The present assistance on oyster culture is a follow up of previous assistance extended from February 19 to June 18, 1983, wherein an oyster farm was established at the estuary of Bagkhali River at Cox's Bazar and a spat monitoring farm along Naf River at Jaliapara, Teknaf. The Cox Bazar Farm was established in the month of March and April and thus having enough time to have some observations. The Teknaf farm was on the first week of June 1983, thereby the consultant was unable to make any observation before his departure.
Oysters are commonly found in Bangladesh attached on concrete sluice gates (on tidal areas, embankment are constructed to protect agricultural lands and the sluice gates are used to prevent entrance of seawater which may damage agricultural crops and also drain excess water durainy months, brackish water fishponds also utilize sluice gates for supply and drainage of water).
The most common species of oysters found in Cox's Bazar and Teknaf areas are the Crassostrea gryphoides (Schlotheim) and C. belcheri (Sowerby). C. madrasensis (Preston) and Saccostrea cucullata (Born) can also be found but in lesser abundance. (See Appendix I for identification of species). On the concrete sluice gates, the oysters are attached on the bottom floring and on the sidings. The upper limit of its attachment is approximately two-third the height of the prevailing tide of the area. The lower level are commonly occupied by C. gryphoides while the upper level (up to the highest limit) are occupied by C. belcheri.
The site of the pilot oyster culture farm is at Nuniachata Village in Cox's Bazar Town, along the southern side of the estuary of Bagkhali River. (This river originate from the east and exit to Bay of Bengal at a south to north direction). The tidal land area has an elevation of 0.0 to 013 meter above the zero tide level. The soil is of soft black mud and a person will usually sink to about 20 cm. while wading to the oyster bed. During high tide high saline water from Bay of Bengal enter the estuary. Aside from oysters which are attached to concrete posts and rocks along the river bank, blood clams (Anadara rhombea and window-pane shells (Placuna placenta are present in the area.
The pilot farm has an area of 5,600 aq. meters, and about 100 meters from the shore. Its corners are marked with tri-bamboo posts with red flag on its top and lighted during the night. Small bamboos were also staked at 10-meter distance to mark the perimeter. These visible marks were installed as the farm site is a navigable water during high tide.
The Cox's Bazar farm area is not the best site for the culture of oysters but was selected for its accessibility. It is affected by low salinity and siltation during rainy season and being a navigable water, boats usually do some damage to the oyster bed. So the farm being at Cox's bazar, it can easily be managed by the Marine Fisheries Station or be inspected/visited by other government and private parties interested on the project.
The other farm site is at Jaliapara in Teknaf, which is along the western side of Naf River. It is accessible by road from Cox's Bazar in 2½ hours. It is adjacent to the WAPDA sluice gate No. 6. The tidal land area has soft muddy bottom exposed during low tide, with elevation of 0.1 to 0.4 meter above the zero tide level of the area.
Both the above sites were described in the report of previous consultancies, Jan. to July 1982 and Feb-June 1983.
From February to June 1983, various methods of oysculture were set up in the two farm sites.
At Cox's Bazar Farm the following were the set up:
At Jaliapara, Teknaf the following were the set up:
The Cox's Bazar farm was established between March and April 1983, wherein the consultant was able to make some observations on the progress of the setup in the farm. The Teknaf farm was established on the last week of May and first week of June 1983, leaving no time for the consultant to make any observation on the setup.
Upon the consultant arrival for the present assignment, he immediately visited Cox's Bazar and Teknaf farms for the purpose of preparing his programme of activities of the oyster project.
On ocular inspection of the two farms, it was noted that there is a need for some maintenance and repair work to be done. At the Cox's Bazar farm site, the following were particularly noted:
The condition of the spat collection site at Jaliapara also need additional maintenance work to be done. Some of the bamboos of the plots were either detached from the posts or broken. Some of the shell cultches fell to the bottom. Due to soft mud at the site, more clay bricks have sunk through the mud.
After the consultant left in June 1983, the personnel of the farm made fifty (50) wooden stakes with shell cultches attached to them. The cultches attracted oyster spats but the wood which were of mangrove tree branches, started to decay after six months duration.
