CHAPTER - IV (Contd.)

4.0 POPULATION COMPOSITION, ABUNDANCE AND MONTHLY DISTRIBUTION OF MACRO-ORGANISMS IN AS REVEALED THROUGH SMALL BEAM NET SAMPLES WITH SPECIAL REFERENCE TO POSTLARVAL AND JUVENILE SHRIMPS

In this study the population composition, abundance and monthly distribution of shrimps, fishes and other organisms including zooplankton were obtained by dragging small beamtrawl along the selected river-bottoms of Chakaria and Satkhira, and these have been shown in detail in Tables 33–38.

During the surveyed period from April to August, 1983 the higher density of organisms was recorded at Chakaria (1622.35/haul, monthly means). At Chakaria, the maximum of 2893.75 indvs/haul was obtained in May whereas at Satkhira it was (310.21 indvs./ haul) recorded in April. The minimum number of organisms was noted in August in both the areas (67.0 indvs./haul at Chakaria and 100.09 indvs./haul at Satkhira).

Among the various organisms mysid was found to dominate both at Chakaria (93.7% of the total organisms) and at Satkhira (31.1%). At Chakaria, Amphipod (1.97%) and Fin-fishlarvae (1.5%) occupied the second and third position respectively. On the other-hand at Satkhira the second position was occupied by Molluscs (30.3%) and Crab was in third position (16.7%). However, at Chakaria, Crab (0.03%) and Molluscs (0.01%) also occured but in poor number.

Abundance of ichthyoplankton found higher at Chakaria (24.25 indvs./ month) than that of Satkhira (7.698 indv./haul/month). In both areas ichthyoplankton were most abundant in April (63.75 indv./haul at Chakaria, 23.75 indv./haul at Satkhira) and the lowest was found in July (4.00 indv./haul at Chakaria; 3.25 indv./haul at Satkhira).

Concentration of shrimps postlarve and juveniles was also found to be higher at Chakaria with a monthly average of 30.25 indv./ haul and it was 26.93 indv./haul at Satkhira. At Chakaria, the maximum number of shrimps were obtained in May (71.00 indv./ haul) and the minimum in August (3.50 indv./haul).

However, at Satkhira the maximum number of shrimp larvae (47.59 indv./haul) and the minimum (10.48 indv./haul) were found respectively in June and May.

Among the penaeid shrimps, twelve species belonging to three genera (Penaeus, Metapenaeus and Parapenaeopsis) were identified of which Penaeus, semisulcatus was only reported from Chakaria (Table 35 & 36).

At Chakaria Penaeus indicus was the dominant species (0.7% of the total organisms) and it was followed by Metapenaeus brevicornis (0.4%) and M. monoceros (0.3%) respectively. However, the highest concentration of M. monoceros was found at Satkhira (2.3%), it was followed by M. brevicornis (2.0%).

The distribution of caridean prawns was found to be higher at Satkhira (8.0% of the total organisms) than that of Chakaria (0.2%).

Monthly percentage composition of post larvae and juveniles of different shrimps at Chakaria and Satkhira areas have been shown in Tables 37 & 38. The distribution of major shrimp fauna has been described in the following lines.

At Chakaria the post larvae of P. monodon was obtained in May and June and 50% catches of May was in juvenile stage.

At Chakaria P. indicus was present all along the sampling period except in August and the juvenile were found to constitute 40% and 7.6% of total catches of April and May respectively. On the otherhand at Satkhira, P. indicus was totally absent in April. The maximum occurrence of juvenile of P. indicus was noted in August (100%) at this area.

M. monoceros was absent in April and August at Chakaria but at Satkhira this species was abundant in beamtrawl catches allover, the period. The juvenile of this species formed 100% and 22.2% in May and June respectively of Chakaria. However, at Satkhira the juvenile of M. monoceros dominated throughout the period except in April when the post larvae of this species constituted 74.3%. M. brevicornis was found to occur throughout the period in both the areas. The juveniles of this species was dominated in May (100%) and August (66.7%) at Chakaria. At Satkhira juvenile were found throughout the period except in April, when the post-larvae were prominent (69.7%).

