light irradiated seawater were effective P. placenta in inducing spawning in ripe male and shells are refined female window-pane shell, P. placenta. and made into Furthermore, UV light irradiated attractive home, office and hotel seawater triggered the synchronous furnishings and release of sperm and eggs in P. decor and have placenta. The technique using UV light become popular in irradiated seawater as inducer of the Philippines spawning is simpler, cost effective, and and abroad. The easier to perform than serotonin lampshade is a injection and can be applied to both popular individual and mass spawning furnishing in experiments. On a routine basis, some tasteful homes and hotels Madrones-Ladja said that she uses this method in producing gametes of the window-pane shell for other larval experiments. The planktonic larva! stages of P. placenta is only 14 days. However, further experimentation should focus on environmental requirements of the larvae to optimize larval survival and improve larva! quality.

Photo courtesy of The Ilonggo Producers Association, Inc.

The impact of shrimp pond construction along the mangrove coastal accretion at southwest Ca Mau Cape, Viet Nam*

Prof. Dr. Sc. Phan Nguyen Hong Director, Mangrove Ecosystem Research Center Hanoi National University, Viet Nam

Ca Mau peninsula of Viet Nam is a According to the laws on land, the houses on stilts made ofRhizophora new, unstable land situated between premature accretion land outside river trunks roofed withNipa palm. the East Sea (South China Sea) and mouths are government property and Analyses of the characteristics and Thailand Gulf. Every year, aliuvia from should remain undisturbed. changes of the soil and water, the the Mekong River system and eroded Notwithstanding these regulations, behavior of creatures in the ponds, as soil from the east of the peninsula since 1991 spontaneous migrants from well as on the tidal mud flats have accumulate at the west coast. The Ca Mau town have transformed ap­ shown that the construction of chain potentially rich Southwest accretion proximately 900 ha of the newly formed embankments for shrimp ponds has led area is an important natural ecosystem land into brackishwater ponds for to severe degradation of the in Ca Mau peninsula. shrimp faming. They have constructed environment and a decrease in the *A complete list of references is available firm chain embankments outside the natural resources of Ca Mau Cape. free of charge upon request. unstable accretion land and built ...to page 4 SEAFDEC Asian Aquaculture Vol. XVIII No. 4December 1996 3 Changes of the physical At the same time, the embankments Geological-geomorphological environment at Bai Bun Bay and Bay Hap River processes. At Ca Mau Cape, all Ca Mau Cape accretion. The mouth has fast encroached upon the geological-geomorphological processes formation of Ca Mau Cape is basically sea while the accretion surface is still such as forming topography, alluvium different from those in large river low and has created an artificial border. current, and erosion accumulation, mouths such as Red River mouth and Along that border, the depth changes have been dominated by the East Sea. Tien River mouth. In such areas, delta suddenly, leading to a decrease in the The northeast monsoon with its high expansion is first the formation of small accumulating process and erosion of wind velocity combined with the coastal sand hills, then alluvium from the embankment and bottom in some dynamic characteristics of the sea (tide, the river heighten the tidal flat inside places. It is evident that this process wave, currents) has been strongly those small sand hills to form the delta. limits development of the accretions. destroying the east-southeast coast; it The amount of alluvia from rivers is not has brought water together with alluvia very changeable and contains a lot of past Ca Mau Cape to Thailand Gulf by coarse grains (sand). the dense network of creeks and canals The accretion at Southwest Ca Mau Analyses of the and sea current flowing from southeast Cape is formed in such a way that the to northwest. materials come from distant points of characteristics and changes The west coast receives a lot of the sea, thus, they are mainly made up alluvia and is strongly accreted in the of fine grains. In the first stage, large of the soil and water, the dry season; the northwest monsoon underwater flats are gradually raised by behavior of creatures in the and any interference with that natural alluvium accumulation. With the sea process should be very carefully encroachment by the delta border, the ponds, as well as on the considered. The construction of many underwater flats are near the coast, kilometers of chain embankments has thus are more quickly accreted. When tidal flats have shown that prevented the creation of tidal rivulets the flats are higher than the low tidal the construction of chain on the accretions and reduced the ways level, Avicennia pioneers begin to that lead seawater to the accretion. The develop and the ecological environment embankments for shrimp paths taken by alluvia that directly add turns into a swamp type, new accretion to the network of creeks and canals delta according to a study by Hong and ponds has led to severe may even increase. An (1994). degradation of the Experience and research show that Moreover, the source of fine grains the rivulet mouths that bring in alluvia from distant points in the