River Brahmaputra is a trans-boundary major river flowing through the northeastern state of Assam, India and is the lifeline of its natural fisheries. In the present article, we discuss eco-hydrobiology (including sediment quality and energy flow) of River Brahmaputra and its important tributaries in the state of Assam, India based on extensive synoptic studies conducted from April 1996 to March 1998, duly collated with past and present studies. Salient aspects of fisheries of the river system in the state (ichthyofauna, biogeography, conservation status, migratory and exotic fish species, trends in fish yield and species composition, fishing crafts and gear) are discussed in light of past and present studies conducted by the Indian Council of Agricultural Research-Central Indian Fisheries Research Institute and others. Fisheries of floodplain wetlands (beels) located in the Brahmaputra valley in the state and their interaction with the river system are also discussed.
River Brahmaputra is a trans-boundary major river flowing through the northeastern region of India and is the lifeline of natural fisheries in the region. It traverses 1,625 km in Tibet (known there as Yarlung-Tsangpo), and 918 km in India, before flowing through Bangladesh for 337 km and emptying into the Bay of Bengal through a joint channel with River Ganga (another trans-boundary major river flowing through India and Bangladesh) (Anon, 2000; Vass et al., 2011). In India, the river flows through the northeastern Indian state of Arunachal Pradesh in a north–south direction for 278 km (where it is known as Siang River), and across the east–west ranges of the Himalayas. After entering the plains of Assam, the Siang River (also known as Dihang) is joined by two large tributaries—Dibang and Lohit near Oiramghat (Lakhimpur District)—and the combined river is thereafter called the River Brahmaputra. It flows westward through the state of Assam for about 640 km up to Dhubri, after which it abruptly turns south and enters Bangladesh. The Brahmaputra valley of Assam is bordered by the sub-Himalayan mountain ranges of Bhutan and Arunachal Pradesh in the north and northeast, the Nagaland Hills in the east and southeast, the Hills zone (Assam) and Meghalaya in the south and the plains of Bangladesh to the west. These mountains/hills give rise to many fast flowing streams/rivers flowing down the gradient and ultimately joining R. Brahmaputra, which has 42 important tributaries in Northeastern India—27 on the north and 15 on the south bank. These rivers crisscross the valley and have numerous floodplain wetlands (locally known as beels), which are created through river meandering action and/or tectonic disturbances. Many of the north bank tributaries (e.g. Subansiri, Jiabharali, Manas) are of Himalayan origin fed by glaciers in their upper reaches (e.g. Dibang and Lohit, Jiadhal, Ranganadi, Puthimari, Pagladia).
The Brahmaputra river system is the lifeline of natural fisheries in the northeastern region of India. However, available scientific information on the hydrobiology and fisheries of the River Brahmaputra in the region is scanty and fragmented (Motwani et al., 1962; Singh et al., 1988; Jhingran, 1991; Yadava and Sugunan, 1992; Yadava and Chandra, 1994; Biswas et al., 1995; Biswas and Baruah, 2000; Bhattacharjya et al., 2000; Pathak et al., 2001; Baruah and Biswas, 2002; Vass and Bhattacharjya, 2008, Vass et al., 2011). A comparative account on the eco-hydrobiology and fisheries of the north and south bank tributaries of the river is lacking, with the possible exception of Anon. (2000). Very little information is available on the influence of the river system on the hydrobiology and fisheries of the associated floodplain wetlands (e.g. Sugunan and Bhattacharjya, 2000; Bhattacharjya, 2002). In view of the above, the present article has been prepared with the following objectives:
To provide a comprehensive account of the eco-hydrobiology (including sediment quality) of River Brahmaputra and its important tributaries in Assam, India
To provide a comprehensive account of the fisheries of the river (including trends in fish catch and species composition) and its important tributaries in Assam, India
To provide an account of the fisheries of floodplain wetlands located in the Brahmaputra Valley in Assam, India and their interaction with the river system
Materials and methods
We conducted field and laboratory studies on various aspects of eco-hydrobiology (e.g. sediment and water quality, biotic communities) and fisheries in River Brahmaputra in Assam, India from Chaikhowaghat (Tinsukia District) in the east to Dhubri in the west (a stretch of approximately 600 km) during April 1996 to March 1998. We made field samplings and estimated fish landings (including catch composition), at eight important fish landing centres of the river in Assam (Chaikhowaghat, Nimatighat, Bishwanathghat, Tezpur, Guwahati/Uzanbazarghat, Goalpara, Jogighopa and Dhubri). We also carried out studies on the hydrobiology and fisheries in the three forerunners of the river (Siang, Dibang, Lohit), 25 north bank tributaries (Depi, Dekari, Simen, Gainadi, Jiadhal, Subansiri, Ranganadi, Dikrong, Buroi, Bargung, Jiabharali, Gabharu, Belsiri, Pasnoi, Jiadhansiri, Puthimari, Pagladia, Bhalukdoba, Beki, Manas, Aie, Champamati, Gaurang, Tipkai, Gadadhar–from east to west direction) and 16 south bank tributaries (Tenga,Kundil, Nadihing, Dibru, Buhridihing, Disang, Dikhow, Dhansiri, Kalong, Kopili, Digaru, Bharalu, Kulsi, Singra, Dudhnoi, Krishnai–from east to west direction). In addition, hydrobiology and fisheries of selected floodplain wetlands (beels) located in both the Brahmaputra and Barak valleys of Assam (56 total) were also studied during April 1996 to March 1998. Sediment, water, and biota samples were collected using wooden plank-build boats (5–6 m) or wooden canoes (3.8–4.3 m). We also conducted studies on the occurrence of indigenous ornamental and exotic fishes in the river stretch and selected floodplain wetlands of the Brahmaputra Valley between 2002–2007 and 2006–2007, respectively. The results were collated with fish catch statistics collected by the Institute between 1972–1979 as well as between 1986–1989 at four landing centers of River Brahmaputra in Assam (Tezpur, Uzan Bazar, Fancy Bazar, Dhuburi) as well as that at Uzan Bazar (Guwahati) centre in 2006–2007 for assessing changes in fish catch and species composition over the years.