COMMENTS

In the present investigation a large number of organisms in beam net catch was recorded at Chakaria area, frequency of occurdance was lesser. The sampling stations at Satkhira being in more up-stream region of the rivers than in Chakaria, abundance of both post larvae and juvenile shrimps was probably lesser, that post larvae do immigrate but in on the otherhand, at Chakaria lesser quantumn because of tidal push and that may be juveniles settle at more down stream area.

Moreover, the two stations at Satkhira were located in river and channel. Wherefrom an extensive collection of shrimp seeds are practised and which minimised the number of shrimp fauna to a greater extent.

Among the penaeid shrimps 12 species were recorded in the present studies. Penaeus indicus was the dominant species at Chakaria. Metapenaeus monoceros occupied the top position at Satkhira. Of the shrimp species, Penaeus semisulcatus was not found at Satkhira.

TABLE 33: SHOWING MONTHLY AVERAGE ABUNDANCE AND DISTRIBUTION OF TOTAL SHRIMPS, FISHES AND OTHER ORGANISMS IN BEAM TRAWL CATCH/ HAUL AT CHAKARIA.

TABLE 34: SHOWING MONTHLY AVERAGE ABUNDANCE AND DISTRIBUTION OF TOTAL SHRIMPS, FISHES AND OTHER ORGANISMS IN BEAMTRAWL CATCH/HAUL AT SATKHIRA.

TABLE 35 : SHOWING MONTHLY AVERAGE DISTRIBUTION OF ORGANISMS IN BEAM TRAWL CATCH/HAUL AT CHAKARIA.

TABLE 36 : SHOWING MONTHLY AVERAGE DISTRIBUTION OF ORGANISMS IN BEAMTRAWL CATCH/HAUL AT SATKHIRA.

TABLE 37 : SHOWING MONTHLY PERCENTAGE COMPOSITION OF POST LARVAE AND JUVENILES OF SHRIMPS IN BEAM TRAWL CATCH/HAUL AT CHAKARIA.

* Figs in brackets ( ) show the percentage composition of Juveniles.

TABLE 38 : SHOWING MONTHLY PERCENTAGE COMPOSITION OF POST LARVAE AND JUVENILES OF SHRIMPS IN BEAM TRAWL CATCH/HAUL AT SATKHIRA.

* Figs in brackets ( ) show the percentage composition of Juveniles.

5.0 ABUNDANCE AND SEASONAL DISTRIBUTION OF BENTHIC MACRO-INVERTEBRATES IN BONDS AT CHAKARIA AND SATKHIRA.

The abundance and seasonal distribution of benthic macro-invertebrates were studied from ponds of Chakaria and Satkhira. These have been shown in detail in Tables 39 & 40 and Fig.8.

However, in this investigation the benthos were not seperated upto different species or even to group level only the total number organisms as found in the collection have been presented to show population density in time and space. A brief description on the abundance and distribution of these organisms is as follows.

In the present investigation a rich population of benthic macro-organisms was noted in ponds of Chakaria with monthly average of 8649 indv/m². The ponds at Satkhira also showed more or less similar richness in the occurrence of benthic organisms (Monthly average of 6512 indv/m²).

In the present study, a clear difference in the seasonal abundance of banthos were observed between ponds of two areas. In the ponds at Chakaria area the number of macro-invertebrates was found to increase in winter with the peak in January (24916 indv/m²). The density of these organisms at Chakaria area was noted to decrease sharply as the monsoon approached. On the contrary, the population of organisms was found to increase in ponds at Satkhira during rainy season. The peak occurrence of these organisms was however recorded in September (19005 indv/m²).

5.1 Comments:

Thienemann (1954) reported that a bottom of a water body containing benthic organisms of 1000 indv/m² might be considered as productive. In the present observation bottoms of ponds of both the area contained more organisms than required to become a productive. Thus the pond bottoms of both the areas may be considered as highly productive for culture fishery.

