The fish resources of tropical estuaries, mostly in developing countries, constitute a vital source of food and em- ployment for millions of people. Hence, in most cases, there are no effective alternatives to maintain the viabil- ity of tropical estuarine fisheries, despite threats to their sustainability from overfishing, industrial and agricul- tural practices [1] . Habitat deterioration mainly from anthropogenic activities, including destructive fishing, pollution and irresponsible coastal development and natural disturbances are declining the fishery with collapse beyond the recovery [2] . Knowledge on the status and trends of inland fisheries is the key to sound policy de- velopment, better decision-making and responsible fisheries management. It is necessary at the national level for the maintenance of food security and for describing social and economic benefits of fisheries [3] . The declining biological integrity and fishery production in the aquatic environment lead to biodiversity loss, fish stock reduc- tion and eventually affect the livelihood support of traditional fishers. An ecosystem approach is important in large catchment area for large lakes and river systems [4] . Fluctuations and trends in fish stocks are difficult to assess. So to facilitate fisheries management the stock assessment process in which the collection of fishery production estimation including multispecies and multi gear fishery is important.

Fisheries activities of the inland sector of India are traditional in nature and fishermen generally carry out the fishing operations without any consideration to sustainability of the resources, which is leading to dwindling catches and poor returns. The fish and fisheries play a crucial role in the Kerala’s economy, particularly among the communities along the coastal belt. The backwaters in Kerala form a habitation for over 200 resident and migratory fish and shellfish species and fishing activities in this water body provide the livelihood to about 200,000 fishers and provide full time employment to over 50,000 fishermen [5] . Fish communities of coastal estuarine environment possess a mixture of ecological groupings that contain euryhaline species adapted to brackish environments, along with true marine and fresh water species. Hence fish assemblages in estuaries are very diverse and are composed of marine, estuarine, freshwater and migrating species [6] . Considering the major contributing species of estuarine fishery, the current knowledge of stock status is poor or missing. Even for those species about which something is known, considerable caution will be needed when making assumptions, at least until stock assessments are better developed. Only with increased monitoring, assessment and research of species will improve the existence of the fishery management system. In this context the present study made an attempt to quantify the fishery resources and propose a management plan for the Vembanad backwater system.

In Vembanad wetland system, the fishery diversity and distribution were studied initially by Shetty [7] ; Kurup [8] ; Kurup et al. [9] -[11] and Unnithan et al. [12] . The crustacean resources in the Vembanad backwater was studied by Menon and Raman [13] ; George [14] , Kuttyamma [15] ; Kuttyamma and Antony [16] and Kurup [8] made a systematic list of 150 species of fishes belonging to 100 genera under 56 families in the Vembanad backwater. Kurup et al. [11] examined the seasonal and spatial variations in fishing intensity and gearwise landings of the Vembanad backwater. Menon et al. [17] pointed out the impact of fluctuations of temperature from premonsoon to monsoon on the seasonal distribution and abundance of fishes in the Vembanad estuarine system. Padmakumar et al. [18] documented the fishery decline in Vembanad wetlands. In Azhikode estuary, the status of exploited fishery resources was studied by Harikrishnan et al. [19] and temporal pattern of fish production was observed by Bijoy Nandan et al. [20] . Fisheries and socio-economic aspects of Vembanad backwater were observed by Anon [21] .

Information regarding the fish production revealed that a drastic decline occurred in the fish production system of Vembanad backwater. It revealed that, fisheries in the Vembanad backwater will eventually collapse due to the unscientific operation of barrage along with illegal fishing activities such as operation of unfriendly gears, poison and other anthropogenic activities. An alarming decline (4387.31 t) in fishery production was observed in the present study. Kurup et al. [10] estimated an annual yield of 7202 t in Vembanad during 1988-1989. He suggested that the northern zone of the backwater having marine and estuarine influences yielded comparatively high production than the southern zone, the fresh water sector of the backwater. Similar trend was obtained during the present study. This may be due to the influence of salinity on fishery. Barletta et al. [30] noted that salinity and distance to the bar mouth were most important in structuring the fish species in an estuary. It was estimated that the annual landing from the southern zone during 1988-1989 and 1995 - 1999 were 500 t [31] and 486 t [12] respectively. During 1999-2000 and 2000-2001 Padmakumar et al. [18] estimated fish landings from southern Vembanad and adjoining lowlands that were 838 t and 687 t respectively. The production from southern zone exclusively from the water body, south of barrage was 584 t and 507 t respectively. The depletion of the fish stock of southern portion of the backwater was mainly due to man-made impacts on the ecosystem such as habitat alteration, reduction of natural grow out systems due to various activities such as intensification of rice cultivation and cropping pattern, physical barriers caused in the migratory pattern, over fishing and pollu-

