Fish of the Congo River

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The secret lives of Congo River fish

A local association of fishermen in Gamba, Gabon, controls fishing nets, and keeps track of the ... rel= © WWF CARPO/Bas Huijbreghts

The bustling underwater world of the Congo River is home to at least 686 species of fish, 80% of which are found nowhere else in the world.

A snapshot of this aquatic habitat reveals the elephant fish, the air-breathing lungfish and the endearing killifishes. But there’s much more to this amazing fish world than meets the eye.

The Congo River is globally important in terms of fish diversity. It is also of great evolutionary importance, as 7 out of 10 fish families in Lake Tanganyika have evolved from its waters.¹

During a survey of the Congo River by the New England Aquarium, it was found that the fish fauna is dominated by the elephantfishes (Mormyridae), followed by cichlids (Cichlidae), characins (Characidae), the Distichodontidae and 2 families of the catfish order (Siluriformes): squeakers or upside-down catfishes (Mochokidae) and bagrid catfishes (Bagridae). Another prominent family included minnows/carps (Cyprinidae).

A closer look at some Congo River fish families

© WWF

Elephantfishes: In the Mormyridae family, fish are slightly electrified, with an electric organ in their tail that they use to move around and communicate. They are primarily active at night.

Cichlids: Cichlidae are unique in that they have teeth both in their jaw and in the throat and a single nostril on either side of the head instead of 2. One of the reasons for the family's massive diversity is advanced care of their offspring compared to other fish.

Lungfishes: Species of the Protopteridae (African lungfishes) family are air-breathing. They can live in anoxic (no oxygen) or hypoxic (low supply of oxygen) water. This allows them to survive dry periods, during which they burrow in a mud hole and are enveloped in mucous.

Bichirs: Fish of the family Polypteridae have the characteristics of ancient fish, with lobed fins and a hard coating on their scales. Really large specimens can reach about 1.2 m in length, but most species are below 30 cm.

Killifishes (toothcarps): Aplocheilidae are an important prey item for other fish species. They feed on insect larvae, such as mosquitoes, and may be important for disease control.

Where they live

Fish don’t just swim anywhere in the rivers of the Congo River Basin. In fact, most of them are picky and will only live in particular areas. These include:

Channels, creeks and oxbows, with shaded calm pools, little current, sand or mud bottoms rich in vegetable debris;
Floating meadows, along banks; and
Inundation zones.³

Fish are particularly fond of the shallow waters along riverbanks and islands, where the slower current and clearer water favour the availability of food.

How fish use the river habitat

Some species have evolved to take advantage of nature’s gifts. For instance, young catfish, characids and mormyrids seek refuge from predators by keeping close to plants such as the water hyacinth, which grows on the riverbanks.

Plants and insects that are found in such habitats provide food for the young and adults of many fish species. For some species of fish, fallen trees and submerged logs from riparian forests are an important habitat.

Other species prefer rocky bottoms, which provide a stable substrate, while others again have adapted to low oxygen and varying water levels in swamps.⁴

Underwater lifestyles in the Congo River Basin

Fish species are not necessarily restricted to one kind of habitat. In fact, many change from one habitat to another during different stages of their lifecycle, for example spawning and feeding. Young may spend their juvenile stage in a flooded zone and migrate to the main part of the river when mature.

The onset of the rainy season signals the beginning of breeding for river fishes. This happens twice a year, the most important for Congolese fishes being during the major rainy season (September-October in general), with a second, lesser breeding period during the rainy season from April to June.

As water levels rise, there is an explosion of microorganisms and aquatic vegetation that leads to an increase in small invertebrates. This coincides with the juvenile stage of fish, when the needs for feeding and growing are the greatest.

The impact of fish capture

While fisher folk usually use gill nets, seine nets, cast nets, hand lines and fish baskets, some have broadened their arsenal to include defoliation of banks (using poisons or herbicides), explosives, poisons and illegally sized nets (mesh sizes of less than 3 cm).

These practices, along with overfishing, the introduction of alien fish species and probably deforestation have led to the decline of several species, including Parachanna spp., Distichodontus spp., Gnathonemus spp. and Protopterus spp. There has also been a decrease in diversity among the Bagridae, Characidae, Cyprinidae and Distichontidae families.⁵

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¹ Shumway C, et al. 2003. Biodiversity Survey: Systematics, Ecology, and Conservation Along the Congo River. Congo River Environment and Development Project (CREDP).
² Shumway C, et al. 2003. Biodiversity Survey: Systematics, Ecology, and Conservation Along the Congo River. Congo River Environment and Development Project (CREDP).
³ Mathes, 1962 in Shumway C, et al. 2003. Biodiversity Survey: Systematics, Ecology, and Conservation Along the Congo River. Congo River Environment and Development Project (CREDP).
⁴ Mathes, 1962 in Shumway C, et al. 2003. Biodiversity Survey: Systematics, Ecology, and Conservation Along the Congo River. Congo River Environment and Development Project (CREDP).
⁵Mathes, 1962 in Shumway C, et al. 2003. Biodiversity Survey: Systematics, Ecology, and Conservation Along the Congo River. Congo River Environment and Development Project (CREDP).