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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

518: Northern Gulf of Guinea Drainages - Bioko

Major Habitat Type:

tropical and subtropical coastal rivers

Author:

Victor Mamonekene, Institut de Développement Rural, Université Marien Ngouabi-Brazzaville, Brazzaville, Congo

Reviewers:

Angus Gascoigne, Environmental Information and Technical Services, São Tomé and Principe and Robert C. Drewes, California Academy of Sciences, San Francisco, CA, USA

Countries:

Cameroon; Equatorial Guinea; Nigeria

Boundaries:

This ecoregion encompasses the coastal rivers and streams that feed the Gulf of Guinea, from the Cross River in Nigeria to the Bay of Cameroon in Cameroon and the freshwaters of the island of Bioko, which is part of Equatorial Guinea. These rivers, streams, and estuaries support a rich aquatic fauna. 

Drainages flowing into:

Gulf of Guinea, Atlantic Ocean

Main rivers or other water bodies:

The Cross, Ndian, and Meme Rivers are the main waterways draining to the Rio del Rey estuary, and the Mungo, Nkam-Wouri, Doula, and Dibamba Rivers drain to the Cameroon estuary. Most of these rivers originate in the Cameroonian highlands. Bioko has numerous, fast-flowing rivers that radiate from the peaks of Pico de Basilé and the Southern Highlands.

Topography:

Bioko, the largest island in the Gulf of Guinea, is part of the volcanic chain that includes Mt. Cameroon, São Tomé, Príncipe, and Annobón. It is dominated by three volcanic peaks, Pico de Basilé, which is the highest at 3,011 m, Gran Caldera de Luba (2,261 m), and Pico Biao (2,010 m). Pico de Basilé is the second highest mountain in western central Africa after Mt. Cameroon. Bioko is approximately forty kilometers offshore of Cameroon, and has a total land area of 2,020 km² (Sunderland & Tako 1999). Rainfall is very high, with the heaviest in the southwest reaching over ten meters per year (Castelo 1994).

Climate:

The ecoregion experiences a tropical, hot-humid seasonal climate. There is one rainy season with the majority of rainfall occurring between July and September, and a dry season that typically lasts from November to February, peaking in December and January. However, some precipitation generally falls within the dry season. The annual temperature varies between 28°C and 30°C.

Freshwater habitats:

Reed beds grow along the rivers and mangroves occur in the estuaries; both of these vegetation types shade many of the smaller rivers and tributaries, limiting light exposure. The water of the Cross River has low conductivity (< 25 µs/cm), low mineral content, and a low pH (pH < 7.5). However, relatively high conductivity values of 60 to170 µs/cm have been documented in the Mungo River, and pH has been recorded there of 7.6 to 7.8. The larger rivers all experience a unimodal flood regime, coinciding with the rainy season. The mean annual discharge for the largest river, the Cross, lies within the range of 173 to 913 m3/s (Teugels et al. 1992).

The mangroves in this ecoregion’s estuaries are extensive. For example, the Rio del Rey estuary contains about 1,500 km2 of mangroves. These highly productive habitats are breeding grounds and nursery areas for crustaceans and fish (Gabche & Smith 2000).

Terrestrial Habitats:

The ecoregion is located within an evergreen rainforest zone and includes parts of the Cameroonian highlands and mountain forests on Bioko. 

Fish Fauna:

The aquatic fauna within this ecoregion is particularly rich, with more freshwater fish species than those with similar drainage areas elsewhere in West Africa. More than 200 species of fish inhabit these waters. Several marine or brackish water fish species occur in the lagoons and estuaries, including Elops lacerta, Ethmalosa fimbriata, Sardinella spp., Pomadasys jubelini,and Caranx hyppos.

Description of endemic fishes:

About 40 fish are considered near or strict endemics and according to Teugels et al. (Teugels et al. 1992)(1992), 11 species of the 132 freshwater fish species that occur in the Cross River basin are probably endemic. Many of the endemic fish are rivulines (genus Aphyosemion). These speciesare adapted for life in small, shaded streams within forests and swamps, and many feed on invertebrates that fall into the water from overhanging trees. 

On Bioko one species of the genus Aphyosemion (A. oeseri) is considered endemic to the northwest of the island and must therefore be considered severely threatened with extinction (Castelo 1994). Barbus thysi is also endemic to the island and A.volcanum is near-endemic, occurring on Bioko and in streams on Mount Cameroon [ecoregion 519]. 

