CHAPTER - IV
BIOLOGY

1.0 INTRODUCTION

In the recent past a rapid expansion of shrimp aquaculture took place in the tidal areas of Bangladesh. It necessitated to carry out the present investigation in these areas which serve as necessary ground of shrimps, fin fishes and other organisms with a view to gather some knowledge on the ecology, community structure of plankton with special reference to shrimp post larvae and benthos of the fish ponds. However, some commendable works have been carried out on estuarine hydrobiology in Bangladesh by Ahmed (1977–81), Mohi (1977), Rahman (1977), Mahmood et al (1978), Mahmood et al (1978), Nath (1979), Bhuiyan & Islam (1981), Das (1981), Miáh (1981) Paul (1981), Quayum (1981), Saha (1981) and Islam (1982). This report includes informations on:

1. Physico-Chemical parameters of waters of the study areas.

2. Population composition and seasonal abundance of zooplankton of the estuaries with special reference to commercially important shrimp post larvae.

3. Population composition and abundance of organisms of beamtrawl catch with special reference to settlement of juvenile shrimps.

4. Abundance and seasonal distribution of macrobenthos in the fish ponds.

5. Analyses of fishery fauna of the ‘Behundi net’ catches in the estuaries.

6. Analyses of harvests from the fish ponds.

1.1 Material and Methods:

The present investigation was made between May 1982 to August 1983 in the three different localities of the tidal area of Bangladesh, viz - (a) Chakaria at Cox's Bazar situated between latitudes 21°40'N longitude 92°OOE (b) Khepupara at Patuakhali situated between latitude 22°55'N and longitude 90°10'E (c) Satkhira at Khulna situated between latitude 22°25'N and longitude 89°5'E. (Map - I). Samples were taken from two stations at Chakaria and three of Satkhira area at an interval of fifteen days in each month. The three stations of Satkhira are located respectively (i) at Satkhira (ii) at Balanapala (iii) at Ichamati.

1.11 Physio-Chemical Observations.

1. Physical:

   Air and water temperature:

   A centigrade thermometer was used to measure the temperatures.

   Rainfall:

   Data on total monthly rainfall was obtained from meteorological Department, Government of Bangladesh.

   Secchidepth:

   Secchidepth reading were obtained by using secchidisc (30 cm diameter).

2. Chemical:

   P^H:

   The determination of P^H of water was made with the P^H paper and for last five months with P^H meter at Chakaria and Satkhira and two estuarine stations of Khepupara.

   Dissolved Oxygen:

   Dissolved oxygen was determined following Winkler's method (Barnes, 1959).

   Salinity:

   For salinity determination, Mohr's chloride titration method (Strickland and Parsons, 1965) was followed where salinities were lower than 10 ppt and by a refractometers when higher.

1.12 Sampling Procedure of Zooplankton:

1. Zooplankton:

   Samples were collected twice a month during fullmoon and new moon phases. Day-night and surface-bottom horizontal haules were done during high tide with the help of a 0.5 mm meshed and coneshaped tow net having rectangular mouth opening and attached to a 185 cm × 55 metal made rectangle. This net was designed after Mahmood and Khan (1982). The net was made up of bolting silk cloth. A flow meter was set up at the month of the net to record the volume of water strained during the collection. Just after hauling samples were preserved in 5% neutralised formalin.

2. Beam Trawl Catch

   The peat larvae of shrimps were collected with a small modified beam net (Mahmood and Khan, 1982), originally designed by W.C. Renfre (Gracia and Reste, 1981).

   The body of the net was cut from a single piece of nylon netting 200 cm x 175 on having 50 opening/cm². The finished net was 150 cm wide on both the head and foot ropes.

   The throat was a canvas cylinder, 20 cm long and 30 cm in diameter, sewed to the end of the body. An ordinary plankton net of 30 cm diameter having 500 meshes with a removable bucket was tied over the throat. A 215 cm nylon rope fitted with local cast net lead weights served as the footrope. The head rope was threaded through five 6 cm diameter sponge floats. Ends of the foot rope and head rope were attached to a 190 cm long, 3 cm diameter brass pipe which serves as the beam.

