Composition and seasonal dynamics of postlarval and juvenile fishes in the Sundarbans mangrove waters, Bangladesh
M. Enamul Hoq*
Bangladesh Fisheries Research Institute, Mymensingh 2201, Bangladesh and
M. Nazrul Islam
Department of Fisheries Technology, Bangladesh Agricultural University (BAU), Mymensingh 2202, Bangladesh
*[E-mail: hoq_me@yahoo.com]
Received 5 June 2006, revised 12 June 2007
The composition and temporal distribution of the postlarval and juvenile fishes in five river systems of Sundarbans mangrove water with different salinities were studied. Monthly or fortnightly day time sampling was conducted with a rectangular drag net. Thirty seven species of fish belonging to 27 families were recorded during the 2 years study. Panchax melastigma, Liza parsia, L. tade and Gobiidae were the most abundant fish species/taxa. In the Sundarbans, species diversity and abundance gradually decreased from the freshwater zone to the saline zone. The highest monthly density of 13 species was observed in the Passur river during November. Liza spp. was the most abundant juvenile catch constituting 29.32%, followed by a small fish P. melastigma (20.13%) and Gobiidae (12.98%). No significant variation in abundance of fish species was observed with lunar cycle, although the annual peak was recorded in winter. Fish abundances varied significantly (p<0.05) among months and rivers. Water temperature and salinity were correlated with the most abundant species.
[Key words: Species abundance; seasonal dynamics; Sundarbans, mangrove; fish; Bangladesh]
Introduction
The Sundarbans is the single largest stand of productive mangrove forest in the world. This area is fed by the river Ganges and spreads to about one million hectares in south-west Bangladesh and south- eastern part of the State of West Bengal in India (Fig. 1). It represents a complex estuarine ecosystem, dominated by dense forest cover, and may be classified as a tropical moist forest ecosystem. Unlike most mangrove forest of the world, the tree vegetation of the Sundarbans is not dominated by members of the family Rhizophoraceae, but by the families Sterculiaceae and Euphorbiaceae. The Sundarbans mangrove ecosystem supports a large group of fish, shrimp, crabs which supply food to 3 million local coastal population. For the management of mangroves, baseline information (including species composition, diversity and biomass of the estuarine community) has been documented in many mangrove estuaries found in tropical areas1-4. Little information is available on seasonal distribution of brackishwater species in estuarine mangroves5-6. The aim of the
present study was to document the composition of fish population, as well as, to examine the effect of lunar cycle, seasonal and water parameters on the distribution and abundance of fish species in Sundarbans waters.
Materials and Methods
The Sundarbans is located between longitudes 89°00'E and 89°55'E and latitudes 21°30'N and 22°30'N. Based on river water salinity zonation7, intensive sampling was carried out in five river systems of the Sundarbans, located about 40-50 km upstream from the Bay of Bengal (Fig. 1). Passur, Sibsa and Koyra rivers represent freshwater (< 5 ppt), Kholpatua river represents semi-saline (5-18 ppt) and Madar river has higher water salinity (> 18 ppt).
These rivers were sampled for fish and water quality monthly for 2 years from June 1998 to May 2000.
Fortnightly samples of fish were also collected in order to find out the effect of lunar cycle on abundance and distribution of fish. A rectangular drag net with a length of 2 m including cod end and a mesh
size of 2 mm, was used for fish sampling. The opening of the net was made of bamboo spilt structure (1.6×0.6m2). A plastic bucket was attached to the cod end for collecting fish samples. A synthetic monofilament net material (high density polyethylene) with knotless webbing was used in sampling net preparation. The net is comparable with sampling nets used by Rajendran & Kathiresan5 and Mohan et al.6.
Fish samples were collected at day time during spring low tides (MH20 high tide) of full and new moons. For each sampling, the net was dragged, starting from shallow waters to near shore and continued to the adjacent mud bank covering a total area of 10 m2. Usually four (and rarely two depending on weather) replicate samples were collected from each sampling site. On shore, the catches were cleaned of any twigs, leaves, large specimens and then preserved.
total body length, in randomly selected individuals per sampling. Species were then identified using a dissecting microscope (Olympus SZ-60, Japan).