Sediment samples from the selected stations of the main river and the selected tributaries were collected using an Ekman's dredge. From each station, three samples were collected, gross plant and animal parts were removed and the remainder was mixed thoroughly to get uniform composite samples of about 5 kg in weight. The samples were collected in black plastic bags and brought to the laboratory. The collected sediment samples were air dried in shade, ground to fine powder by gently pressing with a wooden hammer, strained through sieves having 2 mm mesh size and again air-dried. Laboratory analysis to determine sediment quality was done with these air-dried samples following the procedures described by Jhingran et al. (1988) and the results are expressed on an oven-dry basis. Sediment texture was determined by mechanical analysis using a Bouyocos hydrometer (graduated in g l−1) and the results expressed as percentages of sand, silt, and clay particles in the sediment sample. Sediment pH was estimated by an electrometric method using a WTW pH meter. Organic carbon was determined by Walkley and Black's (1934) method and results are expressed as a percentage. Available nitrogen and phosphorus in the sediment was estimated following the alkali permanganate method and Trough's (Bray's) method, respectively; results were expressed as mg 100 g−1 sediment.
Water samples for determination of various physico-chemical parameters were collected using a clean plastic bucket. The air and water temperatures were recorded at each station using a standard mercury-in-glass thermometer. Transparency was measured at each station using a Secchi disc and the visibility was expressed in metres. The pH of water was estimated in the field by a WTW pH meter. The specific conductivity and total dissolved solids were estimated with the help of a portable Century kit. Dissolved oxygen was determined using Winkler's method, and free carbon dioxide and total alkalinity of water were estimated in the field following standard methods (APHA et al., 1998). Total hardness and total chlorine were estimated following the EDTA titrimetric method and Mohr's method, respectively. Dissolved organic matter was estimated using the permanganate oxidation method and the values expressed as mg O2 l−1 (Jhingran et al., 1988). Silicate–silica and phosphate–phosphorus content of waters were estimated spectrophotometrically. Nitrate–nitrogen was estimated employing the phenol disulphonic acid reduction method (Jhingran et al., 1988).
Phyto- and zooplankton in the surface waters were collected by filtering 100 litres of water through plankton nets having 60 μ and 150 μ mesh sizes, respectively. The nets used were of truncated cone shape having an upper diameter of 20 cm and a filtering cone length of 54 cm. In the laboratory, the phytoplankton samples were filtered through a 150 μ nylon plankton cloth to separate the zooplankton. From the known volume, a sub-sample of 1 ml was drawn for estimating the numerical abundance of different genera of phytoplankton using a Sedgwick Rafter cell. The phytoplankton density is expressed as units l−1 (the units being single cells, colonies, or filament lengths in millimeters as the case may be). For estimating numerical abundance and quality composition of zooplankton, the macro-plankton were separated and counted under low magnification. From the known volume, a sub-sample of 1 ml was drawn. The number of different groups of micro-zooplankton present was counted under high magnification. The values were represented as numbers l−1. Sediment samples were collected for macrobenthic fauna using an Ekman's dredge (area 0.025 m2) and sieved through a filter with 0.5 mm mesh size. Collected macrobenthos were preserved in 5% buffered formalin. These were segregated in the laboratory and identified using standard manuals (Ward and Whipple, 1959; ZSI, 1992). Number of individuals of each taxon were counted and expressed as no. m−2.
Primary productivity and energy flow
Primary productivity of phytoplankton was estimated by the light- and dark-bottle oxygen method using short-term in situ exposure of 4–6 h (Vollenweider, 1969). The energy flow approach was applied for calculating the fish production potential (Odum, 1962; Teal, 1962; Natarajan and Pathak, 1983; Vass et al., 1988).
Available fish species and their relative abundance were observed at the selected landing centres/beels. As far as possible, fish specimens were identified in the field itself. Specimens that could not be identified in the field were preserved in 5% formalin and identified in the laboratory using standard manuals like Sen (1985), Talwar and Jhingran (1991) and Viswanath (2002). The system of classification followed in preparation of the updated list of ichthyo-fauna is as suggested by Nelson (1994). The nomenclature followed in the article is after Froese and Pauly (2011). Field data for estimation of fish catch from the selected fish landing centres followed systematic sampling design. In addition to fish landings and catch composition, observations were made on the fishing crafts and gear employed in the selected river stations and tributaries for harvesting and their catch composition.