A remarkable difference in the seasonal abundance of macro-invertebrates was noted between ponds of two areas in the present studies. The Chakaria area showed rich population during winter whereas Satkhira area during rainy season. Similar observation was made by Ali et al (1980).

FIG. - 8 SHOWING AVERAGE MONTHLY OCCURRENE OF BENTHIC MACRO-INVERTEBRATES AT CHAKARIA AND SATKHIRA.

TABLE 39 : SHOWING MONTHLY FULLMOON-NEWMOON DISTRIBUTIONS OF BENTHIC MACRO-INVERTEBRATES IN PONDS AT CHAKARIA.

F : Full moon
N : New moon

TABLE 40 : SHOWING MONTHLY FULLMOON-NEWMOON DISTRIBUTION OF BENTHIC MACRO-INVERTEBRATES IN PONDS AT SATKHIRA

F : Full moon
N : New moon

6.0 BEHUNDI FISHERY FAUNA OF THE THREE STUDY AREAS

In the present observation the seasonal occurrence, abundance and percentage composition of shrimps, finfishes in “Behundi” net catch of three areas were studied and these are shown in Tables 44–60. The studies on sex-ratio of penaeid shrimps were also noted and this are shown in Tables 56–58.

In all 17 species of shrimps (Belonging to four families), 58 species of fishes (belong to 38 families) and 12 texa of other fauna were recorded. However, some shrimps and fishes of different areas are enlisted hereunder:-

SHRIMPS

6.1 Seasonal occurrence and Distribution of Shrimps of Behundi Catch

(Tables 44, 46, 48, 50, 52, 54 & 59)

In the present investigation the highest density of shrimps were found in Behundi catch at Satkhira (with a monthly average of 505.6 indvs/kg) and it formed 64.63% of total catch of the area. The Khepupara occupied the second position (492.92 indv/kg and 62.33%) of total catchment. The lowest number of shrimps was recorded at Chakaria (172.5 indv/kg and 4.33%) of total catchment (Tables 41–43).

At Chakaria, the maximum number of shrimps was found in May, 1982 (604 indv/kg) and the minimum in June, 1983 (49 indv/kg) whereas, at Khepupara the highest density of shrimps was recorded in November, 1982 (1688 indv/kg) and the lowest in March, 1983 (118 indv/kg).

At Satkhira the peak abundance was recorded in September, 1982 (1070 indv/kg) and the lowest in March, 1983 (194 indv/kg).

In the present investigation ten species (belonging to three genera) of penaeids and five species (belonging to two genera) of carideans were identified from the samples of ‘Behundi’ catch. Metapenaeus monoceros and M. brevicornis were the most abundant species throughout the period in all the study areas. Penaeus monodon and Penaeus indicus were common at Chakaria and Khepupara as well as at Satkhira. Another Penaeid (Parapenaeopsis sculptilis) and two carideans (Palaemon (Exo.) styliferus & a Palaemon sp.) were the most abundant in average distribution at Khepupara and Satkhira. However, at Chakaria P. sculptilis was totally absent and P. (Exo.) styliferus and palaemon sp. were rarely present. Macrobrachium rosenbergii, an important freshwater fauna was rarely found only at Khepupara and Satkhira. On the otherhand, Penaeus semisulcatus was only reported from Chakaria. Parapenaeopsis sculptilis was also found at Khepupara area only. A single species of Sergestids (Acetes sp.) was ound to be the dominant species at Khepupara. Alpheid shrimps were also noted in the samples in this area.

The occurrence and distribution of major penaeid shrimps are described in the following lines:

Penaeus monodon: The highest occurrence of this species was observed at Chakaria area (Monthly average 1.31 indv/kg) which was followed by Khepupa (0.46 indv/kg) and Satkhira (0.13 indv/kg) respectively. At Chakaria, the maximum number was found in April, 1983 (7.0 indv/kg) and the minimum in July, 1983 (1.0 indv/kg).

