Gemeenschapsanalyse en trofische organisatie van de ichthyofauna in een tropische baai (Gazi Bay, Kenya; Community analyses and trophic organisation of the ichthyofauna in a tropical bay (Gazi Bay, Kenya)
De Troch, M. (1995). Gemeenschapsanalyse en trofische organisatie van de ichthyofauna in een tropische baai (Gazi Bay, Kenya; Community analyses and trophic organisation of the ichthyofauna in a tropical bay (Gazi Bay, Kenya). MSc Thesis. Universiteit Gent. Mariene Biologie. Instituut voor Dierkunde. Vakgroep Morfologie, Systematiek en Ecologie: Gent. 136 pp.
Seagrass beds are an important biotope in tropical coastal ecosystems. They are amongst the most productive ecosystems of the world. As nursery areas for juvenile fishes, they contribute indirectly to the productivity of coral reefs. The exchange processes between seagrass beds, mangroves and reefs are complex and fragile. Knowledge about the biodiversity and the interactions, between the different systems, their communities and the constituting species is a prerequisite for the rational management of these biotopes. In this study the fish fauna of the seagrass beds in Gazi Bay (Kenya) was investigated. 8 stations were sampled with a beach seine in the two main creeks of the bay. The spatial patterns of the fish communities were investigated with multivariate statistical techniques (cluster analysis, TWINSPAN, correspondence analysis). A total of 3152 fishes was caught, comprising 75 species (40 families). Based on the density and species composition of the stations three communities were found. The first community occurred in the downstream part of the West Creek and was characterised by a low density and diversity, correlated with a sand substrate and less dense seagrass vegetation. Leiognathus elongathus, Bothus myriaster and Hyporhampus affinis were the dominant species. The separation of this community was based on the absence of Siganus sutor, a typical species for seagrass beds. The community of the upstream part of the West Creek was very different from the community from the downstream part of the same creek. This upstream community was characterised by a high density and diversity. Typical species in this community were Gerres acinaces, Atherinomorus duodecimalis and Stolephorus indicus. The BS2 station, located at the mouth of the West Creek, clustered in this group because of a similar diversity, density and species composition. The community of the East Creek was separated as a third community. This community showed great similarity with the upstream community of the West Creek, especially in terms of density. The diversity was higher in this community because of the denser seagrass beds in the East Creek. The dominant species are typical for seagrasses: Apogon thermalis, Gerres acinaces and Leptoscarus vaigiensis. A lot of species were found with only a few individuals. The high diversity in this community is linked to lower densities per species. The measured lengths of the fishes caught show that all species were present as juveniles. This is a strong indication for the importance of seagrasses as nursery sites. The importance as a nursery site is linked to the structure of the habitat and the shelter against predation, but also the production and the availability of food are crucial. In the East Creek, the former two factors are important, while in the West Creek the input of nutrients from the Kidogoweni river also plays an important role. iv In a second part of this study the feeding ecology of the dominant species was investigated by stomach analyses. The species (and the different length classes) were selected based on their length-frequency distributions and densities. The investigated species were: Herklotsichthys quadrimaculatus, Stolephorus indicus (two length classes), Atherinomorus duodecimalis, Apogon thermalis (two length classes), Fowleria aurita, Paramonacanthus barnardi, Mulloides flavolineatus, Bothus myriaster, Fistularia commersonii (East- and West Creek) and Sphyraena barracuda. The numerical and gravimetrical stomach analysis data were analyzed by the same multivariate techniques as used for the densities in the community analysis. Three feeding types could be distinguished. Herklotsichthys quadrimaculatus, Stolephorus indicus (both length classes) and Atherinomorus duodecimalis are planktivores. The filling index was low and the diet was not divers. The main constituents of the diet were harpacticoids, calanoids, zoea-larvae, megalopa-larvae and ostracods. Apogon thermalis (both length classes), Fowleria aurita, Paramonacanthus barnardi and Mulloides flavolineatus were separated as a second group. The diet of these species was very diverse and it was dominated by Amphipoda, Tanaidacea and Mysidacea. The filling index was low but a little bit higher than this of the planktivores. These species can be grouped as benthivores. A third group was composed of piscivores: Bothus myriaster, Fistularia commersonii (East- and West Creek) and Sphyraena barracuda. The dominant prey-items in the food spectrum of these species were Pisces, Caridea and Mysidacea. The diet was not diverse. The filling index was much higher than this of the other species. Intraspecific competition is possible between both length classes of Stolephorus indicus. Interspecific competition is possible between Herklotsichthys quadrimaculatus and both length classes of S. indicus, between H. quadrimaculatus and Atherinomorus duodecimalis, between both length classes of S. indicus and A. duodecimalis and between Mulloides flavolineatus and the smaller length class of Apogon thermalis. In a last chapter the trofic organisation of the stations, the communities and the entire bay was investigated. The species were classified according to the following feeding guilds: herbivores, planktivores, benthivores (epi- and hyperbenthivores) and piscivores. The composition of the stations in terms of percentage shows that planktivores were dominant in the total density of the upstream stations in the West Creek. Benthivores were dominant in the other stations. Based on the data of this study, we can conclude that the ichthyofauna of the investigated seagrass beds of Gazi Bay was dominated by benthivores.
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