sea is not environment and ashould be densely and evenly stable and changes in its dynamic distributed throughout the whole section characteristics considerably reduce the decrease in the natural in order to promote the accreting amount of alluvia and consequently, the resources of the entire Caprocess (as they were at Ca Mau tip in accumulating process. One of such the past). Filling these rivulet mouths factors is the currents. The Mau Cape.with the constructed embankment may embankment construction has result in partial erosion in places far interfered with the exchange currents from the main rivulet mouths due to lack between the sea and the forest, The increase in the bottom of alluvia. Erosion of the border and between the east and the west coast of excavation outside the embankment bottom (outside the embankment) has Ca Mau Cape, hence, it has limited the can be shown by the composition and also been observed. amount of alluvia brought in. Sedimentssize of grains in the present bottom mud at the accretions are chiefly fine grains sediments - - the sediments reflect Accretion process. The above (0.02-0.01 mm) in pasty mud form, very embankment construction. The mentioned data have indicated that the easily eroded, especially during the sediment size at every point observed accretions southwest of Ca Mau Cape period of southwest monsoon. The increases, sand and powdered sand are now in the process of topographical construction of chain embankments (1.0-0.02 mm) account for more or less surface raising. In order to become a around the accretions has created 50%, while clay makes up the major real accretions delta, they need a waves outside the embankment and proportion, according to the usual law. developed swamp-mangrove environ­ excavation by waves occurs. These coarse-grained materials make ment and fine grain accumulation. The construction of embankments up the bottom sediments, especially in Rational exploitation of the accretions that prevent seaward mangrove the period of southwest monsoon. Thatshould not hinder this process. development also prevents alluvium is the time when the amount of alluvia The building of the embankment accumulation which forms new brought in is small and the currents and along the accretions for shrimp ponds accretions. waves become stronger. has stopped this development process 4 SEAFDEC Asian Aquaculture Vol. XVIII No. 4December 1996 inside the shrimp ponds (near the tide, the sea water gets in slowly and layer full of H2S lies under the thin mainland) because of the decreased the time is too short to change pond brown superficial layer which is about amount of water with alluvia while the water. Thus, the environment inside the 3-5 mm. In the same area but outside accretions outside the shrimp ponds pond conspicuously degenerates after the pond with some Avicennia, depth of near the sea are either excavated or only 1 or 2 years. the superficial layer is 20 cm or more, accreted more slowly. In observing the dry bed of the pond, and the black-green layer lies much one can see Oscillatoria fully covering a deeper. Analysis of the soil samples Changes in soil environment 1 0 m wide stretch along the pond both outside and inside the pond (Table The construction of firm embankments. When the tide comes 1) also shows environmental degenera­ embankments along the southwest into the pond, this alga floats on the tion. coast of Ca Mau Cape outside the water surface, promoting the growth of The table shows that Fe3, Al+3, SO4, premature accretions has limited water lab-lab (Hong and An, 1992). H2S contents and salinity inside the exchange between the sea, the ponds, There are also white patches that pond are higher than outside because and the interior due to lack of sluice stink strongly which means that H2S is the environment lacks tidal exchange gates both in number and size perceptible. Some low places where and water evaporates fast. considering a very small tide amplitude water is stagnant and covered by (0.5-0.8 m). When the tide is low, the lab-lab also stink. Changes in the water chemical ponds lie exposed to the sun. At spring Usually, the very thick, black-green property Inside the pond, the chemical properties o f water are largely different Table 1. Some chemical factors of soil inside the ponds in comparison with that outside from those outside the pond as shown the ponds on the southwest accretion of Ca Mau Cape. A - accretion (outside the pond); B - inside the pond in Table 2. The formation and operation of extensive shrimp ponds on the Sign Percent Al+3 SO4 H2S Sali­ accretions and mangroves have Places of (ppm) (%) (ppm) nity changed the chemical properties of the sam­ N P2O3 CaO MgO Fe (%) natural water: ples • NO-N and N2-N content inside the pond is lower than that of the canals Accretion A 0.14 0.136 2.63 1.71 3.84 27.3 0.32 15.5 2.93 outside the pond. inside Ca • PO4 and NH4 content inside the Mau Bay P 0.14 0.145 4.04 1.88 3.89 84.2 0.62 21.2 4.08 pond is lower than outside. Accretion A 0.11 0.121 1.96 1.54 3.46 25.5 0.33 15.2 2.96 This shows that insufficient water south of exchange has caused contamination of Bay Hap the water. The disintegration of organic Bay P 0.13 0.116 1.73 1.89 3.82 103.9 0.65 20.0 4.74 matter results in the shortage of oxygen and oxidation is weaker than Accretion A 0.11 0.114 2.07 1.67 3.67 76.5 0.3 13.5 2.89 deoxidation. west of Go Cong P 0.13 0.114 2.38 1.95 3.96 111.2 0.68 19.7 4.53 Changes