Effects of mangrove degradation and protection on

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Oct 10, 2018 - breakdown and the flow of nutrients in the system (Lee, 1998). ...... Dewalt et al., 1996; De Graaf and Xuan et al., 1998; Primavera et al., 2000a; ..... to a map a degraded forest, in order to know which parts of the forest must be.

”Effects of mangrove degradation and protection on the crab density, Gazi Bay, Kenya " Anastasija Martjanova 2018

A thesis submitted in partial fulfillment of the requirement of the degree of a Master of Science in Marine Environmental Protection Supervisor: Dr. Martin Skov

In collaboration with the Kenya Marine and Fisheries Research Institute (KMFRI) Kenya School of Ocean Science, Bangor University, Menai Bridge, Anglesey, LL59 5EY, UK

DECLARATION This work has not previously been accepted in substance for any degree and is not being concurrently submitted in candidature for any degree. This dissertation is being submitted in partial fulfillment of the requirements for the degree of MSc Marine Environmental Protection. This dissertation is the result of my own independent work/investigation except where otherwise stated. Other sources are acknowledged by footnotes giving explicit references. A bibliography is appended. I hereby give consent for my dissertation, if accepted, to be available for photocopying and for interlibrary loan, and for the title and summary to be made available to outside organizations. Signed: Date: 10/10/2018


“...аt сurrеnt rаtеs оf dеforеstаtiоn, and in rеspоnsе to rising sеа lеvеls mаngrоvе fоrеsts will bе virtuаllу gоnе by the yеar 2010, and during thаt sаmе уеаr 4.3 milliоn pаpеrs will bе publishеd аbоut thеm.’’ Ellison 2002



This study would not have been possible without the passion, enthusiasm, support, and patience of many people, and I would like to thank them all. Firstly, my supervisor, Dr. Martin Skov for believing in me and supporting at each stage of this thesis, being strict and honest which led to a lot of improvement in my work well beyond this dissertation. I would like to thank my first college and now friend Kirsty Williams because I can not imagine a better person to experience such life event together. A huge thanks go to Laitani‟s useful advice and being cheerful and friendly during the fieldwork sample collection at Gazi Bay; Dr. Kairo, his wife Caroline and staff of KMFRI office, thank you for all your help with logistics, the arrangement of accommodation and laboratory sample processing. Finally, to my father and mother, thank you for your endless support, and to my closest friends for ceaseless believe in me and being out there, when they most needed.


Abstract Globally, despite their ecological and economic importance, mangrove ecosystems face the threat of degradation. Timber and fuel demand have enhanced uncontrolled cutting which results in a rapid decrease of mangrove cover and causes the irreversible change to both biological and physical state of the affected site. In order to identify the effectiveness of management efforts, it is important to look at the significant components of a healthy ecosystem: an abundance of keystone species and ecosystem engineers such as brachyuran among mangrove forest. Various patterns can be responsible for crab abundance and distribution; however, there is limited information on how forest degradation can affect crab density. This study aims to assess crab abundance at one protected and two unprotected sites in Gazi Bay, one of the most studied mangroves ecosystems in Africa. The study has employed a nested sampling design to test the effects of sites protection and degradation on crab density with possible influence from environmental parameters. Crab density was assessed by counting their burrows and by burrow size. Forest floor and soil salinity, temperature, organic matter content, and substrate composition were sampled. Forest degradation was estimated based on major indices such as missing biomass, the proportion of stumps and cuts to undisturbed adult trees. Results have shown significantly higher abundances of both Grapsidae sp. and Ocypodidae sp. at the protected site. A positive relationship was found between adult tree density and total crab abundance at both unprotected sites together. While juvenile tree density showed a positive relationship with Ocypodidae sp. abundance at both unprotected sites together, the density of young stands ( ; Grapsidae sp. : cm 2.7 DATA STANDARDIZATION Fauna and flora parameters have been as burrow per m2 and plant per m2. Degradation parameters such as stumps and tree cuts been analyzed as a proportion to a total alive tree number in the plot, whereas missing biomass been analyzed as a proportion to total aboveground biomass (AGB) per plot and in the results calculation as an average at a particular zone within the site. Environment parameters have been standardized in a different way. Salinity and temperature were taken as the average per zone within the site. Sediment component parameters such as silt and organic matter been taken as a proportion per plot and later in the results analyzing as an average at a particular zone within the site. 2.8 STATISTICAL ANALYSIS Two way nested design analysis of variance (ANOVA) was used to detect statistical significance (p0.25) and significance (p

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