   In operation, the net was pulled by a bridle at ached to the ends of the beam. When pulled slowly, the beam drags along the bottom and to the mouth. Each haul was of 10 minutes in the stations C-1 & C-2 at Chokaria and S-1 & S-2 at Satihira.

3. Collection of Benthos:

   Benthic macro invertebrates in ponds ware collected with the help of Vanveen grab sampler.

4. Behundi Net Catch:

   Two stations were selected from each of the three areas. Two samples, each containing 1 kg, were c collected from each of the stations at fortnightly interval from the catches of behundi nets. Of mesh size 12 mm in codend and 18 mm in the wings. The samples were preserved in 10% neutral formalin. Total length of shrimps and fishes were measured following standard methods. In laboratory two samples were analysed seperately and the result was prepared from the average of the two.

5. Pond Harvest Catch:

   Pond Harvest catch was studied at Chakaria and Satkhira area. Two ponds were selected from each of these two areas. 1 kg. samples were collected fortnightly from each of the two selected ponds and preserved in 10% neutral formalin.

   Identification of zooplankton, larvae of shrimps were made following Davis (1955), Wickstead (1965) and George (1969).

   Fishes and shrimps were identified following Day (1978), Kemp (1915), Holthuis (1950), Munro (1955), Ahmed (1957), Hossain (1971), Howlader (1976) and Khan (1977). Shrimp larvae were identified following Kemp (1925), Khandaker & Patra (1971), Khan (1977), Silas (1978) and Amin and Mahmood (1979).

2.0 HYDROMETEOROLOGY OF STUDY AREA

2.1 General

Physico-Chemical parameters of water in different estuarine stations and ponds of the three areas, viz - Khepupara and Satkhira were studied at an interval of fifteen days and monthly average data on physical and Chemical conditions of water of these areas are shown in Tables 1–13.

2.2 Temperature

(Tables 1,2,7,8 and Figures IIa & IIb).

In this studies air temperature and water temperature at the surface were noted. The highest temperature, both air and water were recorded in May and the lowest in January in all the three areas (Tables 1 & 2).

In all the Stations of the three areas the water temperature was found to follow closely the air temperature.

2.3 Rainfall

(Table-13 and Figure IIC).

In the present observation, the rainfall maxima were recorded as 38.47 m.m. in June, 1982 at Chakaria, 26.69 m.m. in August, 1982 at Khepupara and 14.6 m.m in August, 1983 at Satkhira. Where as the minima were noted as 0.1 m.m and 0.16 m.m in October, 1982 at Chakaria and Khepupara respectively with 0.34 m.m in January in December, 1982 in all the three areas.

FIG. 11 a. SHOWING MONTHLY AVERAGE FLUCTUATION OF AIR TEMPERATURE, (°C) AT CHAKARIA KHEPURA AND SATKHIRA.

FIG. 11 b : SHOWING MONTHLY AVERAGE FLUCTUATION OF WATER TEMPERATURE (°C) IS STATIONS AT CHAKARIA KHEPURA AND SATKHIRA.

FIG. IIC : SHOWING MONTHLY AVERAGE SALINITY (‰) AND RAINFULL (mm) IN STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

2.4 Secchi Depth Range

(Tables-3 & 9 and Figure IID)

Secchi disc visibility was recorded in different sampling stations of the three study areas and results have been shown in Tables- 3 and 9. Of the three areas the highest fluctuation of secchi disc visibility was recorded at Chakaria area (23 cm in August and 97.5 cm in January in river, 35.5 cm in August and 73.8 cm in October in Ponds) Whereas the Satkhira area showed the minimum (7 cm in October and 14 in August in the rivers, 19.5 cm in December and 45.5 cm in February in Ponds).

The secchi disc visibility at Khepupara area was recorded to range between 9 cm in August and 31 cm in January in the river.

2.5 P^H Range

(Tables-4 & 10).