As they were immediately needed at the farm, bamboos ropes and nails were purchased and necessary repair was accomplished in two months between December 1983 to January 1984. Such was done under the supervision of the FAO Expert and so the oyster farm was again operational.
There were four (4) practical methods of oyster culture that were set up in the previous consultancy and these are namely, bamboo plots, concrete structures, clay bricks and tray methods. The process of stake mathod was introduced to the counterparts who tried them after the consultant left, so such was not included in that report. The methods of oyster culture was described in the previous report but since such may not be available to this reader it may therefore be worthwhile to mention here again.
There were two (2) types of bamboo plots constructed. One type was intended for vertical hangings of stringed oyster shells (cultches) and the other for horizontal hangings. Each plot measure 2 by 5 meters and consisting of the following sizes of bamboos;
9 pcs. bamboo posts, cut 2½ meters long (1½ meter of it will be driven to the muddy bottom
3 pcs. bamboos, cut 2 meters long - to serve as the width of the plot, nailed and tied with twine to the posts horizontally
3 pcs. bamboos, 5 meters long - to serve as the length of the plot, nailed and tied with twine to the post horizontally.
There is not much difference between the two types of plots, except that the plot with vertical hangings, the bamboos nailed and tied to the posts were one (1) meter high above the ground, while that for horizontal hangings, 10 to 20 cm high only.
The hanging cultches were made of empty oyster shells. First a hole was punched at the center of each shell. Then 8 to 10 pieces of shells were stringed together with polyethylene twine, spaced with knots at 10 cm. distances. A hanging measure about one (1) meter long.
It was found out that the plots with horizontal hangings to be practical and effective for the condition at the farm site. The shell cultches collected good number of oyster spats. The size of 2.5 to 3 mm. diameter polyethylene rope will be the right size for this purpose. The oldest ones used are more that 1½ years and look to be still be sturdy to last for 2 years or more.
The plots with vertical hanging need further observations. Due to strong tidal current, the bottom end of the hangings need to be fixed so as to prevent entangling with each other. If entangled, the hanging rope may be cut when come in contact with sharp end of the shells. The site being quite elevated will have the upper shell cultches with less oyster spats attached.
The shell cultches that were set in March 1983, collected a good number of spats, but were fouled with barnacles, which settled a month later. On horizontal hangings, there were 4–7 oyster per cultch who were able to outgrow the barnacles. On the vertical hangings, there were oysters at the lower three cultches, deminishing in number at the upper cultches which were highly affected by the fouling of barnacles.
On the new plots, the hanging of which were set on December 1983, there were fifteen to twenty spats on each shell cultch. During field observation on the last week of February, it was noted that barnacles 1 mm in size was so abundant on all cultches including the bamboo plots They were so thightly pack that there were no space left. The oyster spats being only two months old were affected by the fouling of barnacles. Some oysters were able to outgrow them. Occurence of fouling by barnacles at Cox's bazar farm was noted to be in the month of April in 1983 and February in 1984. During this period of consultancy, it was observed that the fouling of barnacles occur ones a year although there were settling of few on each month.
The barnacles that fouled the oyster cultches and the bamboos were all found dead, during inspection on May 1st. These could be due to change on environmental condition wherein the weather was so hot being summer then followed by heavy rain. These occured in the previous week.
In case where barnacles have settled on the cultches just after the latter have been set on the plot, it is possible to eliminate them. This can be done by pulling up the cultches and drying them on the shore. The dead barnacles can be knock or scrape off and the cultches to be set again on the plots.
During the rainy months of June and July, siltation was heavy. As the plots somewhat hampered the flow of water, there was silt deposited underneath the plots. These necessitated the raising of the bamboo plot, 20 to 25 cm. heigher. This was done to prevent the cultches from being buried in the mud, which will result to the death of the oysters.
Barnacles fouling at Teknaf was not heavy and didn't cause any damage to the oysters on the bamboo plots as they were quite big in size. Siltation did not cause any serious problem as the plots were quite high, except one which was so low. Such was raised to prevent the cultches to be buried in the mud.
There were twenty (20) concrete structures constructed and set at Cox Bazar farm in April 1983. No new ones were constructed.
The concrete structure is similar to that used in Thailand for oyster culture, with only little modification, A set of concrete structure is composed of two (2) Y-shaped post holding a bar horizontally. The post measure 0.9 meter long and the bar 1.5 meter, They were made of proportionate mixture of cement and clay brick chips. The concrete structure itself serve as oyster cultches.