Identification was based on morphometrics, meristics, pigmentation and specialized larval characters8-10. Identification was made to species level but in some cases it was confined to genus/family level.
Fig. 1—Study area and location of the sampling stations in 5 river systems of Sundarbans mangrove
The temperature of surface water and salinity were measured directly with a direct reading integrated conductivity meter (Jenway 4200 Conductivity Meter).
In order to estimate the inter-annual density changes of the most abundant species, the coefficient of variation (C.V.) was calculated monthly and annually. Two-way analysis of variance (ANOVA) with habitat (rivers) and times (both month and lunar cycle) as fixed factors, were used to compare the equality of mean numbers of fishes per sampling.
To compare the community changes of most abundant species over time, samples were classified using the agglomerate cluster program, based on root- root transformed abundance data, together with the Bray-Curtis analysis of similarity, recommended by Field et al.11. Statistical analysis i.e. ordination was performed using the STATISTICA (5.5) and ANOVA with SPSS (10.0.1) software package.
Results
Abundance and distribution
During the two years study in 5 rivers of Sundarbans a total of 4,975 fish individuals were collected. At least 37 species of fish belonging to 27 families were identified, of which 10 species are commercially important (Table 1). The mean monthly density was highest (44-130/haul) in Passur river and lowest 14/haul in Madar in 1st year (1998-99) and 10/haul in Sibsa and Koyra rivers in the 2nd year (1999-2000) (Fig. 2). Two major peaks were observed in Passur river- one in January followed by May. No such peak was observed in Sibsa, Kholpatua and Madar rivers with the exception in Koyra with highest occurrence in November. The monthly species diversity (Shannon-Wiener index, H’) was highest in Madar river (0.85) and Kholpatua river (0.75), and species richness (Margalef index, d) was higher in Passur river (13) (Table 2). Species diversity was peak during pre-monsoon period (March-May), whereas species richness was in peak during winter (December – February) in both the years.
Fig. 2—Monthly distribution of average numbers (±SE) of fish in five river systems of Sundarbans during 1998 to 2000
Among 5 rivers, monthly abundance of fish significantly differed in 1998-1999 (F= 3.29, p<0.05).
When both the years are considered, monthly variations in abundance was also significant (F=3.02, p<0.05). Salinity zone did not show significant effect on abundance of fish in rivers under study.
Although number of fish species varied within and among years in rivers of Sundarbans, species richness was highly variable. Maximum numbers of fish species were recorded in winter. The mean total catch ranged from 27 to 421 individuals/haul in monsoon (June-August), 77 to 511/haul in post-monsoon (September-November), 87 to 351/haul in winter (December-February) and 75 to 218/haul in pre- monsoon (March-May).
Community structure
There was a substantial difference between fish abundance in 1998-99 and 1999-2000 in rivers of Sundarbans under study except Passur river. Total catches in 4 rivers were 1,887 and 676 individuals for the first and second year, respectively. In contrast, catch in Passur was 852 individuals during 1st year and 1,560 during 2nd year. Among 37 taxa recorded, 36 species from 26 families were teleosts and single species representing one family was non-teleost (Rhynchobatus djiddensis). The best represented families were Engraulidae and Mugilidae with 4 species each. The individuals of the 4 most abundant species/taxa group (Panchax melastigma, Liza parsia, L. tade, and Gobiidae) comprised almost 40-76% of the total number of specimens. River-wise contributions were 40% by Passur, 61% by Sibsa, 64-76% by Koyra and Madar, and 67-73% by Kholpatua river. A total of 10 species, comprising 21.27-49.77% of the total collection from different rivers, were larvae or juvenile of commercial species.
Commercial species were more abundant (38.36- 49.77%) in Passur, Sibsa and Madar river, and their contribution in total catch were higher in 1st year sampling. According to catch composition, contribution of Liza spp. was maximum, ranging from 14 to 42% in 5 rivers of Sundarbans. Panchax melastigma which is a small weed fish, representing 15-41% of total catch in the rivers under study except Passur. In Passur river this species was less abundant.