Results and discussion
Sediments in the river stretch were, with few exceptions, dominated by sand (78.5–99.5%) having poor organic matter (0.091–0.628%) and available nutrients (N 9.84–22.84 and P 0.28–1.9 mg 100 g−1 sediment). The north bank tributaries joining the upper stretches of R. Brahmaputra in Assam and some tributaries in the south bank were acidic (pH 5.66–6.94). However, the pH was generally in the alkaline range (7.00–7.67). Manas and Aie rivers showed high free CaCO3 (7.5 and 6.25%), probably due to limestone belts located in their catchments.
Salient water quality variables of River Brahmaputra and its tributaries in Assam assessed between 1996 and 1998 are summarized in Table 1. The water was characterized by rich dissolved oxygen (6.73–10.30 mg l−1), alkaline pH (7.15–8.20), high dissolved organic matter (0.98–2.42 mg l−1), and poor nutrients in the water phase. Remarkable variations were observed in respect to alkalinity, conductance, dissolved solids, and hardness, being exceptionally low in Ranganadi (23.5 mg l−1, 52.6 μmhos, 26.6 mg l−1 and 22.4 mg l−1) and high in Manas (207.7 mg l−1, 445.5 μmhos, 224.0 mg l−1 and 155.8 mg l−1). Similar variations were also observed in the south bank tributaries and the two forerunners of the river (Dibang and Lohit). The diverse hydrological setup of the tributaries showed considerable impact on the water quality of the main river, especially at Tezpur and Goalpara, where the tributaries Jiabharali and Manas considerably changed the values of the above chemical parameters. Comparison with past results did not show any sign of deterioration in the water quality of River Brahmaputra over the years, mainly due to the healthy condition of tributaries. One notable change observed was an increase in the average Secchi disc visibility in the river stretch from 26.7 cm between 1972 and 1980 (Yadava and Sugunan, 1992) to 45.2 cm between 1996–1998 (Anon., 2000). However, intensive studies were conducted by CIFRI at three stations (Guwahati, Jorhat and Dhuburi) between 1972 and 1980, whereas between 1996 and 1998, extensive studies were conducted at eight stations (Table 1), which makes such comparison difficult.
Plankton showed considerable variations both in quality and quantity. While many of the tributaries showed dominance of diatoms among phytoplankton, some of them (Subansiri, Manas, Aie, Burgang, Gainadi, Pagladia, Burhidihing, Singra and Krishnai) showed higher percentages of Chlorophyceae (25.02–78.2%). The lower stretch of River Brahmaputra between Guwahati and Dhubri showed dominance of Chlorophyceae (38.2–50.9%), while the upper stretch—including Siang, Dibang and Lohit—showed dominance of diatoms (43.5–90.1%). The numerical abundance of plankton ranged between 22 and 300 ul−1 in the Brahmaputra River stretch in Assam. Zooplankton remained either absent or present in a very small amount (0–5.8%). Numerical abundance of plankton was higher in the north bank tributaries (32–753 ul−1) than that in the south bank ones (20–370 ul−1); wide variations in plankton abundance were observed among the tributaries located along both the banks of the river. Bacillariophyceae dominated the plankton population in all the selected tributaries on both banks (46.7–100.0%), except for Chlorophyceae dominance noted in one north bank tributary (River Subansiri, 78.2%) and one south bank tributary (River Krishnai, 66.2%). Among the benthic organisms, Molluscs remained the dominant group in many landing centres such as Chaikhowaghat, Dibrugarh and Guwahati centres (46.4–92.2%), Oligochaetes in Jorhat (57.2%) and insects in Biswanathghat, Tezpur landing centres, Rivers Jiadhal, Subansiri, Jiabharali, Pagladia, Nadihing, Dhansiri, Siang, Dibang and Lohit (47.9–100.0%). As many as 10 north bank tributaries showed the dominance of insects, whereas none of the south bank tributaries had insects as the dominant group.
The rate of energy transformation by producers (cal m−2 day−1) and photosynthetic efficiency (%) were in the range of 1460–1939 and 0.082–0.0108 in Siang, Dibang and Lohit; 2087–3528 and 0.113–0.190 in Brahmaputra, and 1752–3850 and 0.105–0.212 in the studied tributaries. The fish production potential (kg ha−1 yr−1) was estimated to be (on average) 89.0 in the forerunners, 92.3 in the main Brahmaputra River, and 86.8 in five tributaries. Taken as a whole, the system has a high potential energy resource.