Oysters were abundant on the sides of the posts and underneath the horizontal bars, wherein all the spaces were covered by oyster growth. The top and sides of the horizontal bars have less oysters, which is puzzling being only few centimeters higher.
Infestation of barnacles in February did little or no damage to the oyster growth as they were more than 4 cm in length. It was noted that, with barnacles attached on the posterior margin of the shell, the oysters have grown its outer margin rapidly, 2 to 3 mm. long. As what had happened in the shell cultches, the barnacles attached to the concrete structures were found dead on May 1, 1984.
Siltation did not affect the concrete structures except for five sets which were on lower elevation along the ford that crosed the farm site. Silt had reached the lower portion of the horizontal bars. On the other 15 sets, silt burried the post 2 to 3 cm only.
At Teknaf farm, nine concrete posts, measuring 1.8 meters were constructed. Three (3) of which were set on February 18, 1984 along the canal infromt of the sluice gate. They were found to have negligible attachment of oysters, due to growth of algae induced by the constant flow of water coming out from the sluice gate. There were no attachment of barnacles either.
Four (4) other posts were set at the other side far from the canal, while two (2) others were tied horizontally using three of the posts as support. The setting on April 1, 1984 was sunchronized with the attachment of oyster spats as was noted during inspection of May 3, 1984. A surface of 33 by 10 cm. had 907 spats counted while a 67 × 6 cm surface had 500 spats. During the succeeding inspection on June 8, 1984, there was fouling of barnacles which could have affected about 40 percent of the oysters.
Siltation although present at Teknaf was not heavy so as to affect the concrete posts.
For bottom culture method, clay bricks were used. This material is common throughout Bangladesh for construction of houses, building or roads. A piece of brick usually measure 6.5 cm by 11 cm. by 23.5 cm.
In the previous consultancy one thousand clay bricks were set at Cox's Bazar and five hundred at Teknaf. To prevent them from sinking through the mud as the both sites have soft bottom, the bricks were placed on top of two pieces splitted bamboos, spaced at 15 cm. apart. Small 3-meter long bamboos were used for this purpose and 10 to 12 bricks can be accomodated on each row.
The clay bricks was found to be an effecting clutching material. As seen through the dissecting microscope, the surfaces of each brick are rough with plenty of crevices where oyster larva/spats can squeezed into and escaped predation. Roughness made it easy for the spats to attached.
The one thousand bricks that were set at Cox's Bazar in March and April 1983, collected a lot of spats. Each brick had more than a thousand. But this method being on the bottom is readily affected by siltation wherein mud accumulate on the top. Wave action and boats that run over the bed, had scattered them then sunk in mud eventually. Plenty of spats died from these causes.
The use of clay bricks for oyster culture should therefore be limited to spat collecting. Setting should be done just after the rainy season up to two months before the next rainy season. This can be done, starting from September to March of the succeeding year. Before the rainy months start or three month after the oyster spats have settled, the growing oysters can be pry crf or detached from the bricks. They can then be placed on tray for culturing.
Oysters can be allowed to grow on the clay bricks but they should be raised high above the ground level to minimized the effect of siltation during rainy months. They can be placed on bamboo plots but then these will entail additional cost.
The clay bricks at Teknaf farm were relocated in an area with harder bottom. Mangrove tree branches were used as support. There was also siltation from the Naf River but the bricks were covered by only a thin layer of mud.
Trays were made of bamboo splits, nailed to two pieces of wood as frame. Three new trays were constructed, measuring 60 cm. by 100 cm. To minimized the effect of siltation, the trays were placed on top of bamboo plot so the trays were about 30 cm. above the ground level. These were in Cox's Bazar farm only. Oysters were gathered from from Teknaf and transported to Cox's Bazar for culturing in trays. These will increase the breeding stock of the area.
The hanging shell cultches that were cut from the bamboo plots and oysters detached from attachment were also placed on the trays.
Before this consultant left last June 1983, sample stakes were made. After he left, the farm personnel made 50 pieces of stake using mangrove tree branches, 0.5 to 0.6 meter long, with pieces of shells either tied or nailed to one end. They were then staked to the muddy bottom of the farm sites.