Table 2 ⎯ Monthly species diversity and species richness indices in five river systems of Sundarbans mangrove during two
years study
Shannon-Wiener species diversity (H’)
Margalef species richness (d) Rivers/Year 1998-99 1999-2000 1998-99 1999-2000
Passur 0.62 0.70 10.95 12.58 Sibsa 0.50 0.71 6.38 6.66 Koyra 0.70 0.68 7.73 6.49 Kholpatua 0.75 0.75 7.36 6.58 Madar 0.85 0.68 7.13 7.17
Plotossus canius, Selar spp., Liza surinamensis, Polynemus spp. and Cynoglossus cynoglossus) were classified as rare species which occasionally enter the habitat. All 37 fish species recorded were available during 1st year sampling. On the other hand, Anodontostoma chacunda, Coilia ramkorati, Sctophogus argus and C. cynoglossus were not observed during the 2nd year. C. ramkorati, Satipinna spp. and C. cynoglossus occurred only in freshwater zone of Sundarbans whilst Harpodon nehereus, Mugil corsula and S. argus were restricted mainly in semi- saline or saline zones.
The mean body length (BL) of postlarval and juvenile fish species from 5 rivers was between 8 and 49 mm except Ichthyocampus carce and Hemiramphus georgii. Size of I. carce and H. georgii varied from 42 to 64 mm and from 75 to 155 mm, respectively. BL of frequently occurred L. parsia ranged between 13 and 16 mm, whereas comparatively large sized (18-25 mm) L. tade was abundant at the same time in the rivers of Sundarbans.
However, Tenulosa ilisha including other Clupeidae and Engraulidae with a BL ranging from 20 to 40 mm, were mostly abundant at < 20 mm.
The mean abundance of the most common fish species at 5 rivers were analyzed to show the variability (Table 3). Panchax melastigma and Gobiidae showed higher variability during 1998-99 in all rivers (C.V.w>C.V.a). Stolephorus tri showed such trend in Kholpatua and Madar rivers. Sillago spp.
showed higher variability in Passur, Koyra, Kholpatua during 1999-2000. Liza parsia showed variability in Sibsa, Koyra and Kholpatua during 1st year and in Passur and Madaer rivers during 2nd year including L. tade. There was no difference in abundance of fish among 2 years, although some fish species were absent in the sample.
Seasonal variation in the most abundant species
Maximum 36 fish/haul was recorded during monsoon in Kholpatua, 206/haul during postmonsoon in Koyra, 250/haul during winter in Passur and 450/haul during premonsoon in Passur also. The number of fish individuals were low during monsoon.
No significant seasonal variation in abundance was shown by most fish species in the rivers of Sundarbans. Liza tade was completely absent in monsoon in rivers of Sundarbans and S. tri, Sillago spp. and L. parsia were occurred in small numbers during monsoon. Panchax melastigma, Gobiidae and
in all seasons. Meanwhile, L. parsia was the highly abundant species, majority of other species were more abundant during postmonsoon season. No significant difference in river or lunar pattern of fish abundance exhibited in the rivers studied. Only L. parsia and L. tade showed significant (F= 7.79 & 13.63, p<0.05,) variation in abundance among 5 rivers.
Species abundance in relation to water parameters Water temperature and salinity were the only two water parameters to make a significant contribution to the explained variation in ordination among abundances of fishes in Sundarbans rivers. The correlation coefficients between temperature and salinity and the abundance of 8 dominant species were plotted in Fig. 3. Two (P. melastigma, Gobiidae) out of 8 species were found in the area with high salinity and high temperature and the remaining 6 species in low salinity and low temperature areas during 1998-99. In 1999-2000, some species changed their position, and the corresponding numbers were 4 (P. melastigma, Gobiidae, S. tri, P. koelreuteri) and 4 respectively.
Discussion
The Sundarbans is an unusual mangrove ecosystem. The forest is flushed year-round with upland river water, and salinity remains relatively low. Diversity indices for fishes calculated in the present study were low because of the high dominance of a few non-commercial species/taxa (P.
melastigma, Gobiidae), which are widely distributed in shallow waters. Previous studies have revealed that although many species of fish are represented in mangrove ecosystems, a few are abundant3,12. An increase in the number of species, and hence higher diversity during premonsoon (March-May) is clear from this study.
