Field guide training: Fish – Part 2

The biology of fish – something that a future field guide should be familiar with. In the following blog post you can find further information on this topic.

Naturguide: Ein Walhai von der Seite fotografiert

As diverse as the world of fish may appear, there are certain characteristics which all fish have in common: All fish are aquatic vertebrates and breath through gills. Most fish are ectotherm which means that their body temperature is adjusting to the temperature of the water that surrounds them. All fish are also equipped with fins which allow them to move and navigate through water. The skin of fish is usually coated with scales.


The fish body can usually be divided into three parts: Head, torso and tail. The head reaches from the tip of the mouth up to the gill cover. The part that reaches from the end of the head down to the end of the visceral cavity is called the torso. The end of the body reaches from the end of the anus to the end of the tail. Due to their streamlined shape fish are very agile and perfectly adapted to their habitat: water. On the side of their bodies fish have a special organ called the lateral line system which can detect even gentle currents and movements in the water. This organ allows them to localize their conspecific and to keep the right distance to them.


Fish play a fundamental role in most aquatic ecosystems. Because of their high variety in feeding behaviours fish can be found in every trophic stage: There are primary consumers which for example feed from dead biomass (saprobiont) or from phytoplankton (herbivores). These again are eaten by secondary consumers, for example smaller predatory fish. The last ones in the chain are the tertiary consumers which feed from the other two consumer groups.

Rangerausbildung: Zwei Anemonenfische schwimmen durch Korallen
Naturguide: Ein Rochen mit anderen Fischen
Rangerkurs: Ein Walhai von der Unterseite fotografiert

How close the different species in one ecosystem are connected explains the following example: In the Lowveld region in the north east of South Africa mosquitos are very common especially during the rainy season. These are eaten by the cichlid which again are hunted by catfish. The catfish then is prey for the crocodile. If now this ecosystem is disturbed, and the crocodiles disappear, the catfish population will explode because their natural predator is gone. As a result, the cichlids are eaten in high amounts by the catfish which means that the natural predator of the mosquito is disappearing as well, and the mosquito numbers can rise. In the end the disappearance of the crocodiles can cause an increase in malaria infections.

This close connection demonstrates how important the protection of all species within an ecosystem is – because if one species disappears this can lead to a collapse of the entire food chain.

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