Biogeographical considerations of fish diversity in Bioko
The present study on the fish fauna of Bioko reports a total of 43 species of 21 families; 18 of which are new records for the island. Bioko's icthyofauna is made up of 17 freshwater and 26 saltwater and brackish water species. Freshwater fishes are primarily restricted to a narrow belt along the north and west of the island, similar to the adjacent volcanic region around Mount Cameroon. The rest of the island, however, is inhabited by salt tolerant fishes, as found in São Tomé, Príncipe and Annobón. Fish communities also vary according to elevation since rivers on Bioko are characterized by steep gradients from source to sea. Pressures on Bioko's river systems are not alarmingly high but may increase in the future because of the use of toxic products (lime and insecticides) for fishing. There is no doubt that this is having a significant effect on northern and western rivers which are precisely the most species-rich. Aphyosemion oeseri, which is endemic to Bioko, and which has a distribution restricted to the small northern rivers is in great danger of extinction. P. monkei is in a similarly precarious situation, given that it is extremely rare and will probably disappear altogether in a few years from the island's rivers. Before the situation becomes irreversible urgent measures must be taken to improve matters through educational campaigns as well as through cleaning up heavily-polluted rivers and re-stocking them with fish species from neighbouring river basins.