The distribution pattern of fish larvae and juveniles revealed that the total number of species and abundance in the Sundarbans gradually decreased from the freshwater zone to the saline zone. A total of 37 species/taxa of larval and juvenile fish representing 27 families recorded was much lower than species richness reported for other tropical mangrove habitats of Philippines, Australia and Taiwan, where 105-128 species representing 43-55 families were recorded13-15. The fish species diversity found in subtropical mangroves is lower than that found in tropical
Table 3 ⎯Time of first collection, mean density (Dw= nos./unit effort) and coefficient of variation (C.V.w= %) within each year, and coefficient of variation (C.V.a= %) among years for the most abundant fish species in rivers of Sundarbans
Species 1998-99 1999-2000 Among
Month Dw C.V.w Month Dw C.V.w years
C.V.a
Passur river
Panchax melastigma July 1.43 55.24 August 2.6 43.85 48.02
Stolephorus tri December 16.0 81.25 - - - 0
Sillago spp. October 10.71 113.07 June 29.0 163.52 149.9
Leiognathus spp. “ 3.0 78.24 July 2.0 55 70.31
Liza parsia July 23.3 80.6 June 80.84 149.99 134.45
Liza tade November 21.0 70.29 January 15.34 119.56 91.08
Gobidae October 3.34 88.02 November 1.25 40 74.78
Periapthalmus koelreuteri “ 1.75 39.29 December 1.34 43.28 41.61
Sibsa river
Panchax melastigma November 8.89 189.08 July 3.0 103.67 167.39
Stolephorus tri “ 7.25 163.31 September 2.0 0 255.72
Sillago spp. December 1.0 0 August 1.0 0 0
Leiognathus spp. November 4.75 73.68 March 1.0 0 121.53
Liza parsia “ 35.0 111.26 September 4.38 109.13 111.02
Liza tade “ 8.34 142.09 December 3.0 0 208.99
Gobidae December 5.0 103.6 July 2.38 67.23 91.87
Periapthalmus koelreuteri “ 4.0 117.25 “ 1.5 37.5 98.85
Koyra river
Panchax melastigma June 19.42 191.14 July 6.25 73.12 162.38
Stolephorus tri “ 1.5 38.67 August 2.67 77.9 63.64
Sillago spp. November 2.0 0 March 1.0 0 0
Leiognathus spp. “ 1.0 0 September 1.0 0 0
Liza parsia August 16.63 174.02 July 1.38 53.62 164.71
Liza tade November 3.25 138.46 February 2.5 28.4 90.63
Gobidae June 4.0 174.75 June 3.67 45.23 112.76
Periapthalmus koelreuteri December 1.75 54.86 July 1.2 37.5 47.97
Kholpatua river
Panchax melastigma June 17.82 211.39 June 6.0 132.83 191.6
Stolephorus tri October 2.0 55 August 1.34 43.28 50.3
Sillago spp. August 4.34 118.77 January 1.0 0 181.22
Leiognathus spp. November 5.5 64.36 December 1.0 0 108.92
Liza parsia August 26.0 130.65 July 3.23 96.28 126.81
Liza tade February 4.0 106 February 4.0 0 106
Gobidae June 5.25 138.29 June 8.55 116.02 124.49
Periapthalmus koelreuteri August 2.34 54.86 August 4.4 115.45 98.05
Madar river
Panchax melastigma June 6.18 224.6 June 5.56 106.83 168.82
Stolephorus tri November 2.75 85.82 August 2.0 50 70.59
Sillago spp. February 2.67 57.3 January 1.0 0 83.15
Leiognathus spp. September 1.34 38.81 - - - 38.81
Liza parsia July 7.57 63.28 June 5.5 124.55 88.99
Liza tade February 2.0 70.5 December 11.34 150.26 138.38
Gobidae October 1.75 40.57 June 2.0 57.5 49.47
Periapthalmus koelreuteri “ 1.4 39.29 September 1.0 0 45.83
Fig. 3—Ordination of 8 dominant fish species in the rivers of Sundarbans during 1998-2000 on correlation coefficients (r) for temperature (Y-axis) and salinity (X-axis). (Pm: Panchax melastigma, St: Stolephorus tri, Si: Sillago spp., Le: Leiognathus spp., Lp: Liza parsia, Lt: L. tade, Go: Gobiidae and Pk: Periapthalmus koelreuteri)
systems. Tropical mangrove systems have greater species diversity, with >100 species being recorded1,14,16. Although the proportion of commercial species within subtropical and tropical mangrove habitats differ greatly, both habitats are important in sustaining commercial fisheries. The species richness in different habitats might be due to sampling technique or gear used and also habitat preference by fish species. Mahmood et al.17 record 23 species from 19 families in the Satkhira estuary (Sundarbans). In the Karnafully estuary, Bay of Bengal, Rahman et al.18 reported 12 species of fish larvae belonging to 9 families during high tide in Karnafully estuary with peak in May. In the present study, peak abundance was found in winter (November-January). Many