can navigate by the sun. Even in the dark, fish can use their lateral line to "feel" subtle water pressure differences in their habitat. Blind cave fish can detect differences in the shapes of openings in a simple maze, which allows them to choose the right hole to enter for a food reward.
How good are fish at navigating?
Given the costs associated with getting lost, it's perhaps not surprising that fish are good at navigating. In one experiment, researchers moved some trout 650 feet (200 m) upstream and another group the same distance downstream. In both cases the fish showed an accurate and speedy ability to find their way home.
Goldfish are known to be excellent at using landmarks to find their way about and are also able to remember the locations of several different hidden food patches and how to find each of them. Mosquitofish are able to remember areas associated with risk, such as a place where a predator has attacked them before, and consequently avoid such risky/dangerous places. If placed in a radial maze, where a number of corridors protrude from the center, Siamese fighting fish explore them and can learn a good deal of information about the maze, including which arms have food rewards in them and which do not. When placed in the same maze at a later date, the fish go straight to the places where they have learned a food reward is waiting. In terms of distance traveled, it has been shown that cichlids, such as Pseudotropheus aurora, can accurately travel over a mile (2.1 km) to reach home.
Can fish find their way out of water?
You might imagine that the last place a fish would want to find itself is out of water. And yet, many species are quite adept at crossing land from pool to pool, either to escape predators or leave a shrinking, drying pool. Some species of killifish are good at this, flicking their bodies along the ground in an apparently ungainly way, but showing themselves to be proficient at finding new water bodies. Other species, such as climbing perch and clarias catfish, are far more adept at moving on land to escape unfavorable conditions and can often travel long distances between pools.