Other noteworthy aquatic biotic elements:

Approximately 130 species of frogs depend on the waters of this ecoregion for their survival, with about one quarter (c.30) of the frog species estimated to be endemic. Many of these are not strict endemics, but have small distributions that include parts of other ecoregions, including the Lower Niger-Benue [505] and Western Equatorial Crater Lakes [519].

The amphibian fauna of Bioko is poorly understood due, in part to poor sampling but also to lack of clarity and consensus on the relationships and taxonomy of African amphibians in general. A study in 1965 by Robert Mertens listed one endemic caecilian and thirty species of five families of frogs, none endemic. A recent California Academy of Sciences (CAS) expedition (1998) confirmed the presence of at least 27 frog species including two from the vicinity of the Rio Iladyi in the Moca Valley (central highlands) that are undescribed (within the genera Cardioglossa and Hyperolius), and possibly endemic. Areas of greatest amphibian richness appear to be the well-watered areas of Moka Plateau (Valley), the southern highlands at elevations above 1,000 m (slopes of Gran Caldera de San Carlos, and Lake Biao) and the forest slopes of Pico Basile below 500 m (the northern highlands; the higher reaches of Pico Basile are depauperate). Of the 27 species confirmed by the recent CAS expedition, 21 require freshwater for reproduction, including the two undescribed, possible endemics; the rest reproduce by direct-development.

A total of 48 species of dragonflies (Odonata) have been recorded from Bioko (Dijkstra 2002). Twelve species are endemic to the region, of which four are found only on Bioko (Pinhey 1974). Aquatic mammals inhabiting the ecoregion include the giant otter shrew (Potamogale velox), African water rat (Colomys goslingi), spot-necked otter (Lutra maculicollis), hippopotamus (Hippopotamus amphibius), and African clawless otter (Aonyx capensis). The vulnerable manatee (Trichechus senegalensis) also occurs within the ecoregion, sometimes ascending the rivers for considerable distances.

Justification for delineation:

This ecoregion is defined by the coastal rivers and streams that feed the Gulf of Guinea, from the Cross River in Nigeria to the Bay of Cameroon in Cameroon and the freshwaters of the island of Bioko. This region acted as a refuge during the last ice age. Today, many species of trees and vertebrates occur only within this ecoregion, particularly in Korup and Cross River National Parks. This ecoregion has part of its ichthyofauna in common with the Nilo-Sudan and Congo bioregions, in addition to possessing several of its own endemic species (Reid 1989). Bioko was connected to the continent some 10-12,000 years ago and the island shares a majority of its fish species with those of the mainland (Kamdem Toham, pers. comm.).

References/sources:

Castelo, Ramon (1994). "Biogeographical considerations of fish diversity in Bioko" Biodiversity and Conservation 3(9) 808-827.

Dijkstra, K. D. B. (2002) "Odonata of the Gulf of Guinea islands. Gulf of Guinea Islands' Biodiversity Network" <http://www.ggcg.st/Species_Lists/odonata.htm>(2003)

Gabche, C. E.,Smith, S. V. (2000) "Cameroon estuarine systems. Land-Ocean Interactions in the Coastal Zone (LOICZ) Project of the International Geosphere-Biosphere Programme: A study of Global Change (IGBP) of the International Council of Scientific Unions (ICSU)" <http://data.ecology.su.se/mnode/Africa/Cameroon/cameroonintro.htm>(2002)

Pinhey, E. (1974). "Odonata of the Northwest Cameroons and particularly of the islands stretching southwards from the Guinea Gulf" Bonner Zoologische Beiträge 25 179-212.

Reid, G. McG (1989) The living waters of Korup rainforest: A hydrobiological survey report and recommendations with emphasis on fish and fisheries. Godalming, UK: World Wide Fund for Nature Publication.

Sunderland, T. C. H.,Tako, C. T. (1999) "The exploitation of Prunus africana on the island of Bioko, Equatorial Guinea. A report for the People and Plants Initiative". Germany. WWF-Germany//IUCN-SSC Medicinal Plant Specialist Group.

Teugels, G. G., Reid, G. McG, et al. (1992). "Fishes of the Cross River Basin (Cameroon-Nigeria): Taxonomy, zoogeography, ecology and conservation" Annales du Musée royal d’Afrique centrale (Sciences Zoologiques) 266 1-132.

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