TDA-IME Project Final Report June, 2013 it is very difficult to compare fishery yields from sites with and without mangroves that are also similar in these other respects. While it is clear that many commercial species of fish and crustaceans enter mangroves, and they are linked to other fish species through the coastal food web (e.g. Salini et al., 1990), the key question is to what degree are mangroves “critical” to the productivity of coastal fisheries? This question is fundamental to decision-making about transboundary mangrove management, because aquatic species move across boundaries and are commonly landed and sold at considerable distances from where they are caught. Moreover, mangroveassociated fisheries have immense, but not yet quantified, socio-economic value throughout Indochina. In a recent analysis of this issue, Manson et al. (2005) showed that mangrove characteristics (especially area and perimeter) were the dominant parameters accounting for differences in the catch per unit effort (CPUE) for three mangrove-associated commercial species (mud crab, banana prawn and sea bass) along the Queensland coast of Australia. In contrast, CPUE for a fish species that does not use mangrove as nursery habitat (the coral trout) was not related to the mangrove parameters, while estuary-associated species (tiger shrimp, blue swimming crab and blue threadfin) showed a weaker correlation to the mangroves than the mangrove-associates. Manson et al. concluded that the results represent evidence of a causal link between mangroves and fishery production for some species; and that mangrove nursery habitat is critical to sustaining the fisheries of these species. The best indicator species to select in order to monitor the relationship between mangrove ecosystem health and fisheries productivity are probably the mud crabs (Genus Scylla). These large, edible swimming crabs are intimately associated with mangroves throughout Indochina, and support the economy of many coastal fishing communities. In the Pagbilao mangroves of Luzon in the Philippines, for example, mud crabs make up about 95% of the value of the artisanal fisheries (Gilbert and Janssen, 1998). Four species of Scylla are recognized (Keenan et al., 1998), with the mud crab populations studied in the South China Sea region usually comprising of two or even three of the species (Overton, 2000). They are identified locally by their colour differences: S. olivacea (black crab); S. paramamosain (white crab); and S. tranquibarica (green crab). Mud crabs are an important transboundary group ecologically because the adult crabs migrate offshore, up to 50 kilometres (Hill, 1994). The berried females release planktonic zoea larvae that are transported back inshore by tidal currents (Ong, 1964), where they moult to become megalopa larvae, which is a settlement stage. It is not entirely clear where the megalopa larvae settle, but they then recruit into mangrove habitats as juvenile crabs (Moser, 2001), where they stay for three to five months until they reach the sub-adult stage (Figure 2). 43
Transboundary Diagnostic Analysis of Indochina Mangrove Ecosystems
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