The P^H of the surface water of the three areas was observed and the monthly fluctuation in P^H is shown in Tables 4 and 10. In river of Chakaria area the minimum P^H (6.5) was recorded in April and the maximum (7.5) in September to January whereas in pond the minimum P^H (6.9) was noted in June and the maximum (7.6) in January and February. In river of Khepupara the P^H fluctuated between 6.5 (August and June) and 7.0 (March to May and July and August).

The P^H was recorded to vary between 6.8 (June and July) and 7.8 (January) in river at Satkhira area whereas in the ponds the lowest P^H was noted to be 6.2 in July and the highest value of 7.9 in May and June.

Fig II d - Showing Monthly Average Fluctuations Of Secchidepth (cm) At Chakaria, Khepupara and Satkhira.

2.6 Dissolved Oxygen

(Tables-5 & 11)

The dissolved oxygen content at surface water in the three areas were recorded and the monthly variation in DO content is shown in Tables-5 and 11. At Chakaria area D.O. content was found to vary between 3.7 – 6.9 ml/l in river and 3.7 – 7.9 ml/l in pond. At Satkhira the amount D.O. fluctuated from 3.6 ml/l to 5.5 ml/l in river and from 4.1 ml/l to 5.9 ml/l in ponds. At Khepupara the maximum D.O. content was recorded to be 7.4 ml/l whereas the minimum was 5 ml/l.

2.7 Salinity Range

(Tables-6 & 12)

In the present studies the salinity of surface water was recorded and the fluctuation in salinity in different months is shown in Tables 6 and 12. During the present studies the salinity of water showed the highest fluctuation (0.4‰–32.6‰ in river and 1‰–35.7‰ in ponds) at Chakaria area. The minimum fluctuation in salinity was observed (5‰–21.5‰ in river; 5‰–18.9‰ in ponds) at Satkhira area. At Khepúpara area the salinity of water was noted to vary between 0.5‰ and 17.8‰.

2.8 Comments

The temperature is generally regarded as one of the important factors in aquatic biology (Ali, 1980). In all the investigated areas the average air temperature was observed always higher than average water temperature and a close parallel relationship existed. Thus the water temperature and air temperature was found to go hand in hand. So, the present study showed that the water temperature in these areas was mainly influenced by air temperature.

TABLE-1: SHOWING AVERAGE AIR TEMP. (°C) IN STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 2: SHOWING AVERAGE WATER TEMP. (°C) IN STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 3 : SHOWING AVERAGE SECCHIDEPTH (CM) IN STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 4 : SHOWING AVERAGE p^H IN STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 5 : SHOWING AVERAGE DISSOLVED OXYGEN (ml/l) IN STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 6 : SHOWING AVERAGE SALINITY (‰) IN STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 7 : SHOWING MONTHLY AVERAGE AIR TEMPERATURE (°C) IN DIFFERENT STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 8 : SHOWING MONTHLY AVERAGE WATER TEMPERATURE (°C) IN DIFFERENT STATIONS AT CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 9 : SHOWING MONTHLY AVERAGE SECCHI DISC DEPTH (IN CM) IN DIFFERENT STATIONS IN CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 10 : SHOWING MONTHLY AVERAGE PH IN DIFFERENT STATIONS IN CHAKARIA, KHEPUPARA AND SATKHIRA.

TABLE 11 : SHOWING MONTHLY AVERAGE DISSOLVED OXYGEN CONTENT (ml/L) OF WATER IN DIFFERENT STATIONS AT CHAKARIA KHEPUPARA AND SATKHIRA.

TABLE 12 : SHOWING MONTHLY AVERAGE SALINITY (‰) IN DIFFERENT STATIONS AT CHAKARIA KHEPUPARA AND SATKHIRA.

TABLE-13 : Showing Average Monthly Rainfall in MM at Chakaria, Khepupara & Satkhira

3.0 POPULATION COMPOSITION, ABUNDANCE AND SEASONAL DISTRIBUTION OF ZOOPLANKTERS WITH SPECIAL REFERENCE TO PENAEID POST LARVAE.

In the present investigation the population composition, abundance and seasonal distribution of shrimp larvae, finfish larvae and other zooplankton were studied and these are shown in detail in Tables-14 – 32 and Figs. 1 – 6 and the replicate of the some studies are given in the Tables Rl - R16.

