Onchocerciasis control on the island of Bioko (Republic of Equatorial Guinea) by vector elimination: Environnemental impact assessment (EIA) part 2c macro invertebrate indicator species

A literature search has established that the freshwater macro-invertebrate fauna ofBioko is extremely poorly studied and that relatively few species of freshwater insects have been recorded from the island (Appendix 1). Nevertheless, it is apparent from this searc[ and from insects collected on the island during the present study, that the ocp identification manual to aquatic inseas from the Ivory coast @ejoux et al., lggl)and the standard recording form 5620 are adequate for family level recording of specimens collected on the island because all families known from Bioko are covered. There are genera of aquatic insects which are found on Bioko and are not covered, but this is unlikely to seriously distort estimates of community structurq for which they are designed. l'2' The impact of the inseaicide Temephos, used to eradicate Sirrulium dannowms.l. the vector of onchoceriasis, on chironomidae (non-biting midges) genera and odonata (dragonflies) species on three streams on Bioko has been investigated as part of an environmental impaa assessment. 1'3' Adult and larval odonata and larval Chironomidae were surveyed throu-ehout Bioko between 23 Marchand 9 April 1999 to establish the distribution ofthese groups and to indicate whether recolonisation oftreated strezmrs from untreated refugia would be possible to re-establish any non-target ta:ra that were eliminated following rhe Simuliumeradication progriunme.l'4' Treatment of all three trial sfeams with Temephos had an adverse effect on the chironomid fauna with an elimination of up 5u/o of thegenera at sites immediately downstream ofthe treaunent point. Insufficient nurnbers of odonata larvae were present in the samples to draw meaningful conclusions about the direa impact ofthe insecticide on the odonate fauna. However, if the insecticide causes a general reduction in abundance and biodiversity of aquatic macro- invertebrates, the odonate fauna could be expected to decline within a few weeks due to a shortage ofprey l'5 Twenty-five species of Odonata were recorded during the present survey, including six species not previously found in Bioko. The total number of odonata species now known from the island is 48 including one species (Trithemis hartwigt)and one subspecies (Chtorocypha cancellata insulua)that are cunently thought to be endernic. Trithemis hartwigr was not collected during the current survey but has previously only been recorded in the highland region around Moca. Chloruyplw cancellaninsalqwwas collected from several lowland streams during the present survey The dragonfly fauna has two elements, an upland element which includes species that occur above 1000 m only, and a lowland element including several species that do not occur above 500 m. Only three species occurred in both these faunistic elements, but there was a transitional fauna at 600 m that included a few species from each elernent. Much ofthe adult dragonfly fauna is probably seasonal in occurrence and so was not recorded during this brief survey. Certain Libellulidae probably migrate to Bioko from mainland Africa to breed in temporary pools during the wet se.$on. ffis migration us likely to be wind assisted. 1.9. Downstream recolonisation by Odonata of streams treated with inseAicide below 1000 m will not occur since upland strearns do not act as refugia for most ofthe lowland odonate fauna. However, if some permanently flowing lowland sffeams are left untreated these could act as refugia for the lowland fauna. Simultaneous inseaicide fieament of the lowland sections of all rivers on Bioko is likely to result in the elimination of a significant proportion ofthe dragonfly fauna on the island, although recolonisation by some species from the African mainland might be possible over time. The chironomid fauna appeared to be fairly uniform in lowland rivers but samples from the high altitude site included four tu