Edinburgh Napier University

  Healthy mangroves are important for the provisioning of ecosystem goods and services that contribute to human wellbeing. Productive fisheries have been associated with healthy mangrove forests which function as nursery habitats for juvenile reef species. The threats facing mangroves and other nearshore habitats like seagrass beds are still high. Despite these threats, the rate of loss for instance for mangroves is on the decline due to increased global awareness. Replanting and reforestation of degraded areas has also been on the increase. Greater attention is now paid on the quality of the remaining forests and of restored areas and how this affects the capacity to provide ecosystem goods and services. The current study sought to explore the role of forest quality in fish and crustacean community structure (biomass, abundance, species diversity) in the Vanga mangrove ecosystem, in south coast Kenya. Further, it aimed to explore the role of the seascape (a mosaic of connected habitats), with a focus on seagrass beds, on the faunal community assemblages sampled in mangrove areas.
Chapter one discusses mangrove goods and services and especially the role they play in supporting healthy fisheries. It explores the definition of 'nursery habitats' and the key elements that define them. It also discusses roles of mangroves as refugia and feeding grounds for juvenile fish and the links between mangroves and offshore fisheries.
Chapter two describes fish and crustacean community assemblages in Vanga. Fyke nets were used to sample fish at 14 mangrove sites between September 2015 and September 2017. Fishes were sampled once every three months. A total of 59 fish and 16 crustacean species were encountered with 50% of these species (both fishes and crustaceans) being of commercial importance. Circa 70% of the catch was dominated by six species, as is common with nearshore habitats. Over 95% of the fishes caught were juveniles, further strengthening the argument for mangroves as nursery habitats.
Chapter three addresses the importance of mangrove forest quality on faunal communities. The Complexity Index (C.I.), a product of forest structural features (stem density, mean tree height, basal area and no. of species) was used as a proxy for habitat quality of mangroves. Broad scale and fine scale forest features were regressed against fish and crustacean variables: biomass, abundance and number of species. Fine scale forest features were not important in structuring fish and crustacean communities whilst broad scale features did show significant relationships. Mean fish abundance decreased with increasing C.I. while mean crustacean abundance increased with increasing C.I. There were sites that showed high diversity and corresponding high fish biomass for fishes and high abundance and biomass for crustaceans over time.
Chapter four discusses the diet of six of the most abundant fishes that were sampled. A total of 193 stomachs were analysed using the stomach content analysis method. Nine food categories consisting of 36 different prey items were ingested and crustaceans were the most dominant food category. All six sampled species ingested insects, which they most likely got from the mangrove forests. This indicates that some feeding took place in the mangroves. The diet breadth was narrow and ranged between 0.08 – 0.45 revealing that most fish species fed on a low variety of prey items.
Chapter five explores the influence of seagrass metrics on fish and crustacean assemblages sampled in the mangrove forest sites. The spatial analysis and calculation of seagrass geometry was done using ArcGIS. Seagrass metrics - area, perimeter, perimeter/area ratio, cumulative area and cumulative perimeter - were regressed against fish and crustacean variables. Fish variables were positively correlated with seagrass area and responded negatively to increasing perimeter/area ratio. On the other hand, crustaceans increased with increasing perimeter/area ratio; hence there was a tendency for fishes and crustaceans to respond in opposite ways to seagrass seascape metrics. In this respect, these results mirrored those found when exploring the effects of forest quality metrics. Most fishes and crustaceans responded to seagrass metrics measured within a distance of 3.5km from the catch sites. From this study, seascape features of the seagrass beds are found to be important for some of the fish and crustacean species caught in the mangroves meaning that it is not sufficient to explain faunal assemblages with a focus on a single seascape habitat.
Chapter 6 is a synthesis of all the chapters in this thesis bringing together all the findings and generating general conclusions and their implications on management approaches. Multiple linear regression models that include both forest and seascape variables were tested against fish and crustacean variables. The strongest significant relationship (p=0.001) was between the Indian white prawn Penaeus indicus and perimeter/area ratio at 2.5km, and mean tree height, number of tree species and stem density of mangroves. About 87% of the variation was explained by these predictor variables. This study suggests that, given the increasing interest in a holistic approach to seascape management and conservation, seascape habitats can no longer be studied in isolation. In addition, the current work has shown that different faunal groups and even individual species respond differently to forest and seascape features and therefore, the notion of a single 'nursery habitat ecosystem function' is simplistic; rather the mangrove/seagrass seascape provides a range of different nursery services for the species present.