Penaeus indicus: The juveniles of this species were the most abundant at Chakaria (monthly average 21.94 indv/kg) with the peaks in May, 1982 (118 indv/kg). At Khepupara and Satkhira this species was rarely present.

Metapenaeus monoceros: Metapenaeus monoceros occurred abundantly in all the three areas of which the highest population was noted at Satkhira monthly average of 65.6 indv/kg) whereas at Chakaria and Khepupara (Monthly averages of 42.5 indv/kg and 17.27 indv/kg were found respectively.

At Chakaria, the maximum number of M. monoceros was recorded in November, 1982 (85 indv/kg) and the minimum in May, 1982 (12 indv/ kg). The highest number of this species was recorded in October, 1982 (49 indv/kg) at Khepupara. At Satkhira the highest abundance was observed in August, 1982 (monthly average of 315 indv/kg). The population gradually declined in July, 1983 and onwards.

Metapenaeus brevicornis: This species was also found in large number in all the three areas. The highest population, however, was recorded at Satkhira (monthly average of 158.07 indv/kg). The Khepupara occupied the second position with monthly average of 73.38 indv/kg. At Chakaria monthly average of 42 indv/kg was noted.

The abundance of the species reached the peak in November, 1982 (99 indv/kg) at Chakaria and the lowest number was found in January and March, 1983 (6 indv/kg).

At Khepupara, the peak abundances were observed in December, 1982 (203 indv/kg respectively). More or less similar trend of distribution of this species was found at Satkhira, where the peak was noted in September, 1982 (665 indv/kg).

6.2 Distribution of the Males and Females (Sex-Ration) of Penaeid Shrimps in Behundi Catch.

The distributions of male and female sexes (sex-ration) of different penaeid shrimps the three areas were studies and these are shown in Tables 56–58 and Appendices B-VIII - X.

It was observed that the males of Penaeus monodon was dominant in June, 1982, May & August, 1983 at Chakaria and in August, 1982 at Khepupara. However no males of this species was reported from Satkhira.

The female of Penaeus indicus was noted in large number in all the three areas. However, at Chakaria the males of Penaeus indicus were found to be the maximum in June & July, 1982.

It was revealed from the present investigation that the males of Metapenaeus brevicornis were more abundant than the females of the same species in the months of May & December, 1982 at Chakaria whereas the females were dominating in other months except in August, 1983 when both the males and females equaled in number. At Khepupara, females were dominant throughout the period, except in the months of July, September & October, 1982 and March and May, 1983 females were found to occur in higher percentage at Satkhira. Females of Metapenaeus monoceros were found to be dominating throughout the period at Chakaria except in July, 1982. Similar observation was recorded at Khepupara, where males were dominating only in November, 1982 and January, 1983. At Sathira, males of the species were found to dominate in September through November, 1982 and February through May, 1983 whereas the females occupied the top position during the rest of the period.

6.3 Seasonal occurrence and Distribution of Fishes Found at Behundi Catch.

In the present study fifty eight species of fishes were identified from the samples obtained at Behundi catch. Of these species three were Chondrichthyes (belonging to two families) and the remaining fifty five were Osteichthyes (belonging to 26 familes). The seasonal abundance, distribution and percentage composition of different fishes are given in Tables 45, 47, 49, 51, 53, 55 & 60.

In general several species of fishes were found to occur abundantly in one or two areas but rarely present or totally absent in other area. For example, Leiognathus sp. and Mystus spp. were dominant at Chakaria and Khepupara though these frequently occured at Satkhira. At Chakaria, Mugil tade, Apocryptes sp. Glossogobius sp. and Anguilla sp. were the dominating species whereas at Khepupara, Illisha filigera, Gadusia sp. Chirocentrus dorab, Mugil spp. Otolithoides sp. and Periophthalmus sp. were most abundant. However, Coilia dussumieri, Harpodon nehereus and Mystus guilio were the major species at Satkhira area (Table-60).