in the characteristics of Source: Hong and An, 1992. aquatic organisms Analysis of the aquatic organisms on the accretion and inside the ponds Table 2. Chemical factors of water in ponds and in adjacent canals in Ca Mau Cape, Viet Nam in June 1992 showed the following: Zooplankton and phytoplankton. In Chemical Canals and creeks Forests all the shrimp ponds on the accretions factors (accretions) and in the mangroves, a decrease in Canals Ponds Canals Ponds the diversity of the organisms in com­ parison with the natural environment PO4 (mg/l) 0.030 - 0.077 0.147-0.168 0.055 - 0.097 0.076-0.114 outside has been noted (Table 3). In the pond, the predominant species belong NH4 (mg/l) 0.029 - 0.042 0.047-0.106 0.024 - 0.045 0.061-0.119 to Oscillatoria, a phytoplankton that N2-N (mg/l) 0.015-0.068 0.006-0.138 0.050-0.162 0.007-0.072 develops quickly in the contaminated environment. Only zooplankton such as NO3-N (mg/l) 0.054-0.138 0.028-0.059 0.027 - 0.273 0.038-0.165 Paracalanus parvus, Acartia clausi, Oithona nana, and Masopodosis Source: Hong and An, 1992. ...to page 6 SEAFDEC Asian Aquaculture Vol. XVIII No. 4December 1996 5 slabbari can withstand the environmen­ with regular water exchange. When the pioneer Avicennia alba. This is due tal changes. Oithona has been rapidly shrimp ponds are constructed and to the water and alluvium exchange developing and is now predominant. water exchange becomes limited, which decreased when the The accretion at Ca Mau tip is a Oscillatoria develops and produces embankment was constructed. Poor highly nutritious area with the lab-lab. Lab-lab die and when water exchange does not facilitate predominant growth ofOscillatoria and decomposed, take a lot of oxygen from Avicennia growth and regeneration. Oithona nana in most of the places the pond. Plenty of H2S is generated, It was noted that Avicennia where samples were collected. The thus, the water smells. Consequently, regenerated very quickly after 8 months development takes place in two benthos cannot survive. Only 1-2 in parts of the pond embankment where directions during the construction and species, such as Nepthys oligobranchia the sluice gates were broken in order to operation of shrimp ponds: or Masopodopsis slabbari (motile ones), get tidal water in, while they did not - Oscillatoria develops fast and the out of 22 survive. grow in ponds where the sluice gates number of zooplankton falls with the were retained. predominant species being Oithona Waste. Due to the degenerated, Even when planted outside the nana and Nauplius larvae. toxic environment inside the pond, embankment, mangroves do not take - The shrimp ponds risk being shrimps live for only 15-17 days. The root because of the great depth and big organically contaminated. The size and weight are small (800-1,200 waves that uproot the trees near the predominant zooplankton areAcartia shrimps/kg), hence, a big waste. embankment. clausi and tiny shrimps. Changes in vegetation. Abnormal growth vegetation. The Benthos. The three groups of Vegetation at embankments. The construction of embankments both polychaetes, crustaceans, and bivalves embankment along the new accretions outside and inside the forests with small are suitable only in a clean environment is a hindrance to the development of sluice gates has reduced tidal entry from Thailand Gulf into the forests inside the accretions. This has led to Table 3. Diversity of the phyto- and zooplankton in the southwest coast of Ca Mau abnormal growth of the vegetation, and Cape, Viet Nam in June 1992 the death of trees in the dry season due to the high salinity. The embankment Aquatic Number Percentage Number of Number Dominant has also prevented the entry of environment of species of all species species in of cells/m3 species vegetative detritus from the forests (%) 360 m3 (max/min) which are food for creatures in the (max/min) accretions or shallow waters near the A. Phytoplankton Oscillatoria Rivers, 55 83.33 23/12 848.000/ subbrevis coast. Some species are likely to rivulets, 13.000 O. limosa migrate to more favorable areas. accretion Coscinodicus Meanwhile, plankton and benthos astrophalus, which are shrimp food also decrease. Nitzschia lorenziana Arising Social Problems Ponds in 37 56.06 29/12 183.000/ Oscillatoria The exploitation of the southwest Ca accretion 28.000 subbrevis, Mau coastal accretion for shrimp ponds O. limosa by the construction of surrounding Ponds in 31 46.97 16/10 400.000/ O. subrevis, forest 57.000 O. limosa, embankment has caused conflict Leptocylindus between people who get large benefits lanicus from collecting shrimps at the accretion 9. Zooplankton Oithona nana. ponds and local shrimp farmers whose Rivers, creeks, 26 100.00 14/9 9996/323 Oithona nana crops fail because of the shortage of accretions Acarthia clausi water and seeds. Ponds on 9 34.62 9/3 22.253/ Oithona nana, Management of the population and accretions 850 Acartia clausi protection of forest resources has also Nauplius become more difficult. copepoda The new community at the accretion Ponds in 12 46.15 10/8 1,768/ Oithona nana, forests 498 Acartia clausi is a complex mix of people who are not Mesopodopsis local inhabitants. Besides, the benefit slabbari from shrimps has attracted an influx of transmigrants from other areas. They Source: Canh, Mien, and Loc, 1992. have to build houses, make furniture, 6 SEAFDEC Asian Aquaculture Vol. XVIII No. 4December 1996 burn firewood, cut trees for