Of the three areas the highest occurrence of organisms was withessed at Chakaria (6691.968/m³ in 16 months with monthly average of 418.248/m³) and the lowest at Khepupara (1634.885/m in 13 months with monthly average of 125.760 organisms/m³). The Satkhira area stood intermediate between Chakaria and Khepupara with total number of 4647.609 organisms/m³ in 15 months with monthly average of 309.841 organisms/m³ (Fig.1. Tables 14–16). The Satkhira and Chakaria areas showed a simillar trend in seasonal occurrence of total organisms. A large number of organisms was found between May and July in 1982 and between May and August in 1983 at Chakaria with the peak (729.311 organisms/m³ in 1982, 881.353 organisms/m³ in 1983) in July during both the year. The Satkhira area was witnessed a high density of organisms from June to August in 1982 and April to August in 1983 with the highest occurrence in August (1023.67 organisms/m³ in 1982, 497.39 organisms/m³ in 1983) during both the years.

On the contrary, at Khepupara a rich populations of organisms was noted from January to May with the highest number of 352.12 organisms/m³ in May (Tables 14 – 16 Fig. 4).

However in the replicate sampling the highest occurrence of organisms was noted at Satkhira (4943.027 indvs./m³ in 15 months with a monthly average of 329.534 indvs/m³) and the lowest at Khepupara (1267.086 indvs/m³). The Chakaria area stood intermediate nryerrm Satkhira and Khepupara with total number of 4720.34 indvs/m³ in 16 months with a monthly average of 295.021 indv/m³. But when the average of the orginal and replicate samples were considered it was noted that the Chakaria area contained the highest populations of zooplankton (356.635 indvs/ m³) and the second position was occupied by Satkhira (319.688 indvs/m³).

Among the various organisms Mysid was the dominant fauna at Chakaria (46% of the total organisms) and also at Satkhira (63% of the total organisms) whereas Chaetognatha occupied the top position (40.5%) at Khepupara area which was closely followed by Copepods (39%). The Copepods also occupied the second position in Chakaria and Satkhira area (Table 14–16). Simillar observation was noted in replicate samples (Tables R 1 - R 3).

The density of the organisms during full moon and new moon, at surface and bottom during day and night was observed and these are shown in appendices B I - VII & BR I - VII. In these studies no clear trend was noted in the occurrence of organisms during full moon and new moon both in orginal and replicate samples. However, the number of organisms was found greater during night in all the three areas in both the samples (Appendices B 1 - VII; BR 1 - VII).

The Chakaria area occupied the top position in the occurrence of shrimp larvae (Monthly average abundance of 4.328/m³) whereas Khepupara took the last position (Monthly average abundance of 1.085/m³). At Satkhira area a rich population of shrimplarvae was observed during the present studies (Monthly average abundance of 4.22 organisms/m³) (Figs 2–3, Tables 17–19).

However, in the replicate sample the highest occurrence of shrimp larvae was found at Satkhira (4.877 indvs/m³) and the Chakaria area stood the second position with monthly average of 3.37 indvs/m³. The Khepupara area showed the minimum number of Shrimp larvae (Tables R 4 – 6).

At Satkhira area 1.36% of total organisms were shrimp larvae. At Chakaria area 1.035% of total organisms were shrimps larvae whereas at Khepupara only 0.862% of the total organisms were shrimp larvae (Fig. 2). More or less similar observation was found in replicate samples (Tables R 7 – 9).

The maximum number of shrimp larvae was recorded from June to August at Satkhira with the peak occurrence of 16.73 /m³ in June, 1982. At Chakaria area the shrimp larvae were found to be maximum also during June to August with the highest occurrence (27.272/m³) in July, 1982. The maximum density of shrimplarvae was noted between November and January at Khepupara with the peak (3.316/m³) in November (Table 23–25). Similar trend was witnessed in replicate samples (Tables R - 7–9).