In the present observation the fishes were noted to be the most abundant at Khepupara (monthly average 279.08 indv/kg and 35.29% of total catch of the area) whereas the Chakaria area was ranked the second position (monthly average 249.3 indv/kg and 6.26% of total catch of the area). The lowest number of fishes was recorded at Satkhira (monthly average 244.5 indv/kg and 31.26% (Table 41 – 43).

At Chakaria, the highest number of fishes was found in March, 1983 (.93 indv/kg) and the lowest in June, 1982 (75 indv/kg). Whereas at Khepupara the maximum number of fishes was observed in February, 1983 (822 indv/kg) and the minimum in July, 1983 (77 indv/kg). However, at Satkhira the peak of population was recorded in October, 1982 (667 indv/kg) and the lowest number of fishes was found in September, 1982 (103 indv/kg).

A large number of species of fishfauna occured in the Behundi fishery during the present studies of which a few common species of commercial importance are described hereunder.

Hilsa ilisha: This species was dominant at Khepupara (monthly average 91 indv/kg) whereas it was rarely present at Satkhira (1 indv/kg) and Chakaria (0.68 indv/kg) respectively. At Khepupara it appeared in most of the months except in May to August, 1983. However, it was first reported in the catch in August, 1982 and February, 1983 at Khepupara and Satkhira respectively. The highest number of the species was noted (49 indv/kg) in January, 1983 at Khepupara; in April, 1983 at Chakaria and in February, 1983 at Satkhira.

Hilsa Kanagurta: It was only reported from Khepupara (monthly average 0.46 indv/kg).

Harpodon nehereus: This species was the most abundant at Satkhira (monthly average 111.87 indv/kg). But it was noted frequently at Khepupara (5.15 indv/kg) and very rarely at Chakaria (0.25 indv/kg). At Satkhira the highest number was recorded in April, 1983 (418 indv/kg). Whereas, the maximum number of this species was obtained in May, 1983 (17 indv/kg) and at Chakaria it was only found in August and October, 1982.

Mugils: Mugil tade was recorded only from Chakaria (monthly average 38 indv/kg). The species first appeared in the catch in January, 1983. Other species of the genus Mugil were found abundantly at Khepupara (27.61 indv/kg) they also frequently occurred at Satkhira (1.27 indv/kg) and Chakaria (1.25 indv/kg).

Lates calcarifer: It was noted in the Behundi catches only at Chakaria (monthly average 2.94 indv/kg) in the months of November & December, 1982 and July & August, 1983.

Cynoglossus spp.: Cynoglossus spp. was the dominant fauna at Chakaria (monthly average 7.813 indv/kg) and Satkhira (5.67 indv/ kg). They were recorded to be poor in number was Khepupara (2.46 indv/kg). At Chakaria the highest number was recorded in April, 1983 (19 indv/kg) whereas at Satkhira this species was found throughout the year except in the month of January, 1983 with the peak in September, 1982 (30 indv/kg).

Mystus spp. : Mystus gulio was found to be the most abundant at Satkhira (monthly average 11.6 indv/kg). A poor occurrence of the species was recorded at Chakaria (1.25 indv/kg) and whereas it was net found at Khepupara. However, other species of the genus Mystus was noted at Khepupara (13.23 indv/kg) and Chakaria (10.25 indv/kg). These were also present at Satkhira (1.4 indv/ kg).

6.4 Seasonal Occurrence and Distribution of Organism (Other Than Shrimp and Fishes) in Behundi Catch.

Twelve texa of different organisms (other than shrimps and fishes) were recorded in the present investigation of which crabs were the most abundant in all the three areas throughout the period. Other major fauna were Mysids and squilla sp. (Tables 41–43).

Mysids: Mysids were found to occur dominantly at Chakaria (monthly average 3516.56 indv/kg) only in the four months with the highest number in May, 1982 (9792 indv/kg) and November, 1982 45824 indv/kg). However they were not recorded in behundi catches of other areas.

Crabs: The highest number of crabs was found at Satkhira (monthly average 26.13 indv/kg). At Chakaria (21.50 indv/kg) and at Khepupara (17.15 indv/kg) comparatively a lower density of this species was recorded in the present investigation.