Shallow bays with mangroves and seagrass beds are considered to be important nursery grounds or feeding areas for many fish species. Such a nursery role of mangroves and seagrasses is widely accepted for tropical mangrove and tropical and temperate seagrass habitats in the USA, Africa, Asia and Australia (Austin, 1971; Middleton et al., 1984; Wright, 1986; Thayer et al., 1987; Little et al., 1988; Chong et al., 1990; Blaber et al., 1992; McNeil et al., 1992; Laegdsgaard & Johnson, 1995). Although seagrass meadows and mangroves often coexist in close proximity, in many cases species assemblages in seagrass beds differ from those in nearby mangroves and sand habitats (Pollard, 1984; Ferrell & Bell, 1991; Blaber et al., 1992). To date only few studies have evaluated the importance of mangrove habitats as nursery areas relative to adjacent habitats such as seagrass beds and mud flats that are also associated with estuarine areas. Robertson and Duke (1987) and Thayer et al. (1987) have undertaken comparisons of mangroves and proximal habitats, and both studies recorded more number of fish species in mangroves than in surrounding seagrass habitats. It is also reported that the sub-tropical seagrass systems harbour fewer number of species than tropical systems (Stephenson & Dredge, 1976; Quinn, 1980; Thayer et al., 1987; Morton, 1990; Laegdsgaard & Johnson, 1995). Stable carbon and nitrogen isotopes can be used as a tool to trace the flow of organic matter through ecosystems (Peterson & Fry, 1987). The combined measurement of d13C and d15N provides a powerful tool to determine the sources of nutrition for consumers and indicates trophic relationship among organisms. Stable isotope abundance of carbon and nitrogen in animals are largely determined by isotope composition of their diet. Heavy isotope enrichment occurs between animals and their food. Such an isotope enrichment is estimated at about 1‰ per trophic level for carbon and at 3-4 ‰ per trophic level for nitrogen (Fry & Sherr, 1984; Minagawa & Wada, 1984). In general, d13C measurements primarily indicate which sources of carbon are important to consumers in food webs, rather than indicating trophic level. This study elaborates on the findings outlined in Netherlands Indian Ocean Programme (NIOP) and STD III Programme. The main objective of this study is to compare the species composition, the abundance, biomass and size ranges of different fish species in the mangrove creeks, seagrass meadows and sand flats of Gazi bay in Kenya. The evaluation of the feeding behaviour of different size classes of fishes was also investigated using stable carbon and nitrogen analysis. |
