Title: A Comparative Analysis of Mangrove Forest Structure and Fish Assemblages in Kwale County, Kenya. Case of Gazi and Vanga Mangrove Forests
Author: Moses Mathenge Githaiga
Supervising Institution: IHE Delft - Institute for Water Education
Mangrove forests are characteristic coastal wetlands occurring in tropical and sub-tropical regions along the intertidal zone. For optimal development, mangroves require fresh water input and less wave action from the open sea. While mangroves support a wide range of ecosystem functions and services, there is evidence that globally, mangroves disappear resulting in loss of services. There is also evidence that mangroves support fisheries. Therefore, there is a risk that livelihoods will be affected. Although several studies have been conducted to determine the relationship between mangroves and fisheries, it is still unclear which particular mangrove forest variables support fish assemblages. Besides, a collaborative research approach between mangrove researchers and direct resource users such as fishers is rarely adopted. In Kenya for instance, participatory mangrove management and research is at its infancy stage hence the need for more collaboration among stakeholders. As such, this study compared fish assemblages and forest structural attributes in Gazi and Vanga mangrove sites, using quantitative biological approach and social ecological approach. Fish assemblages were characterised by fish species richness, abundance and diversity index. Forest structure was characterised by complexity index, species importance value (I.V.), juveniles’ natural regeneration, roots’ density and sediment composition. Structured questionnaires were used in the social ecological survey to determine mangrove fish catch throughout the year and to gauge fishers’ perceptions on benefits of mangroves and level of involvement in mangrove conservation. The findings indicated that fish species composition and their abundances were significantly different between Gazi and Vanga (ANOSIM, P < 0.05). The main contributors of fish community differences between the sites were Acropoma japonicum (35%) and Sphaeraena jello (22%). Additionally, Gazi had lower fish species richness but a higher diversity index as compared to Vanga. Complexity index revealed that Vanga (19) was structurally more complex than Gazi (12). There was more organic matter present in Gazi soil samples (28.7 % ± 7.2) than in Vanga’s (8.8 % ± 0.8). However, there was no significant difference in root density between sites (t-test, P > 0.05). This study concludes that Vanga mangrove forest is structurally more complex than Gazi. There are also different fish assemblages in the two sites. Additionally, complexity index is perhaps the main determinant of fish species and abundance compositions. It is also noted that most fishers are not adequately involved in mangrove conservation and hence potentially contribute to unregulated mangrove extraction.