Connectivity and interaction of the river system with floodplain wetlands
Investigations on floodplain wetlands (beels) carried out by CIFRI during the past two decades (Sugunan et al., 2000; Sugunan and Bhattacharjya, 2000; Bhattacharjya, 2002) showed that the ‘extent of riverine connection’ was the single most important factor affecting their ecology and fisheries. Open wetlands retain connection with the parent/adjacent rivers either for the whole year (perennially open wetlands) or only during the rainy season (seasonally open wetlands). Such wetlands have continuous exchange of water as well as fish fauna with the parent river (e.g. Sone beel in Assam). In open wetlands, the continuous water exchange affects the nutrient input–output ratio. While this adversely affects the biological productivity of the ecosystem, it helps in delaying the eutrophication process. Open wetlands receiving water from rivers having high sediment load (e.g. north bank tributaries of R. Brahmaputra), are likely to be silted up as seen in Gorjan-Bullutjan beel in Assam. Rapid water renewal also helps in breaking the thermal stratification that may develop in deep wetlands, which is beneficial for nutrient recycling and gaseous exchange. At the same time, continuous water flow does not allow the plankton species to stabilize, resulting in lower plankton density and primary production by phytoplankton in such beels. It may be noted here that lower fish production (874 kg ha−1 yr−1) potential was estimated in a seasonally open wetland of the Brahmaputra Valley (Charan beel, Morigaon district) than that in a closed wetland (Jaluguti beel) of the same district (1063 kg ha−1 yr−1) (Bhattacharjya, 2002). Many open wetlands act as breeding grounds for commercially important fishes during the southwest monsoon season and invariably most open wetlands act as nursery grounds for offspring of such fishes. Migration for spawning or feeding of Indian major Carps (Gibelion catla, Labeo rohita, L. calbasu, Cirrhinus cirrhosa) and minor Carps (L. bata, L. gonius and C. reba) is significant from the point of view of recruitment/auto-stocking of these wetlands as well as that of the parent river.
Closed wetlands are completely cut-off from the nearby rivers and receive water mostly from their catchments following monsoon rains or during high floods. Riverine embankments constructed to prevent floods along R. Brahmaputra in Assam between 1954 and 1956 and those along its tributaries, which were constructed in 1964–1965, have converted most open wetlands into closed ones by blocking the riverine connections. Since closed wetlands receive nutrient-rich water from their catchments following monsoon rains year after year, they usually have higher primary productivity leading to higher fish production potential than open wetlands (Bhattacharya, 2002). In addition, these wetlands present relatively more stable hydrobiological regimes than open ones and facilitate colonization of plankton and benthos, resulting in higher natural food availability. However, lack of water renewal and replacement advance the eutrophication process in these wetlands. As a result, most closed wetlands are infested with large aquatic macrophytes, which, if unchecked, adversely affect their fisheries in the long run. Closed wetlands receiving surface run-off from surrounding agricultural fields also receive sediments from their catchments; however, sedimentation is less intense and more gradual in such wetlands compared to that in the open ones.
Our field studies have revealed the presence of 141 finfish species belonging to 84 genera and 29 families occurring in the river stretch. Motwani et al. (1962) reported 126 species of fish belonging to 26 families in R. Brahmaputra in Assam, of which 41 species sustained fisheries of some commercial importance. We also recorded two commercially important Prawn species (Macrobrachium gangeticum, M. malcolmsonii) in the river stretch, of which the former was more abundant. A total of 216 fish species belonging to 36 families have been recorded and reported from the Brahmaputra and Barak River systems in Assam, India (Bhattacharjya et al., 2003). Fish fauna of the river system are comprised of torrential, plain water, and cold-water forms (Sinha, 1994; Sen, 2000; Vishwanath, 2002). Kottelat and Whitten (1996) estimated the Brahmaputra and Irrawaddy River drainages to contain 200 species of fish. The river dolphin, Platanista gangetica, is the most important aquatic mammal inhabiting the river (Biswas and Baruah, 2000).
Owing to the diversity of topographic and climatic features, the pattern of species distribution and abundance varies considerably in different stretches of the river. In the upper stretch extending up to Dibrugarh, commercial catches are dominated by Labeo gonius, Wallago attu, L. bata, Notopterus notopterus, etc. Indian major Carps (L. rohita, Gibelion catla, Cirrhinus cirrhosa, L. calbasu) are generally less abundant. In the lower reaches, Indian major/minor Carps, Catfish, and Tenualosa ilisha are adequately represented.
Commercially important fish species of the R. Brahmaputra include Indian major Carps (Labeo rohita, Gibelion catla, Cirrhinus cirrhosa, L. calbasu), minor Carps (L. gonius, L. bata, Bangana. dero and Cirrhinus reba), major Catfishes (Wallago attu, Sperata seenghala, S. aor, Rita rita, Pangasius pangasius, Bagarius bagarius), minor Catfishes (Eutropiichthys vacha, Ompok pabda, Clupisoma garua, Ailia coila, Setipinna phasa, Mystus tengara, M. carcio, M. bleekeri, M. cavasius), knifefishes (Chitala chitala, Notopterus notopterus), Hilsa (Tenualosa ilisha), Prawns (Macrobrachium gangeticum, M. malcolmsonii) and miscellaneous species (Cabdio morar, Gudusia chapra, Barilius barilius, Puntius spp., Trichogaster spp.). At all the centres, small economic finfish species like C. morar, A. coila, Neotropius atherinoides, C. reba, Gudusia chapra, Puntius spp., Mystus carcio and M. tengara were numerically dominant. This indicates decline in abundance of major Carp and Catfish species, apparently because of their selective overfishing and/or disruption of the natural stock replenishment process.