The penaeid shrimp occupied 0.919% of the total fauna at Chakaria with the maximum density of 26.807/m³ in 1982 and 11.838/m³ in 1983. The Satkhira area was found to hold (0.92% of total organisms) more or less similar concentration of penaeid shrimps with the peak (11.37/m³) in June in 1982 and in July (10.106/m³) in 1983. At Khepupara the penaeid shrimp was recorded to constitute only 0.29% of the total organisms. More or less similar trend was recorded in replicate samples (Table R 1–3).

Of the penaeid shrimps four species viz. Penaeus monodon, P. indicus, Metapenaeus monoceros and M. brevicornis were noted to be the major fauna at Chakaria and Satkhira area. Among these organisms Penaeus monodon was found to be dominant fauna at Chakaria area with monthly average occurrence of 1.707 organisms/m³ (Table 20 and 22). At Satkhira area Metapenaeus monoceros was the dominant organisms among the penaeid shrimp (Monthly average abundance of 1.142/m³). In Khepupara three major species among the penaeid shrimp were recorded. These are, Penaeus monodon, P. indicus, Metapenaeus monoceros (Table 21). P. monodon was observed to be the dominant fauna among the penaeid shrimp at Khepupara area.

However in the replicate samples Metapenaeus monoceros occupied the top position both in Chakaria and Satkhira area and P. indicus took the highest position at Khepupara area (Table R 4–6).

The caridean prawns were witnessed to form 0.109%, 0.569% of the total fauna at Chakaria and Khepupara area respectively. Comparatively, a rich population of the organisms was noted at Satkhira area which constituted 3% of the total fauna. The highest density was found to occur at Satkhira in June (5.36/m³) in 1982. On the contrary in replicate samples 0.16%, 4.87% and 1.55% of the total organisms was caridean prawn found respectively from Chakaria, Satkhira and Khepupara (Table R 1-3).

In the present investigation the highest number of finfish larvae was noted from Chakaria area (Monthly average of 6.096/m³ and 1.456% of total organisms). More or less similar occurrence of finfish larvae was witnessed at Satkhira (Monthly average of 2.631/m³) and Khepupara (Monthly average number 2.346/m³). Thus the Satkhira area was characterized by rich populations of shrimp larvae and a poor concentration of finfish larvae in comparison to the other two areas (Tables 23–25). More or less similar observation was recorded in replicate samples (Table R 1–3, 7–9).

At Chakaria the number of finfish larvae was noted to be high between June and July in 1982 and between April and August in 1983. The peak in occurrence of finfish larvae was observed in August (28.835/m³) similar trend was noted in replicate samples. At Khepupara the density of these organisms was high during September and November with the highest number of 7.516/m³ in November. At Satkhira area the density of finfish larvaê was witnessed to increase during June and August in 1982 and during March, April and June in 1983. The highest number was recorded in April (8.34 organisms/m³) (Tables 23–25). On the other hand the highest occurrence of finfish larvae in replicate samples were recorded in October, June respectively from Khepupara and Satkhira areas.

The seasonal abundance, monthly average and percentage composition of shrimp larvae, finfish larvae, other zooplankton and total organisms at the different stations of the three areas were studied and these are shown in detail in Tables 23–25 and those of the replicate samples were shown in Table R 7–9. The monthly average distribution and percentage composition of major species of penaeid shrimp larvae was also noted at different stations in these areas and these are shown in Tables 23–32. The data of the replicate samples of the similar studies are shown in Tables R 10–15. However, these are described briefly in the following lines.

3.01 Chakaria Area: (Tables 23, 26, 27 and R-7).

In the present investigation no marked difference in abundance of total organisms and their seasonal distribution was recorded from the two stations at Chakaria. At this area the yearly average density of 482.108/m³ was noted from station - 1 whereas the station-2 was found to have 354.388 organisms/m³. The both the stations showed a similar trend in seasonal occurrence.

A high concentration of total organisms was found during May to August in both the stations at Chakaria. The peak abundance of total organisms was also recorded in July at both of these stations. More or less similar trend was recorded from station-1 of the replicate samples. However the station-II showed lower number of organisms (184.624 indivs/m³) with the peak in August (Tables R-7).