tion hazards due to excessive use of chemicals and pesticides in the paddy field of Kuttanad [9] . A stagnant water condition in the southern zone during the closure period of barrage stimulated the intensification of sewage and agriculture wastes in the southern zone causing threats to fisheries. According to Anon [32] , obstruction to the natural and free flow of the river-backwater system and the ensuing interruption to the seasonal mixing of saline and fresh water have affected the natural cleansing mechanism of backwaters. It was reported that opening of Thaneermukkom barrage after the closure period benefited fishermen in the southern area from marine fish catch, Anon [33] . During the period 1988-1989, 115 species of fishes belonging to 84 genera, 6 species of penaeid prawns, 4 species of palaemonid prawns and 3 species of crabs were reported by Kurup et al. [11] . But in the present study only eighty species of finfishes, five species of penaeid shrimps, three species of palaemonid prawns and two species of crabs were reported, in which three species were classified vulnerable. The modification in the ecology of Vembanad backwater due to Thaneermukkom barrage is believed to have lost about 23 species of fishes, preventing migration of about 13 other species [32] . During 2008, Atree [34] reported 51 species of finfish representing 26 families and 35 genera, as well as 11 species of shell fish belonging to 6 families and 7genera from the southern part of Vembanad. It includes two species of critically endangered, four species of endangered and five species of vulnerable fish. In 2009, Atree [35] reported a total of 65 finfishes and 14 shell fishes from the southern part of Vembanad backwater.

Comparing the previous studies in the southern sector, the fishery contribution by major species has changed over the years. The Channa species (27.1%) and Etroplus suratensis (27.5%) followed by Metapenaeus dobsoni (12.06%) and Macrobrachium rosenbergii (6.01%) formed the dominant fishery in the Vembanad backwater during 1990s. Now the fisheries of Channa sp., Macrobrachium rosenbergii, Metapenaeus sp. were dominated over by cyprinids, Hemirhamphidae, ambassids, and Bagridae. In cyprinids, Labeo dussumeri, Amblypharyngo- don microlepis and Puntius sp. formed the major fishery. The catch of Macrobrachium rosenbergii in Vemba- nad declined over a period of 4 decades (Figure 5). In the early period, the highest catch (429 t) was recorded in 1960 and lowest (189 t) in 1962. The average for the period 1960-1964 was 300 t [36] . During 1988-1989, the catch declined to 39 t [9] . During 1994-1995 and 1995-1996 the exploited stock from Vembanad backwater were 112.85 t and 129.44 t respectively [37] . During 1995-1996 and 1996-1997, the catch of M. rosenbergii in the southern zone was reported to be 57.93 and 36.33 t respectively [12] whereas in 1999 to 2000 it was 65.2 t and it reduced to 26.72 t during 2000-2001 [18] . In the current study the exploited stock of M. rosenbergii from the whole backwater was 57.69 t. The landing of M. rosenbergii increased from July onwards and the maximum occur on September. Compared to the northern zone (39%), the southern zone (61%) contributed maximum production. M. rosenbergii has been exposed to various changes over the years in Vembanad backwater mainly due to the operation of Thaneermukkom barrage. The salinity barrier was responsible for the alarming depletion of M. rosenbergii. After commissioning of barrage over the backwater in 1976, fisheries at upstream of the barrage was negligible in most of the fish landing centres. It obstructs downstream migration of berries and the upstream migration of post larvae of M. rosenbergii and found that recruitment over fishing of M. rosenbergii occur in the backwater and also responsible for the depletion in fishery [37] . They proposed suitable management practice for M. rosenbergii fishery, which include ban on fishing in the breeding ground, creating sanctuaries and hatcheries

near the breeding ground etc. The present study reveals that fishermen capture berried female during their breeding migration. This has a tremendous adverse impact on population recruitment and declining of M. rosenbergii fi- sheries. Comparing the seasonal fishery production it was observed that during monsoon season a reduction in the catch was observed. The heavy rainfall during the South West monsoon and associated physical stress made fishing activity difficult for the fishermen, which reflected the decreased fishing days and production during monsoon period.