Comparative accounts of the occurrence of fish fauna of the north and south bank tributaries of R. Brahmaputra are lacking. Studies conducted by us between 1996 and 1998 showed the occurrence of Indian major Carps and minor Carps (Cirrhinus reba) only in a few south bank tributaries (e.g. Nadihing, Buhridihing, Dikrong, Disang, Kulsi). This situation needs to be periodically reassessed, as the occurrence and abundance are likely to change over time.
A detailed discussion on the biogeography of the freshwater fishes occurring in the river stretch in Assam, India is beyond the scope of the present study. According to available information, most of the present-day fish fauna of India descended from Pleistocene migrants (Jhingran, 1991). The geological upheavals in the past in the Eastern Himalayan region resulted in mixing of drainages and their fish fauna in NE India. The Eastern Himalaya is considered as a biodiversity rich region (Kottelat and Whitten, 1996). The diversity is attributed to the recent geological history (the collision of Indian, Chinese, and Burmese plates) and the Himalayan orogeny, which played an important role in the speciation and evolution of groups inhabiting mountain streams (Kottelat, 1989). The evolution of the river drainages in this part of the world has been the subject of several studies that utilized geological evidence to reconstruct the palaeo-drainage patterns during much of the Cenozoic Era (65.6 million years ago to the present) (Brookfield, 1998; Clark et al., 2004). Molecular phylogenetic studies of the fishes of this region (Guo et al., 2005; Ruber et al., 2004) have indicated that vicariance events in the Miocene (23.0 to 5.3 million years ago) may have played a substantial role in shaping the current distribution pattern of freshwater fishes of the region.
According to Viswanath et al. (2008), the fish fauna of the Eastern Himalaya region may be subdivided in to three drainage-based geographic units: (1) the Ganga–Brahmaputra drainage, that floods the Ganga Himalayan foothills, Ganga Delta and plain ecoregions, as well as the Upper and Middle Brahmaputra; (2) the Chindwin–Irrawaddy drainage in the Chittagong–Irrawaddy freshwater ecoregion; (3) the Kaladan/Kolodyne drainage and a number of short drainages along the western face of the Rakhive Yonad of Myanmar in the Chin Hills–Arakan freshwater ecoregion. The present-day fish fauna of the Brahmaputra Valley can be grouped into three broad groups (Jhingran, 1991): (i) widely distributed (in Sri Lanka, India, Myanmar, and farther east) (e.g. Wallago attu), (ii) species of northern India including Myanmar and farther east (e.g. Chitala chitala) and (iii) species of the Himalayas (e.g. Tor tor, Schizothoraichthys progastus).
In recent years, habitat modification and overexploitation have resulted in considerable depletion of fish stocks in the river system. According to Yadava and Chandra (1994), six Carp and five Catfish species were threatened at various levels in the Brahmaputra River system. Similarly, Sinha (1994) listed eight threatened coldwater species occurring in the northeastern region. Habitat destruction, indiscriminate fishing, poisoning and blasting reportedly endangered Korang (Barilius spp.), Mahseer (Tor spp.) and other sport fishes occurring in the region (Agarwala, 1994). Bhattacharjya et al. (2000) identified 25 fish species occurring in the state as threatened. As per the CAMP report (1998), 76 threatened fish species occurring in the state (3 critically endangered, 26 endangered and 47 vulnerable) have been assessed. However, the assessment of 170 species for this purpose was based on their distribution in the whole of India and not exclusively in the northeastern region (Sen, 2000).
Vishwanath et al., (2010) assessed the status and distribution of freshwater fishes of the Eastern Himalayan Region. According to their findings, Tor putitora occurring in the Ganga–Brahmaputra River system is an endangered species, whereas about 27% of the 520 fish species assessed were data deficient. They observed that the number of endemic species in the lower reaches of the Brahmaputra basin is comparatively small and the labyrinth fish Ctenops nobilis was a characteristic example.
Motwani et al. (1962) reported the occurrence of Hilsa (Tenualosa ilisha)—an anadromous migrant—in the Brahmaputra river. Considerable quantities of Hilsa (forming 11.06–15.98% of the total fish catch) were landed in the lower reaches of the river in Assam, between Tezpur and Dhubri. Bhattacharjya et al. (2003) reported the occurrence of Anguilla bengalensis bengalensis, a catadromous migrant. We recorded juveniles (measuring 8–10 cm in total length) of A. bengalensis ascending the river near Guwahati in the month of February in 2005. Mahseers (Tor tor, T. putitora, T. chelynoides and Neolissocheilus hexagonolepis), Labeo dyocheilus and Barilius spp. have also been recorded in the river and its tributaries during the winter months (Anon, 2000). Apparently, these fishes undertook local migration in response to temperature regimes, descending to the foothills/plains areas within the river/tributaries during the winter months (December to February). Biswas and Baruah (2000) reported that the giant Catfish viz. Bagarius spp., Sperata spp., Silonia silondia, and Pangasius pangasius migrated upstream for spawning.