The number of finfish larvae was observed to be higher at station 1 (42/m³) whereas the number of shrimp larvae was almost equal at the two stations (Table - 25). However at station-I the number of shrimp and finfish larvae was noted to be greater in replicate sample (Table R-7).

At both the stations Metapenaeus monoceros was the dominant fauna among the penaeid shrimps.

3.02 Khepupara Area: (Tables 24, 28, 29, R-5, 8, 12 & 13).

The two stations of Khepupara contained almost the equal number of total organisms. They also showed the similar trend in seasonal occurrence. Penaeus monodon was found to be the dominant among shrimp larvae at both the two stations of Khepupara (Tables 28, 29). In the replicate samples the station-II of Khepupara showed the highest occurrence of shrimp, finfish larvae and also the total organisms.

M. monoceros was the dominat among penaeid shrimps at station-I whereas P. indicus occupied the top position at station-II in this area.

3.03 Satkhira Area: (Tables 25, 30, 31, 32 and R-6, 9, 14–16)

A wide variation in abundance of total organisms between station-1 and rest of the two stations was observed at Satkhira.

The station-1 constituted quite a large number of organisms (Monthly average 586.275/m³) whereas the station-II (136.396/ m³) and station-3 (103.27/m³) borne the minimum number. The three stations at this area were also noted to differ in the abundance of panaeid shrimps. Similar observation was found in replicate samples (Table R-9). The density of finfish larvae showed a remarkable variation in these stations. At the station-I the total number of finfish larvae was 5.037 organisms/m³ whereas the station-2 and 3 were recorded to contain 1.18/m³ and 0.218/m³ respectively. Similar observations was found in replicate samples.

Penaeus monodon was the dominant shrimp at station-1, whereas Metapenaeus monoceros was found to be the dominant at station-2 and 3. However, M. monoceros was recorded to be the dominant at the three stations in replicate samples.

3.1 Comments:

In the present studies the highest occurrence of organisms was noted at Chakaria area (Monthly average 418.248/m³). The Satkhira area also supported a reasonably high concentration of organisms (Monthly average 309.841/m³). Inspite of the rich occurrence of organisms at Satkhirà there exists a noticeable gap in the abundances between these two areas. However, the lower density of organisms which was recorded during the period of studies, does not possibly focus the exact potentiality of Satkhira area to bear organic life. If all the three sampling stations at Satkhira were chosen at same latitude or close to the vicinity of the mangrove forest, as it was done at Chakaria area, the picture in the occurrence of organisms would have been quite different. The sampling station-1 (Ellarchar) at Satkhira though is located upward to the station-2 (Kaliganj) and station-3 (Ichamati) it was found to be highly productive as almost a negligibel seed collecting practice has been developed in the channels those feed the area. It is presumed that if this station was chosen further down at more or less same latitude of other two stations or close to the mangrove forest the number of organisms would have been increased to a great extent. The station-3 was selected at Ichamati river and station-2 is feed partly by the same river. The Ichamati is a wide river and an extensive fishing for shrimp larvae has been practiced both at Indian and Bangladesh side. These are the possible reason for getting low density of organisms at Satkhira. The minimum number of organisms at Patuakhali may be due to the fact that the sampling stations located at wide channels which further increase in size during monsoon and thus the number of organisms become very poor and become the minimum at the rainy season (Tables 14–16).

The concentration of total organisms was recorded to be high during night at surface in all the three areas of observation (Appendices B-I - VII). The above observation is quite natural as some of the crustacean fauna are photophobic in nature.

The Chakaria (Monthly average abundance of 4.328/m³) and Satkhira (Monthly average of 4.22/m³) areas exhibited almost similar concentration of shrimp larvae (Fig. 3). The density of shrimp larvae was found to increase considerably (Monthly average of 4.5/m³) at Satkhira if we avoid the observation at station-3 of the area. The station-3 as described earlier was located at Ichamati river. The Ichamati is a wide river and it is subjected to the maximum pressure for collecting shrimp seeds from both Indian and Bangladesh side. Thus the number of shrimp larvae decreased to a great extent in the river which ultimately lowered the total population of shrimp larvae at Satkhira.