Life in a river isn’t always easy and there are a number of problems fish must overcome in order to survive in a habitat that is constantly changing. River fish species have certain requirements, many of which are required by all animals, but others are specific to life in a flowing water. Physiological, behavioral and morphological characteristics give an indication of the needs of a fish with respect to its habitat.
Fish are cold-blooded animals, meaning their bodies are heated by their surrounding environment rather than being heated internally as humans and many other animals are. Being cold-blooded is generally an advantage living in water because it uses less energy and the daily and even seasonal temperature changes in water are much smaller compared to the changes in air temperature land animals must endure. This means that fish can become well adapted to living at a relatively constant temperature. If water temperature within a stream changes too much, too quickly, it can end up killing fish as their bodily functions may not be able to withstand the drastic changes in their metabolism as a result of the changes in temperature.
Requirements for life in a stream
Living in water, obviously river fish need a certain depth and volume of water and the water must also be of a certain quality. As water levels rise and fall, fish must move to suitable habitats where water levels are safe for them to survive. In some cases, for example when the water level decreases after a flood event, fish can get caught in spots that are disconnected from the main stream or river and, unless there is another flood soon, they face an unpleasant end as the water dries up around them.
Nutrient fluctuations are another concern for many fish populations. Increases in farming and many other practices have led to an increase in nutrients finding their way into streams and rivers. If nutrient concentrations become too high in a stream it can lead to the water becoming depleted of oxygen due to algal blooms which consequently ends in the death of the fish within the stream.
As with any animal in nature, freshwater fish need protection from predators. This could come in the form of structures in the water such as logs, boulders or undercut banks or it may be an issue of timing. For example, certain fish species have developed behavioral patterns that help them avoid predators by only coming out to forage for food at night. Many fish have a cryptic camouflage to help them blend into their environment and reduce the chances of being spotted by a predator.
River Fish species
Species of river fish differ significantly from marine fish. One big difference between freshwater and marine fish communities is the prevalence of eels in freshwater environments. Eels are found in the ocean but they are far more common in most of the world’s rivers and streams and are all believed to have originated from the tropics around south-east Asia. All freshwater eels belong to a single genus called Anguilla.
Common fishing species are trout and salmon which are also found through most parts of the globe, either naturally or due to being introduced by humans. Catfish are another common group of fish found in many parts of the world. Some species of catfish can grow into giant freshwater monsters and reach a length of 2.5 m (8 ft.) long and weigh more than 100 kg (220 lbs.).
Carp are a family of freshwater fish that have become a massive problem in many parts of the world after being introduced and then completely taking over many rivers and streams through extremely prolific reproduction. Once carp have established themselves in an environment they often then go and completely alter the quality of the water through their eating methods and can entirely change the ecosystem. Despite intensive efforts to control carp populations in a number of streams and rivers, attempts more often than not appear to be unsuccessful as carp numbers continue to increase.
Last edited: 20 December 2015
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