They had collected oyster spats and was found to be an effective method, being off-bottom. The only problem was that, the mangrove branches decayed within six months, so that the cultches fell down the bottom. Falling cultches were retrived and tied or hanged on the bamboo plots.
Some stakes have grown abundant oysters that a sample had a cluster of oysters measuring 21 cm. in diameter and having more than 50 pieces of 5–7 cm size. Some of the oysters were dead having been outgrown by the others in the clusters.
There is a need to find a kind of wood that will last for more than one year, which will be use as stake.
Measurement of the rate of growth of oyster is difficult since there is considerable variability between oyster specimens and when the oyster is grown in relation to the others (Quayle 1980).
From visual observation at a sluice gate at Shaphuri dwip in Teknaf, oysters that settled at a lower level are bigger than those at the upper layer. Growth rate of oyster differ depending on their level of attachment. Oyster at the lower level have longer feeding time being exposed during low tide for a shorter period, so grow faster than those at the higher level.
From the oysters collected from the above mentioned sluice gate, the biggest sample of Crassostrea gryphoides measured was 14.15 cm. the bigger size between 11 to 13 cm. These were two years old oysters as the sluice gate was constructed and finished in July 1982. The samples were those left behind after the office that maintained the sluice gate, eliminated/cleaned the sides of the gate. C. belcheri measured were 8 – 10 cm. in size.
At Teknaf farm, C. gryphoides on shell hangings have grown to 6 – 8 cm. in one year and 5 – 7 cm for C. belcheri.
At the Cox's Bazar farm, C. gryphoides on shell hangings have grown to 7 – 11 cm. in 1 ⅓ year and for C. belcheri 7 – 9 cm.
One method of determining the spawning season of oyster is by spat attachment. The others are by plankton sampling and gonadal examination. Plankton sampling will be discussed on such item below. Gonadal examination was undertaken only whenever oysters are available, due to limited quantity at the farm. At times oysters have to be taken from Teknaf but these were stock at Cox's Bazar farm to increase the breeding stock of the area.
In the collection of spats, various cultching materials were used. These are oyster shells, concrete bricks and the bamboos of the plot itself. In the Philippines oyster shells and bamboos are widely used; in Thailand, concrete structures while in Australia, wooden sticks.
Oyster larvae will settle on any hard object. On the observation at Cox's Bazar farm, clay brick was found to be the best material to attract spats due to the roughness of its surfaces. The spats also redily attached to concrete and empty shell cultches, as these materials contain calcium and so the larvae can easily cement themselves.
On the bamboos of plots settling of spat were less although there were numerous attached to shell cultches set at the same time. This was due to smoothness and the nature of the bamboo material.
In the Philippines, bamboos are used as stake culture and there was no difficulty of attracting oyster spats. The reason was that oyster culture is a developed industry in that country, with wide areas of culture bed so there were plenty of breeding stock. In Bangladesh it is only in sluice gates where they can attached naturally and all shoreline areas are muddy.
Silted and unclean cultches do not attract spats. Observation showed that spat settling occurred during rainy months but it is not adviceable to set cultches as the water contain high silt load. At times the water is of low salinity and the new spats will readily be affected when submerge for extended period.
Fouling of barnacles happened and is a nuisance. In countries with developed oyster industry, this rarely happen as there were plenty of breeders thus great quantity of larvae in the water, to offset the settling of barnacles and other marine organisms. In Bangladesh, all other marine organisms may settle on the cultches.
Oyster spat attachment occur throughout the year in Bangladesh. The temperature of the sea during winter do not go down below 22° Centigrade so they do not hybernate and can breed.
Plankton study is necessary for predicting the spawning season of oysters. Presence of early stage larvae indicate recent spawning and the number of will show whether it is a major or minor one. (Quayle 1980). Knowledge of this information will help in programming the setting of cultches to collect the spats.
Quantitative analysis for the presence of larvae in a given volume of water was not undertaken due to limited facility and equipment. These include boat, flow meter, plankton splitters and counting chambers.