sluice gates and construct wave-buffer fences for The construction of many kilometers of chain shrimp ponds. This makes the protection of forests and the primary embankments has prevented the creation of tidal natural mangroves at Con Trong, Con rivulets on the accretions and reduced the ways that Ngoai Islands at the Ong Trang river mouth meet with a lot of difficulties. If lead sea water to the accretion. protective measures are not immediately taken, this area will be the nursery grounds for many importantfor extensive shrimp ponds with high void of forests or perhaps only stunted marine organisms. It can be considered productivity on a small area. Permission forests would remain. a “green wall” to protect the coast from for the development of these ponds The law and regulations are defined wave and tide effects. Forests cannot should be in accordance with the but the accretion encroachment is survive without adequate water and comprehensive plan and given only difficult to stop. Once embankments nutrition from tide and sea currents. after careful study. are built on the accretions, the ecologi­ A break in this relationship will •Fishermen should be encouraged to cal consequences will be serious. damage the highly productive rear cockle-shells (on sandy soil) and From the evaluation and analysis of ecosystems and valuable natural oysters (on muddy soil) at the the trends of environment change, we resources in tidal areas. newly-formed accretions. The provincial believe that the exploitation of the •The sea encroachment by the authorities should assure consumption young southwest coastal accretions of pioneer Avicennia forests should be and export of these shells and oysters Ca Mau Cape by building chain guaranteed because these forests not so that growers can concentrate on long embankments for shrimp ponds can only attract marine creatures but also term work. The regulation on the marine bring profit in the early years to the reduce erosion, protect the product catching area ought to be more spontaneous migrants. Nevertheless, it accumulated soil, and promote the practical. The complete prohibition at has negative effects on the physical accumulation of sediments to raise the the moment makes local inhabitants and ecological environment. These accretion surface. jobless so they have to clear forests for chain embankments interrupt the •The relation between the east and shrimp ponds to earn their living. succession and development of the the west coasts should be kept. Due to •To solve the problem of environment mangrove ecosystem and affect the the special position of Ca Mau Cape, and resources at the Ngoc Hien protection and development of with different tide regimes and the accretions and the adjacent areas, the resources, not only at the accretions strong effects of various monsoons, thegovernment should help the province to but also in adjacent forest and marine eroded materials from the east coast draw up a production plan. This problem areas. Moreover, it results in come to accrete on the west coast. affects not only Minh Hai Province but complicated social problems which The water and material exchange also the utilization of wetlands with badly influence the security, discipline, between the east and the west coasts mangroves in the country and and laws of the society. From this should be facilitated to transport shrimp Southeast Asia. viewpoint, the clearance of shrimp seeds, shrimp food, and other essential ponds on the accretions is reasonable nutrients into the mainland, thus, the Problems identified and necessary. stable productivity of shrimps and of the This paper hopes to encourage forests and the expansion of the west cooperation among the countries in the Solutions to rationally utilize the accretions are assured. region and generate interest from accretions •Avicennia forests which are rapidly international organizations to conduct Cardinal principles and actions toencroaching be on the sea should be further research on the sustainable taken. A solution to rationally utilize the carefully protected. The swamp stage utilization of coastal resources and the accretions should be part of an overall ought to be hastened by planting environment. The following problems plan for the whole mangrove area. To Rhizophora apiculata together with that require further research have been contribute to the building of that plan, natural Avicennia alba so that the latter identified: (a) the hydrological and we present some important will protect Rhizophora from being dynamic processes in the coastal zone recommendations in utilizing the uprooted by waves or damaged by in rivers, creeks and canals; (b) the accretions. clinging creatures. In addition, relation and appropriate proportion of •The close relationship between the Avicennia should be planted at forests, aquaculture area, and mangroves and the sea should be accreting places to limit local erosion. agriculture land in topographically maintained. Forests supply the •After 3-4 years, with the protective different areas; and (c) building of important source of detritus for Avicennia forest outside, a certain appropriate agro-fishery-forestry creatures in the shallow sea as well as portion behind forests (which are youngproduction models for high economic at the river mouths: Forests are also Avicennia at the moment) can be used and environmental efficiency. SEAFDEC Asian Aquaculture Vol. XVIII No. 4December 1996 7