Occurrence of exotics
We recorded the occurrence of exotic Common Carp (Cyprinus carpio var. communis), Grass Carp (Ctenopharyngodon idella) and Silver Carp (Hypophthalmichthys molitrix) from R. Brahmaputra and in selected floodplain wetlands in Assam state during 2006–07 (Vass and Bhattacharjya, 2008). Fortunately, none of these exotic Carps has established naturalized populations in the main river and the surveyed wetlands. However, the study confirmed the establishment of Common Carp in Umiam and Nongmahir reservoirs in Meghalaya state. The Common Carp has also reportedly established itself in Ranganadi reservoir (Arunachal Pradesh state) and Daiyang reservoir (Nagaland state), a situation which needs confirmation. In addition to the above three exotic Carps, Bighead Carp (Hypopthalmichthys nobilis) and Tawes (Barbonymus gonionotus) are regularly stocked in many of the floodplain wetlands of Assam. According to Vass and Bhattacharjya (2008), these exotics did not established their naturalized populations in the River Brahmaputra and its wetlands in Assam at that time.
Current status of fisheries of the river system
Fish yield and fish catch composition
There was almost a total lack of information on the fish production and abundance of commercial fisheries of the Brahmaputra River system until the Brahmaputra Survey Unit was established by ICAR-CIFRI at Guwahati in October, 1972 (Jhingran, 1991). Out of 42 fish landing centres located between Chaikhowaghat to South Salmara in Assam (a stretch of about 600 km), four important centres—Tezpur, Guwahati (Uzan Bazar and Fancy Bazar), and Dhuburi—were selected for recording fish catch statistics between 1972–1979 and 1986–1989. Studies conducted by ICAR-CIFRI from 1974 to 1979 showed that the average fish catch from River Brahmaputra at the Tezpur, Guwahati and Dhubri centres was 50.15, 110.54 and 54.93 t yr−1, with a total of 215.62 t (Table 2). Landings at Uzanbazar centre declined from 111.8 t in 1973 to 16.3 t in 1979 at Fancy Bazar centre (Guwahati) from 121.6 t in 1973 to 22.7 t in 1979 and at Dhubri centre from 70.7 t in 1974 to 36.8 t in 1977. Landings at Tezpur centre declined from 90.2 t in 1974 to 25.6 t in 1977 and from 192.6 t in 1987 to 57.1 t in 1989 at Jorhat fish assembly centre. The total fish landings at Tezpur, Guwahati and Dhubri centres declined considerably from 215.62 t between 1974–1979 to 150.42 t between 1996–1998 (Table 2). ICAR-CIFRI continued to record fish landings at Uzan Bazar (Guwahati) centre until 2006–2007. Fish landings at this centre considerably improved during the 1980s and 1990s after the merger with the Fancy Bazar landing centre. Landings showed a more or less upward trend since 1986–1987 (65.09 t), reaching a peak in 2002–2003 (954.2 t). This was apparently due to more stretches of the river being covered by fishers to cater to the demand for fish in Guwahati city, coupled with intensification of fishing. However, fish catch at this centre has gradually declined since 2003–2004, stabilizing at 225.4 t (2005–2006) to 236.5 t (2006–2007).
The Institute estimated the average daily catch from Siang, Dibang and Lohit Rivers (the three forerunners of R. Brahmaputra) for the first time during 1996–1998. These estimates were 91, 102.2 and 62.5 kg, respectively. Their fishery was dominated by Mahseer (Tor putitora and Neolissocheilus hexagonolepis), Snow Trout (Schizothorax richardsonii) and other coldwater species (e.g. Bangana dero, L. dyocheilus) contributing 80.2–92.4%, with the remainder of the catch being miscellaneous fishes. At Chaikhowaghat landing centre, out of 69.7 kg daily catch, 68.6% was contributed by coldwater species, 19.8% by Catfishes and 11.6% by miscellaneous species. The fishery showed considerable qualitative shift at Dibrugarh, where coldwater species disappeared completely. Out of 90 kg daily catch, major Carp, Catfishes and miscellaneous fishes contributed 21.9%, 12.0% and 57%, respectively. The contribution of Featherback was 6.6%, but minor Carp and Prawn were insignificant. From Jorhat to Dhubri landing centres, the average daily catch ranged between 53.8 kg (Goalpara) and 198.7 kg (Guwahati), mainly represented by miscellaneous species (44.8–61.1%), major Carps (11.5–20.5%) and Catfishes (8.7–19.4%). Minor Carp, mainly Cirrhinus reba, represented a sizeable percentage at Guwahati (16.4%). Among the major Carps, Gibelion catla and Labeo rohita were dominant in almost all the stretches, while L. calbasu was found only in Tezpur and Guwahati landing centres. The order of abundance of Catfishes was Sperata aor, S. seenghala, and Wallago attu (Anon., 2000). Three north bank tributaries—namely Jiabharali, Manas, and Subansiri—as well as Kopili, a south bank tributary of the river, are famous for Mahseers and attract anglers from all over the country and even abroad. Daily fish catch from these tributaries in the upper stretches ranged from 30 to 100 kg during 1996–1998, mainly represented by Tor putitora, Tor tor, Neolissocheilus hexagonolepis, Labeo dero and L. dyocheilus. The catch from Subansiri in the lower stretches (Majuli River Island) during the same period was 50–200 kg d−1, mainly represented by Labeo calbasu, Sperata seenghala, S. aor, Wallago attu, Systomus sarana, L. gonius, Prawn, and a good amount of miscellaneous fishes. The daily catch from Buhridihing, the most productive south bank tributary, was 60–100 kg during 1996–1998. Other tributaries also contributed significant amounts to the fishery of the Brahmaputra system.
