Search

The diversity, abundance, biomass and community structure of crustacean and molluscan macrofauna were studied in the Ranong mangrove forest ecosystem on the Andaman Sea coast of southern Thailand. After a history of commercial exploitation the mangroves along the Klong Ngao tidal creek have been assigned conservation status within a new Ranong Biosphere Reserve established in 1997. Over the past 12 years, several areas of mangrove destroyed or degraded by wood harvesting, tin mining and aquaculture, have been rehabilitated on a pilot basis by planting monocultures of mangrove seedlings using four common local species (Rhizophora apiculata, R. mucronata, Bruguiera cyclindrica and Ceriops tagal). These plantation forests with different past management histories were compared with a natural, mixed, mature mangrove forest which has been conserved for about 40 years. Macrofauna were sampled within a 100 m2 vegetation quadrat in each study site. Crustaceans were sampled quantitatively by 3×15 min timed hand catches per site. Molluscs were sampled in 3× m2 quadrats positioned around three randomly selected trees in each vegetation quadrat. The lowest crustacean and molluscan diversity was recorded from the former tin mining site. The highest diversity was recorded from aRhizophora plantation in the natural mixed forest area for both crustaceans and molluscs. The vegetation community structure was not correlated with the environmental variables measured, or with macrofauna community structure. Of the environmental parameters chosen, the crustacean community structure was best expressed by shore level, while for molluscan diversity and abundance it was soil moisture content. The macrofauna community structure at the tin mining site was significantly different to the other sites, and was dominated by a single species of crab, Metaplax elegans. Grapsid crabs, especially sesarmid species, dominated over ocypodid crabs in the mature forest site, whereas Uca species and other ocypodids were more abundant than grapsids in the degraded concession forest area. Snails of the families Neritidae and Ellobiidae were the most abundant molluscs in the mature forest, whereas Littoriinidae, Assimineidae and Potamidae species were more representative of the younger plantation sites. The findings from this study suggest that some of these well represented families of mangrove macrofauna could be used as indicators of ecological change as part of a long term environmental monitoring programme in Ranong and other areas in Southeast Asia where mangroves are being rehabilitated. [Copyright &y& Elsevier]

The development stage and number of seed (eggs and fry) removed from individual incubating Oreochromis niloticus (L.) broodfish at 10-day interharvest intervals (IHI) was compared with the harvest of free-swimming fry from undisturbed fish held in a tank within the same system. Spawning synchrony and productivity were compared for (1) total exchange of females for 'conditioned' females after each harvest, (2) exchange of only spawned females, and (3) return of females back into the spawning tank. Improved synchrony and seed production were positively related to removal of eggs and fry from incubating females and could have important implications for broodstock management in commercial hatcheries. Allowing fish to remain undisturbed in clear water tanks may lead to inhibition of spawning activity related to the formation of hierarchies. Individual weight loss by females during incubation was correlated with stage of seed harvested and thus with timing of spawning during the interharvest interval. [ABSTRACT FROM AUTHOR]

Mangrove crabs, Scylla serrata (Forskal). represent one of the main economic resources in the Ranong mangrove ecosystem on the Andaman Sea cost of southern Thailand. However, there is a serious problem of overfishing which has caused fishermen increasingly to sell undersized crabs to meet market demand and maintain their income levels. The crab population and fishing activities were studied to assess the scope for better management of the fishery, especially protection of female broodstock, and the feasibility of using young crabs for aquaculture. Berried crabs examined were always larger than 10.0cm (carapace width), with the great majority of females maturing in the size range 10.0-11.5cm. Gonosomatic index values peak in September, within a main egg carrying and spawning period from July to December. Protecting female crabs from fishing pressure during this season, plus limits on the harvesting of crabs less than 10cm. or using them for fattening in ponds, could lead to better utilization of this resource. Crab farming in Ranong is constrained by climatic factors, the presence of acid-sulphate soils and lack of technical knowledge among the local fishermen. An attempt initially to culture crabs from Ranong under more favourable conditions at Surat Thani showed limited success. Nonetheless, there is a considerable price differential between wild and pond-fattened crabs to encourage further interest in crab farming in Ranong. [ABSTRACT FROM AUTHOR]

An investigation was conducted under laboratory conditions to determine the optimum food particle size of an inert diet for common carp fry ranging in size from 15 to 466 mg body weight and 13 to 31 mm total length (TL). Speed of consumption of a standard quantity of food was used as a measurement of feeding preference/efficiency for different particle size ranges. It was observed that for carp of this size range preferred food particle size increases with fish size. On the basis of ingestion time and feeding behaviour, food particle sizes in the ranges 125–300 μm. 300–500 μm, 300–790 μm and 500–1000 μm diameter were found to be most appropriate for carp fry weighing 15–23mg(TL 13–18), 46–97mg(TL 17–22), 105–209 mg(TL 20–25) and 210–466 mg (TL 24–31) respectively. This study also demonstrated that for carp fry of these size ranges the mean preferred food particle sizes are about 0.2–0.4 that of mouth size. Based on this relationship, general recommendations are made on the optimum food particle sizes for different size classes of carp fry reared under hatchery and nursery conditions. [ABSTRACT FROM AUTHOR]