Samples of plankton were taken for qualitative analysis. Sampling were taken at the farm site on waist-deep water (0.6 – 0.7 meter) using plankton net with 30-cm. diameter opening. Two persons accomplished the process. The plankton net with 30-meter rope is held by one man. Extending the other end of the rope, the other man will pull the net toward him. This process will approximately strain 0.7 cu. meter of water (with the formula pi × r2 × 10) Such is only approximation as not all water on the 10 meter tow will pass through the net. Such will need flowmeter.
The sampling being on the shore area collected sand and silt (suspension by wave action on the shore). Oftentimes the drain tube at the end of the plankton net was clogged with sand. Sampling during rainy days clogged the net with silt suspension on the water.
Laboratory analysis was therefore limited to the identification of oyster larvae and the stage of its development. Dimension and shape of specimens seen through the microscope were refered to literature of Loosonoff, Davis and Chanley 1966). This activity was as part of the training of the counterpart officers.
The pest that greatly affect the oyster culture farm was the fouling of barnacles. They were so abundant at times that they competed with space for attachment, hampered the growth or caused mortality especially on young spats. Two major occurence of barnacles was noted at Cox's Bazar which were on April 1983 and February 1984. Barnacle fouling was a real problem on young spats. When newly attached to the cultches they can easily be eliminated, by lifting the cultches and drying them on the shore for days. The cultches will then be free of the dead barnacles by scraping or knocking them off. On the bigger size oysters, barnacles caused lesser damage. The occurence of barnacle fouling varies from year to year, so this need further study.
Algae and corals (coelenterates) do foul the oyster cultch but was not experience at any of the farms. as a big nuisance. The bamboo plot and the concrete post on the canal fronting the sluice gate at Teknaf were the only ones affected. These usually become a problem when the cultches were always submerged in water and not exposed to air even for few minutes.
Predatory snails and fishes although present at the farm site did not gave serious problem.
Water temperature and salinity (Fig. 8 & Table 4) were taken regularly at least once a week. Due to unavailability of boat for this purpose, the samples were taken on the shore at 0.5 – 0.7 meter depth, a person wading to collect the samples. In order to have a near accurate record, samplings were taken during high tide, within one hour before or after. Occational samplings were done during low tide although these unimportant, the oysters being exposed to tide. Readings were plotted in a graph, disregarding those taken during low tide.
Salinity of water at the farm site as shown in the graph, were usually high within the range suitable for oyster growth. The salinity went down as the tide recede, due to discharge from river, which were almost near zero salinity at low tide. This has no effect on oysters, as they were exposed during low tide or when not can tightly close their shells. Near zero salinity was recorded at high tide but this occured when heavily raining for days and during neap tide when tide fluctuation is narrow.
On one occation during neap tide which was on Aug. 6, salinity taken at 1715 hrs. was near zero. It was noted that the tide at that time was the lower high. It was then decided to take another reading on the coming higher high tide which was in the morning of the succeeding day (Aug. 7). Sample taken at 0.7 meter depth had a salinity reading of 10 ppt. and could be much higher if to be taken at the farm site where the water is deeper.
The average salinity of water at the farm site is between 25 to 30 ppt. which is suitable for oyster growth.
Water temperature affects the growth and breeding of oysters. At temperature below 20°C. oyster metabolism is considerably decreased. Bangladesh being between 21° to 26° North latitude, has a narrow fluctuation of seawater temperature. Per record taken, the temperature did not went down below 23°C. even during the cold winter months. On the graph, it can be noted that the temperature on Jan. 11 was 19°C. The consultant doubted this reading as it went down so abruptly from 23° to 19° C. and there was no reading between such figure. Upon inquiry from the data collector, it was found out that the water sample was taken only at the surface and on the shore which was not accurate, being exposed to strong cold winter north wind.
From the record taken on temperature, it can be concluded that growth of oysters is continuous and spawning occuring throughout the year.
The counterparts were trained in the field and laboratory on various aspects of oyster culture. These include the construction, operation and maintenance of the oyster farm and in carrying out hydrobiological studies both in the field and laboratory which are necessary to collect information on oyster behavior. It is to be noted that the consultant was always with the counterparts in all the activities he had accomplished. One counterpart, Mr. Fuchrul Alam, had been guided closely on his activities on blood clam (Anadara spp.) Other government officers at Cox's Bazar Station, although not counterparts were also assisted on their research activities on other field of fisheries.