fish species spawn during spring (July-September), which coincides with the influx of postlarvae and juveniles into estuarine areas in late spring/summer (April- June) after their planktonic phase. Ichthyoplankton richness showed seasonal variations with high value during premonsoon and lowest during monsoon in Sagar Island, Indian Sundarban, and about 22 species of fish were recorded during 199819. Although they support fewer species, subtropical mangrove systems yield a higher proportion of species of commercial value20,21. This is evident in the present study, where about 25% of the abundant species are of commercial importance. Species composition from five year study in Indian Sundarbans22 was very much similar with
that of the present study, although per cent abundance of fish was different. Moreover, they did not find any difference in fish abundance during full or new moon phase, which was evident in this study also.
Differences in community assemblages among sites that are separated even by short distance have been reported elsewhere, and the magnitude of the dissimilarity may vary from year to year3,23. Mangrove fishes from Sundarbans are dominated by species belonging to families Engraulidae, Sillaginidae, Leiognathidae, Sciaenidae, Mugilidae and Gobiidae which are typical of tropical mangrove system elsewhere in the Indo-Pacific region24. Because of the lack of detailed observation on large- scale variation in recruitment patterns of most of the juvenile fish in the present study, it is difficult to speculate whether offshore processes influenced the observed site (river) variation in fish abundance.
Many of the small fish (P. melastigma) which dominated mangrove catches in this study are important prey species for higher consumers. The generalization that the ichthyofaunal composition in estuaries is dominated by marine, then estuarine and finally freshwater species was true for the fishes in the mangrove of Sundarbans.
Most of the fish species recorded in the present study seem to be temporary resident or rare species, because the number of individuals of most species was less than 6 per sample. Species like Lates
calcarifer, Liza spp., Mugil spp. and Gobiidae seem to be permanent resident, because their occurrences were very frequent and available not only in rivers, also in coastal shrimp farms in large numbers. Most fish larvae and juveniles in the near-shore waters of the Sundarbans spawned in adjacent marine waters, their larvae passively drift with the current in the rivers, where they stayed through the early stages of their life history17. On the other hand, Mugilidae and Gobiidae are estuarine spawning and may remain in the estuary throughout their life cycle. Juveniles of commercial fish species enter tropical mangrove forests less frequently3,4 but are often found in the subtidal waters along the mangrove fringes25. The nursery and feeding areas provided by subtropical mangroves support intermediate carnivores that constitute the majority of the catch in these areas, whereas the carnivores sustained by tropical mangrove systems provides a food source for species that seldom enter the mangrove forest.
Fish species composition and richness in any one tropical mangrove system will depend primarily upon− i) its size and diversity of habitats together with its flood and tidal regimes, ii) its proximity to mangrove and other systems, and iii) the nature of the offshore environment, particularly depth and current patterns. The standard mesh size commonly used26 for taking fish larvae samples is 0.505 mm, but in the present study due to practical reasons nets with a mesh size of 2 mm had to be used. The community structure of fish postlarvae and juveniles in the Sundarbans water in relation to abiotic factor was very complex and changed with time. Some correlation may be considered as primary relationship based on cause and effect, whereas others may be accidental or secondary relationship. Not only physical factors, but also the development and ontogenetic behavior of the species played an important role in their abundance.
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