Individual study: Replanting of mangroves restores mud crab Scylla olivacea populations in mangrove rehabilitation efforts, Panay Island, Philippines
Walton M.E., Le Vay L., Lebata J.H., Binas J. & Primavera J.H. (2007) Assessment of the effectiveness of mangrove rehabilitation using exploited and non-exploited indicator species. Biological Conservation, 138, 180-188
The value of mangroves is increasingly appreciated (e.g. in terms of providing timber and non-timber products, and for nature conservation) and mangrove replanting in cleared or degraded areas is often used as a restoration method. However, once diverse forests are often replanted with mono-genus mangrove stands, thus when established, whether their ecological function is restored is unclear. In this study, four sites on Panay Island (Philippines) were selected to represent different types of mangrove habitat (a replanted area predominantly of Rhizophora spp.; a natural area predominantly Sonneratia spp.; a diverse natural basin mangrove; and a degraded mangrove). The abundance of the commercially important mud crab Scylla olivacea, and two non-exploited species, Baptozius vinosus and Thalamita crenata were used as indicators of rehabilitation success.
Study sites: Four sites with similar topography and tidal elevation representative of mangroves on Panay Island in the central Philippines were selected:
1) Replanted coast-fringing mangrove in Buswang (11º42′N 122º23′E) where only isolated trees remained. An area of mudflat was planted with 65 ha of Rhizophora spp. and 5 ha of Nypa fruticans up to 1993-1994. In 1997, 43 ha had established, with colonising Avicennia marina and Sonneratia alba increasing the area to 75.5 ha. Three distinct areas were present: Rhizophora spp.; mixed A.marina/S.alba; and a patch of N.fruticans;
2) Ibajay (11º48′N 122º12′E), a 72 ha natural basin mangrove with 27 tree species, the main creek draining and flooding through a narrow opening;
3) Colong Colong (10º47′N 122º39′E), an area of natural 100 m-wide fringing mangrove dominated by A.marina (with Aegiceras corniculatum, Ceriops decandra, Rhizophora apiculata, R.mucronata and S.alba) draining directly into the sea;
4) degraded mangrove in Dumangas (10º52′N 122º45′E) with only a few Avicennia spp. trees remaining. The habitat has been further degraded by the removal of large quantities of mud.
Crab population assessment: Relative abundance (CPUE) and changes in crab density with tidal elevation, mangrove type and time was compared using trapping grids of commercial bamboo traps. Crab density was also estimated in the replanted mangroves at Buswang using mark-recapture studies.
The trapping grids were deployed at the three mangrove-species-distinct replanted sites at Buswang in 2002. In 2004, trapping was undertaken in the replanted mangrove at eight locations, including Buswang. The grids were placed at a range of tidal elevations and in areas of different mangrove species to investigate the effects of the decreased abundance in S.olivacea as indicated by CPUE initial data, and on the abundance of the other two study crab species (B.vinosus and T.crenata).
The relative abundance of mud crabs was fairly similar in the natural fringing mangrove (1.89 crabs/trap/day) and the replanted mangrove areas (1.71 and 0.81 crabs/trap/day). Lower densities of mud crab in the basin mangrove area (0.33 crabs/trap/day) were possibly due to low recruitment; at this site there was a higher abundance of the two non-commercial crab species. No mud crabs were caught in the degraded mangrove; here CPUE for the two other crab species was also low.
Relative abundance as indicated by trapping of S.olivacea was supported by the mark–recapture studies conducted in the replanted mangrove site at Buswang.
Conclusions: Overall, the authors conclude that replanting of mangroves, even in mono-genus stands, was effective in restoring mud crab populations, indicating recovery of an ecological function to a level approaching that of natural mangroves.
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