Surviving in Harsh Environments: Some Freshwater Fish Are Undaunted

Refuges may prevent detection or prevent attack. Those that prevent detection by a predator are usually hiding places in one form or another. But prey animals sometimes head for areas where they know that predators may not be able to follow.As well as physical shelters, such as rocks, crevices, and weed beds, hunted fish often use their environment in remarkable ways.

Are rapids good refuges for fish?

Whitewater rapids are an inhospitable place for fish, but those that adapt to live in such places gain the added benefit of escaping many of their predators. Several of the armored catfish come from exactly these conditions, where they are free to browse on algae and aufwuchs unmolested by other fish. Their powerful mouths are ideal, not only for feeding, but also for providing a great deal of purchase on the rocky substrate to prevent themselves from being washed away. Interestingly, female guppies are known to move into faster-flowing water to escape the constant persistence of males. The males cannot follow because their smaller body size limits their swimming ability and they would be simply swept away.

Does migration work as a defense against predators?

Some species brave the physiological rigors of an estuarine environment where, again, many of their predators are unable to follow. And it is not only predators that find it hard going in the constantly shifting conditions at the sea's edge; parasites, including the dreaded freshwater variant of white spot, are swiftly killed by exposure to salt. But this ability has a downside — populations of mollies that have become adapted to brackish water are less resistant to white spot and can succumb to it if kept solely in fresh water.

Can other tough environmental conditions be used by prey fish for shelter?

Moving to water that is low in dissolved oxygen may be one means by which prey fish can escape their predators. The swamp areas that fringe many larger lakes often act as a nursery for young fish, and the low oxygen levels found in many of these areas can sometimes prove too much for larger predators. In the mid-twentieth century, the introduction into Lake Victoria of the Nile perch brought about the extinction of hundreds of species of endemic cichlids similar to those found in the other Rift Lakes. But the story would have been far worse if the fish had not been able to seek refuge from the huge predator by moving out from the lake to the swampy margins and into a series of smaller satellite lakes, where the perch was unable to follow and where the fish were able to live unmolested.

Do any fish take even more drastic measures to survive?

Extreme circumstances call for drastic action and on occasion, fish have been observed surviving against all odds, having managed to wait out some individual or collective disaster. In doing so, they have demonstrated themselves to be among the most adaptable animals on the planet — one reason for their success. The abilities of fish such as climbing perch and walking catfish are well-known, but perhaps the most footloose of fish is the mangrove killifish (Kryptolebias marmoratus). It makes its home in the smallest of water bodies, ranging from crab burrows to mere root holes in the mud and can move easily between each and every situation with flicks of its body. It can even withstand extended periods of emersion, surviving in damp leaf litter for days on end — one specimen survived happily for 66 days out of water.