|Het verschil in gebruik van natuurlijke, heraangeplante en gedegradeerde mangroves door de ichtyofauna in Gazi Baai, Kenia = The difference in use of natural, replanted and degraded mangroves by the ichthyofauna in Gazi Bay, Kenya|
Reubens, J. (2007). Het verschil in gebruik van natuurlijke, heraangeplante en gedegradeerde mangroves door de ichtyofauna in Gazi Baai, Kenia = The difference in use of natural, replanted and degraded mangroves by the ichthyofauna in Gazi Bay, Kenya. MSc Thesis. Universiteit Gent. Faculteit Wetenschappen: Ghent. 77 + 2 appendices pp.
Universiteit Gent; Faculteit Wetenschappen; Vakgroep Biologie; Onderzoeksgroep Mariene Biologie (MARBIOL), more
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VLIZ: Non-open access 121446
|Document type: Dissertation|
Feeding behaviour; Feeding ground; Fish; Mangroves; Pisces [WoRMS]; ISW, Kenya, Gazi Bay [Marine Regions]; Marine
This study handles about the importance of mangal as a feeding ground for fish. The impact of degradation and reforestation of mangal on the associated ichthyofauna is also investigated. Mangrove forests fulfil numerous important socio-ecological functions and provide a diversity (of economically important) products, with a direct or indirect human value. In many places in the world the surface of mangal decreases much faster than natural regeneration can take place. More than 50% of the global surface of mangal is lost due to anthropogenic influences (Spalding et al .1997). The environmental consciousness about the importance of mangrove forests made a spectacular rise in the past 30 years (Field 1999). Many initiatives to prevent further loss of the ecosystem have been undertaken, of which Gazi Bay, Kenya is an example. This thesis fits within the follow-up of a reforestation project carried out in 1994 in Gazi Bay.The purpose of this essay is to find out if there are differences in use of the mangal, in different stages of degradation or reforestation, by the ichthyofauna.The samples were taken at the south coast of Kenya, in Gazi Bay, in September 2006. Five sites were selected: a natural Sonneratia alba site (NAT), a 12 year reforested S. alba site (RFOR), a degraded site (DEG), a fragmented S. alba site (FRAG) and the sandy beach (SB). The different sites were sampled with fyke nets and a seine net.In the first part of this research the community structure was investigated. Clear differences were found in density, species richness and biomass between the different habitat types, caused by the environmental factors. Each habitat is characterised by biotic and abiotic factors, of which each fish species has it’s preference and a certain tolerance. Small-scale habitat differences play an important role here. Each estuary has quite some microhabitats. Each microhabitat houses its own community, influenced by the biotic and abiotic environmental factors.The ichthyofauna observed in the reforested site can serve as an indicator for (partly) recovery in function and provision of the mangrove forest in the site. This however, does not mean that the reforested site is the equivalent of a fully recovered ecosystem. The time needed for functional equivalence at the natural site is much longer than the time needed for visual recovery of the habitat itself.The fragmented site is the transitional situation from natural to degraded. Relative small changes in the structure of the mangrove forests can lead to significant changes in abundance and diversity of macrobenthic organisms. These changes, at their time, can lead to a whole cascade of effects on higher tropic levels. As a result, the quality of these habitats as nursery and feeding grounds firmlydecreases. In the second part of this research the feeding ecology was investigated.The possibility of differences in feeding activity in different habitat types depends on the species.In general we can say that there are few differences in fullness index between the sites. This has two possible causes: or the species does not feed in the mangal, or the species does feed in the different habitats and possibly changes the diet between sites. For most species the feeding hypothesis was not confirmed. Three diet types were defined: a detrivorous, a carnivorous and a piscivorous diet. Besides four trophic classes were defined: an omnivorous, detrivorous, carnivorous and piscivorous class. If smaller trophic classes are a must, prey-items should be classified in smaller groups by determination till genus- or species level.There exists a serious dearth in scientific knowledge on distribution patterns of the vagile fauna in mangrove forests, due to the time consuming and often difficult sampling methods. Rönnbäck et al. 1999 gave some suggestions and remarks about the sampling methods to form a platform forfuture studies. Via some remarks from the present research this platform is extended and some suggestions are made.
- The ichthyofauna is vagile and can enter and leave the mangal at any place, so the selected sites need to be large enough and distributed over different locations. In this manner fishes that fed in one site do not become part of the analysis in another site.
- The different sites need to be sampled simultaneously, because temporal variations in tide amplitude, currents and weather conditions may influence the catches (Rönnbäck et al .1999). However, the number of sites that can be sampled simultaneously is limited by manpower, time and available materials.
- Gut content analysis my give some unreliable results with respect to the diet compositionand the source of the food (Lugendo et al. 2006). It is the only method however that gives some detailed information about the prey-type and the number of individuals taken. Stable isotope analysis gives an average of the diet over a period of weeks to months. A combination of both techniques will give the most reliable results.
- Some more remarks to take into account:
- In mangal you find both diurnal and nocturnal fish. Nocturnal fish hide in the mangrove forests during the day. At night the move to their feeding grounds in the mangroves and the seagrass beds.
- There is possibly a difference in activity of the ichthyofauna during spring- and neap tide.
- It could be that the ichthyofauna moves along a certain path trough the estuary.When tide comes in they follow the creeks and channels before moving into theforest. When tide goes out, they leave the forests at a random place and re-enterthe creeks and channels.
If we want a durable forest management, it’s important to take into account the social aspects.The human population must be seen as an inherent part of the mangrove forests (Field 1999).Anthropogenic activities have large impacts on the forests. At the same time we depend on these forests to survive. The local communities and governments should be firmly involved in the conservation- and rehabilitation projects. They should be an active participant and have responsibilities regarding the project (Yap 2000)
- Difference in usage of natural, reforested and degraded mangroves by the icthyofauna in Gazi Bay, Kenya, more