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. 

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).

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. 

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]. 

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.).