The ichthyofauna of a tropical mangrove bay (Gazi Bay, Kenya): community structure and trophic organisation

The spatial and temporal community structure and the trophic organization of the fish fauna in Gazi Bay (Kenya) are discussed. Gazi bay is presented as part of the entire Eastern African coastal area subjected to the oceanographic and atmospheric processes in the Western Indian Ocean region (WIO). The aim of this study was to collate and synthesise the available information, to collect a time series of new, quantitative data on the fish fauna of Gazi Bay and also to attempt to identify observable patterns in relation to the biotic and abiotic environment of the bay. In total, 346 species in 72 families of fish have been reported from Gazi bay. Of these, only 11 species and 20 families were common to all the six campaigns. A total of 180 species and 20 families were unique to one or the other campaign. It was characteristic of all the campaigns that very few species dominated the catch, the majority of species representing Sphaeramia orbicularis, constituted 96 % of the total catch in fykenets and dragnets deployed in the mangrove swamp. Atherinomorus duodecimalis, A. lacunosus, Herklotsichthys quadrimaculatus and Gerres acinaces were the most abundant species caught in beach seines, where only 3 species made up to 70 % of the total catch per campaign. The dominant species in beam trawls were Leptoscarus vaigiensis, Scarus sordidus, Plotosus lineatus, Parascorpaena mossambica, Sebastapistes strongia, Apogon fragilis, A. nigripes, Fowleria aurita, Siganus sutor, Lethrinus harak, L. nebulosus and Cheilia. This work is a major contribution to the understanding of the fish ecology and the diversity of the fish fauna in an intimately placed tropical coastal ecosystem

Diets of abundant fishes from beach seine catches in seagrass beds of a tropical bay (Gazi Bay, Kenya)

The composition of the diet of 14 fish species that were common in beach seine catches over the seagrass beds of Gazi Bay (Kenya) was investigated. Three trophic guilds could be distinguished based on dietary diversity and on the numerical and gravimetrical composition of the diet. Herklotsichthys quadrimaculatus, Stolephorus indicus and Atherinomorus duodecimalis were planktivores. Their stomach fullness index was low and the diet was not diverse. The main food items were harpacticoid and calanoid copepods and brachyuran zoea and megalopae. Apogon thermalis, Fowleria aurita, Paramonacanthus barnardi, Mulloides flavolineatus, Lutjanus fulviflamma, L. argentimaculatus and Gerres acinaces were benthivores, mainly feeding on small epi- and hyperbenthic prey. Their diet was very diverse and it was dominated by Amphipoda (Gammaridea), Tanaidacea and Mysidacea. Their fullness indices were low, but a little bit higher than those observed for the planktivores. A third group were the 'piscivores': Bothus myriaster, Fistularia commersonii, Sphyraena barracuda and Plotosus lineatus. The dominant items in the food spectrum of these species were postlarval fishes and large nektonic invertebrates (gammaridean amphipods, mysids, shrimp and crabs). Their diet was not diverse and the fullness index was much higher than that of the other species examined. All other species caught were further classified according to the following feeding guilds: herbivores, planktivores, benthivores (epi- and hyperbenthivores) and piscivores. The ichthyofauna of Gazi Bay was clearly dominated by benthivores

Aspects of the biology and feeding ecology of the orbiculate cardinal fish <i>Sphaeramia orbicularis</i> (Cuvier, 1828) (Teleostei: Apogonidae) in a Kenyan mangrove forest

The orbiculate cardinal fish Sphaeramia orbicularis is the most abundant teleost among the root system of the extensive mangrove forests bordering Gazi Bay, Kenya. The species was never recorded from the bay proper and it can thus be considered to be a true mangrove resident. The sampled population clearly consisted of two cohorts: the modes were approximately 65mm and 80 mm. Most individuals with standard lengths >40mm had mature gonads ; the number of eggs ranged from 4,700 to 10,000. S. orbicularis are carnivores, mainly feeding on small epi- and hyperbenthic crustaceans. Numerically, gammaridean amphipods and tanaids were the dominant prey categories in the stomachs of both size classes. Individuals belonging to the smaller cohort mainly supplemented their diet with harpacticoid copepods, while larger fishes also fed on postlarval brachyuran crabs and caridean shrimp. The latter two taxa were important prey items in gravimetrical terms. A preliminary analysis of the otoliths revealed 21 stress marks and 20 striations. An attempt to validate these growth rings indicated that the average age of fishes in the samples ranged from 11 (smaller cohort) to 15 (larger cohort) months

Fish communities in a tropical bay (Gazi Bay, Kenya): seagrass beds vs. unvegetated areas

The fish fauna of seagrass beds and unvegetated areas in Gazi Bay (Kenya) was sampled in 9 stations with a beach seine. A total of 3601 fishes (>95 % juveniles) was caught, comprising 75 species (40 families). The spatial patterns in the fish communities were investigated with multivariate statistical techniques.A first community occurred in the downstream part of a major river-fed creek and was characterized by a low density and diversity. These were sandy bottom stations with sparse seagrass vegetation. The dominant species of this community were Leiognathus elongatus and Bothus myriaster.A second community occurred in the upstream part of the same creek, and was characterized by a high density and diversity. Gerres acinaces and Atherinomorus duodecimalis were the dominant species. A third community occurred in the stations of the shallow part of the bay and was characterized by a high diversity but a lower density. The dominant species were Apogon thermalis and G. acinaces. Both latter communities occurred in stations with dense seagrass beds

Species richness of the genus Molgolaimus (Nematoda) from local- to ocean-scale along continental slopes

This study investigated the distribution of Molgolaimus species (Nematoda) at different hierarchical spatial scales and observed the turnover of species along bathymetrical transects and among transects in two separate geographical regions. Samples from six transects (200-2000 m) from the Southern Oceans (SO) and four bathymetric transects (50-2000 m) from the Western Indian Ocean (WIO) were compared. Of the 30 species recorded, only one was common to both regions. WIO had higher local species richness than the SO. In both regions, the local scale was the greatest contributor to the total species richness. In the SO, there was no difference between species turnover at the different spatial scales, however, in the WIO, the turnover along bathymetrical transects was higher than among separated transects. For the particular genus studied, the evidence suggests that the study area in WIO has more widespread species and was better sampled, while the SO has many restricted species and it is most probably characterized by different biogeographical provinces. At the ocean scale (i.e. WIO versus SO), evolutionary histories may have strongly influenced nematodes species composition, while at local and regional scales, ecological processes are probably promoting species co-existence and speciation. The high co-existence of certain species at local scale is partially explained by species preference for different sediment layers