Fish catch composition of the river changed over the years. Of the total landings, Indian major Carps contributed 11.65% at Uzan Bazar, 25.25% Carps at Fancy Bazar Carps, 16.41% at Dhubri, 21.03% at Tezpur and 10.2% at Jorhat landing centres. Major Carps, minor Carps and Catfishes, which together contributed 56.4% during 1974–1979, decreased to 36.1% between 1996–1998, and Hilsa fishery reached its minimum. However, the fishery of small-sized species showed considerable increase from 27.9% to 56.3%, with an overall improvement of 41%. A similar trend was observed at all three landing centres. Observations at other centres also showed a considerable increase in miscellaneous species, which stood at 57% at Dibrugarh, 51.5% at Jorhat, 44.8% at Biswanathghat and 51.5% at Goalpara landing centres during 1996–1998. The average contribution of major Carp, minor Carp, Catfishes, Featherbacks, Hilsa and others between 1973 and 1979 at all the centres were 19.4%, 14.1%, 23.8%, 3.6%, 11.2% and 22.9%, respectively (Tables 3 and 4), the inter-stretch variations being of smaller magnitude. The contribution of Hilsa at Tezpur was lower (4.2%) than that at Guwahati and Dhubri (13.7–15.5%). Miscellaneous species have started dominating the total catch; their contribution increased from 22.9% between 1973 and 1979 to 55.2% between 1996 and 1998 (Table 3). Aspidoparia morar, a minor Carp, has emerged as the most dominant fish species at all major landing centres. The qualitative shift in fisheries over the years has severely affected the pattern of utilization of available potential energy and energy harvest from the river.
Vass et al. (2011) reported a similar decline in fish landings and changes in catch composition in the Ganga River over the years. Fish landings at major commercial fish landing centres in different stretches of River Ganga showed a decline in fisheries, of which the worst sufferers were major Carps and Hilsa. From 1961 to 1968, the fishery in the Kanpur to Bhagalpur stretch had a high annual yield rate of 1169 kg km−1 at Bhagalpur. Major Carps dominated from Kanpur to Allahabad (Jhingran and Ghosh, 1978). After 1972, Ganga River fishery started declining, with sharp changes in stock structure. At Allahabad, the yield decreased from 935.2 kg km−1 in the 1960s to 389.6 kg km−1, along with a drastic decline in catch of major Carps and large Catfishes (S. aor, S. seenghala, W. attu). However, the catch of smaller species remained at the earlier level, with only slight changes in catch composition. From 1991–2000, the contribution of L. calbasu declined sharply and the share of major Carps slipped to only 28.9 kg km−1. Between 2001 and 2008, the fisheries improved in general, mainly due to invasion of exotic species, C. carpio and O. niloticus, which have constantly increased over the years (Vass et al., 2011).
There has been a significant decline in the fishery in its many stretches of River Brahmaputra in Assam. The average catch recorded at Tezpur during 1973 to 1979 was 196.9 kg day−1, which consisted of 19.4% major Carps. Presently, the catch at this stretch of River Brahmaputra has declined to 137.3 kg day−1 with an 11% contribution of major Carps. Changes in fish catch structure of the Brahmaputra are summarized in Tables 3 and 4. At Guwahati, the share of major Carp declined from 19% during the 1970s to 15% during 1996–1998, while the decline was from 18% to 14% in Dhubri. In both the stretches there was a corresponding increase in the catch of miscellaneous fishes. On the whole, there was a 30% fall in the catch of major Carps in River Brahmaputra in Assam. The minor Carps, Hilsa and Prawns also declined significantly during this period. Miscellaneous fishes filled the niches vacated by the Carps and registered an increase. The study also revealed large-scale destruction of brood fishes and juveniles.
ICAR-CIFRI studied fish and fisheries of important tributaries of R. Brahmaputra from 1996 to 1998. It estimated the average daily fish landings from Siang, Dibang and Lohit (forerunners) at 91.0, 102.2 and 62.5 kg day−1, respectively. The daily fish catch from major tributaries varied between 20 and 200 kg (Anon., 2000). Among the southern tributaries, Buhridihing was the most productive and almost 50% of the total landing at Dibrugarh centre was contributed by this river. The tributaries of River Brahmaputra have considerable fishery potential and play a vital role in the qualitative and quantitative fluctuations in the fisheries of the main river. The fish production potential of some of the tributaries were: 65.7 kg ha−1 in Subansiri, 60.7 kg ha−1 in Jiabharali, 106.9 kg ha−1 in Manas, 133.3 kg ha−1 in Buhridihing and 67.2 kg ha−1 in Kolong. If the fish production potential of tributaries and the main river are taken together, the system as a whole has fish production potential.
