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Connectivity of Scylla serrata in Kenya and the Indian Ocean
Mascaux, N. (2012). Connectivity of Scylla serrata in Kenya and the Indian Ocean. MSc Thesis. ULB: Bruxelles. 46 pp.

Thesis info:

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    VLIZ: Non-open access 247984
Document type: Dissertation


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    Scylla serrate is an important and valuable crab specie in the Indian Ocean, particularly in East Africa and in small island country where it seems to be actively harvested by local population. Despite its economic importance as well as its ecological importance in the mangrove, there are very few regulations to prevent its overexploitation in East Africa. In consequences crab size risks to decrease and harvesting will progressively target a broader fraction of crab populations including all size classes (Dumas et al. 2012, Fondo et al. 2010). Crab Fisheries in East Africa seem to not be overexploited yet but the tourism activity is increasing and many peoples move to the coastal from neighbouring country, so the pressure on marine resources is likely to increase (Barnes et al. 2002) Marine protected areas (MPA) are considered as a tool for protecting biodiversity and enhance fisheries of many species (Palumbi 2001). It has been shown that MPA based on two type of catch restriction, sex limit (e.g. no female crab can be taken) and size limit (e.g. legal size for male is 15cm carapace width) are beneficial for fisheries. Crab catch, size and biomasses where higher in protected area compared to non-protected area (Pillans et al. 2005). Although MPA with no take zones are protecting mud crab from exploitation and these areas contain greater number of crab in all size classes. Furthermore this protection strategy is an immediate and effective management tool for the recovery of fished mud crab stocks when populations become overfished (Butcher 2005). Implementing no catch reserves in a fisheries and increasing the size limit of catch seem to have the same effect of increasing the age of first capture and therefore to allow more individuals to achieve reproduction (Botsford et al. 2003). Design of MPA requires a strong knowledge on larval dispersal and transport in and around the reserves (Palumbi 2001). Nevertheless, behaviour, dispersal and direct measurement of larval transport are not well known because of the difficulty of tracking larval stages before they settle in the mud flats and mangroves. More studies focused on the larval behaviour and dispersal of the mud crab are still important to understand the long term dispersal of the specie. Genetic patterns and Isolation by distance are a complementary way to direct measurement of larval dispersal and could help to set the appropriate geographic scale on which Marine reserve system will function well (Palumbi 2003). The problem is that for creating a reserve characterised by self-recruiting of larvae, the protected area must to be very huge. This is especially the case for large scale dispersal specie where reserves must be as large as the mean larval dispersal distance of that specie (Botsford et al. 2003). Another possibility of sustainable management of that specie could be the development of network of non self-seeding reserves. In this network, each marine reserve might serve as stepping stone area for recruit from distant population. Our study provides information about connectivity of Scylla serrate and some could be useful in the process of design of MPAs. The presence of genetically heterogeneous populations in WIO would permit to separate populations as isolate stocks. Populations along the East African coast seem to be interconnected and represent a large stock which has to be managed specifically. Gene flow appears to be independent of geographic distance and high enough to spread larvae all along the coast, maintaining connectivity between remote areas. Implementation of local management seems to be crucial to maintain this connectivity and to allow populations to play a role of source of juveniles for the other populations. Populations from West Indian Island such as Mauritius and East Madagascar seem to be separates from the East African stock and should be managed independently. Further studies in the Indian Ocean should be leaded to highlights the extension of this new stock. Nevertheless, connectivity and larval dispersal are quite large in WIO and the implementation of MPA network seems to be adequate for a sustainable management of Scylla serrate. Additional strategies such as mud crab aquaculture may be useful for managing and restocking endangered populations (Le Vay 2008) but further studies must be carried out on larval rearing techniques, broodstock maturation and spawning in order to struggle against limitation of seed supply (Keenan and Blackshaw 1999, Davis 2004). However, mangroves of WIO are more and more fragmented and their role as habitat and as stepping stone area for this crabs risk to be endangered. The disappearance or overexploitation of one population could compromise the connectivity of the entire metapopulation and consequences would be disastrous for the specie. Therefore, protection, management and rehabilitation of WIO mangroves are critical for the sustainable welfare of Scylla serrata. It is not conceivable to protect Scylla serrata without considering parallel effective conservation policies of mangroves habitat.

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