Fisheries were nearly similar in all the rivers, represented mainly by Mahseer, Tor putitora, Neolissocheilus hexagonolepis and Schizothorax richardsonii among Trout and cold water species (Bangana dero and L. dyocheilus). Among the stretches of River Brahmaputra, only Chaikhowaghat showed considerable representation of the above species (68.6%) in the total catch. In other stretches, miscellaneous fishes were dominant, followed by Catfishes, major Carps, minor Carps and Featherbacks. In the stretch between Dibrugarh and Dhubri, the contribution of Mahseer and other cold water species was negligible. The average daily catch in the entire stretch between Chaikhowaghat and Dhubri fluctuated between 20.2 and 53.8 kg day−1, with maximum being in Dhubri and the minimum in Goalpara. Although the qualitative picture of the fishes was similar between Dibrugarh and Dhubri with an overall dominance of miscellaneous species in the commercial catch, the percentage composition of various groups showed considerable variations between the stretches. Almost 80 to 90% of the catches from the upper stretches of Subansiri, Nadihing, Jiabharali, Manas and Beki (north bank tributaries) was represented by Mahseer Tor putitora, N. hexagonolepis and the cold water species Bangana dero and L. dyocheilus, while the catch from the downstream stretches towards the confluence with River Brahmaputra mainly comprised of major Carps, minor Carps, Catfishes and miscellaneous species.
Fishing crafts and gear
Fishers operate a large variety of fishing crafts and gear in the river for exploiting its multi-species fisheries; the main gear are gill nets, Chinese dip nets, bag nets, cast nets, drag nets, hooks and line. The entire fleet of wooden fishing crafts is nonmechanized and ranges from small dugout canoes (3.5–4.5 m) to bigger plank-built boats (5.0–6.5 m).
Fisheries of floodplain wetlands
Our studies showed that fish landings of closed wetlands of Brahmaputra Valley of Assam were dominated by barbs (Puntius spp.), Rasboras (Danio, Rasbora and Devario spp.), small Catfishes (Mystus spp.), Murrels (Channa spp.), Notopterus notopterus, Wallago attu, etc. Fish species that spawned in flowing waters (e.g. major/minor Carps, Bagarius bagarius, Pangasius pangasius, Chitala chitala, Ompok spp., Aspidoparia spp., etc.) were usually not found in closed wetlands. In the absence of significant recruitment from rivers, fishes that spawned in stagnant waters (usually minor/small fishes) inhabited such wetlands. In general, the natural fishery of closed wetlands was overwhelmingly (50 to 90%) dominated by small economic fishes (Puntius spp., Rasbora spp., Trichogater spp., Mystus spp., Notopterus notopterus, etc.). Insectivorous and air-breathing fishes (Channa spp., Anabas testudineus, N. notopterus, Clarias magur, Heteropneustes fossilis, etc.) dominated the landings in macrophyte-choked closed wetlands.
If managed along scientific lines, fish production from floodplain wetlands can be increased significantly. For example, studies conducted by the ICAR-CIFRI have shown that the fish yields from floodplain wetlands of India can be raised to 1,000–1500 kg ha−1yr−1 from their estimated average yield rates of 172.9 kg ha−1yr−1 (Sugunan et al., 2000; Sugunan and Bhattacharjya, 2000) with scientific management. Many floodplain wetlands are in a transient phase of their evolution into marshlands. Such wetlands, apart from attracting migratory and resident waterfowls, support a rich faunistic diversity in the form of plankton, macrophytes, benthic organisms, insects and other macrophyte-associated fauna, as well as a rich variety of air-breathing and small-sized fish species, some of which are threatened. Thus, conservation of these dynamic and productive habitats has become necessary for in situ conservation of threatened aquatic species, including fishes and waterfowls. In addition, these wetlands also regulate the water regime and nutrient exchange and act as natural filters. However, a combination of the processes of river bed evolution and the effects of extensive flood control and irrigation works have reduced the fish production of many wetlands through siltation, habitat destruction, macrophyte infestation, and isolation from the seasonal floods restricting entry of riverine fish stocks. The floodplains with their associated lentic waterbodies are essentially a continuum of the rivers. Habitat modifications of the parent/adjacent river (e.g. siltation, aquatic pollution) and indiscriminate killing of brood and juvenile fishes of commercial species during the breeding and recruitment seasons badly hampers fish production in the wetlands as well as in the parent/adjacent rivers.
Construction of large river valley projects in a number of tributaries of R. Brahmaputra (e.g. Ranganadi HE project in Arunachal Pradesh, Kurichu HE project in eastern Bhutan) have already changed the flooding pattern in the floodplain wetlands located downstream, the precise effects of which on their ecology and fisheries are yet to be studied. Thus, there is an urgent need to formulate sound management norms for sustainable development and optimal utilization of the Brahmaputra river system in the region including the floodplain wetlands, keeping in view the proposed construction of a large number of river valley projects in northeastern India.
We are thankful to Drs. V. Pathak and M. Choudhury (Ex-Principal Scientists), Mr. Alok Sarkar (Ex-Senior Technical Officer), and other staff members of the ICAR-CIFRI Regional Centre, Guwahati who participated in the 1996–98 studies. Thanks are also due to the anonymous referees for their valuable suggestions.
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