In the northern sector the stake net operation during monsoon was not much prominent. Strong under water current in the monsoon season may damage the fishing gear. The rapid proliferation of the aquatic exotic weed, water hyacinth, Eichhornis crassipes during the monsoon season had negative impact on fisheries in the whole backwater. The fishes were subjected to overfishing and under sized fishing by stake net, which negatively im- pacts the fish population. Over exploitation of fishery resources due to its high economic value leads to the vul- nerability of the population in different ecosystems [38] . “Padal fishing”, an illegal fishing method was em- ployed in Vembanad backwater during the monsoon season. It is the most destructive fishing method caused to the destruction of juvenile population of commercially important fish, such as Etroplus suratensis (pearlspot), mullet, shrimp and perch [39] . The Vembanad backwater has been reclaimed over the past 150 years for various purposes such as agriculture expansion for paddy cultivation, aquaculture, urban development activities and for public and private uses. The progressive shrinkage of the estuarine system has resulted in the decline of fishery areas and estuarine dependent fishery resources particularly the migratory fishes and shrimps.

Humans have profoundly altered the aquatic environment in many regions of the world, in the form of hydroelectric dams and barrages for power generation, irrigation, drinking water and shipping. The opening and closing of barrages adversely affect the migratory organisms. The building of the High Aswan dam in Egypt had important impacts on the marine life in the south eastern Mediterranean Sea. The decrease in sardine catch is generally attributed to alterations in Nile outflow, in which the total catch of sardines declined by 90% (Halim [40] ; Dowidar [41] ) and 75% reduction in Egypt’s annual shrimp landings was reported by Wadie and Abdel Razek [42] . According to Quraishee [43] , the catch rate of all fish species in Pakistan dropped significantly in the late 1950s due to the completion of the first large barrage across Indus River. Before the construction of the Thanneermukkom barrage the entire estuarine area was saline or brackish during the pre-monsoon period. The barrage divide the estuary into two zones, where the southern area was rich in estuarine fish fauna earlier, con- sisting of species originating from the sea, fresh water species and true estuarine species. The latter reproduce in the estuarine environment and find their physiological optimum in brackish water. The marine fish and prawns reproduce in the saline waters of sea and migrate to the estuary after completing their pelagic development. Some of the commercially important species of this area were Mugil sp., Chanos chanos, Tachysurus sp., Etroplus suratensis, Macrobachium rosenbergii, Scylla serrata, in which some of these species have now va- nished from the area, while others have become a rarity. The native fish species such as Horabagrus brachysoma and Hyporhamphus xanthopterus of Vembanad estuary were vulnerable and other species suffered a decline in both number and distribution. The operation of Thaneermukkom barrage and its impact on the depletion in catch of Macrobrachium rosenbergii was reported by Kurup and Harikrishnan, 2000 [37] . They pointed out the obstruction of barrage on the downstream migration of berries and the upstream migration of postlarvae of M. rosenbergii. It was confirmed by releasing marked post larvae into the estuary north of the barrier during the period of its closure. M. rosenbergii requires both freshwater and saline areas for the completion of its life history. Regular conflicts occur between farmers and fisherman on the issue of regulating the operations of the iron shutters of Thaneermukkom barrage; the fisher folk demanded entry of salt water in the southern part of barrage and fishes to enter for breeding and for better catch. A complete barrage operation strategy was needed for the ecosystem balance of Vembanad estuarine fishery.

The present study revealed that a drastic reduction of capture fishery in the Vembanad wetland system. The de- clining fish stocks in the estuary could be recognised due to the lack of sufficient fishery resource management in the system. Adequate enforcement or acceptance of both fishery management and conservation laws is im- portant for the protection of fishery resources. To achieve the optimal ecological balance, the implementation of fish ways in the barrage and special design features helpful to enable migration of fish and other aquatic organ- isms. Among these are fish ladders or other device which allow the passage of fishes. Harvesting fish in an un- sustainable manner also means that harvesting them inefficiently. In order to protect the lake fisheries a holistic approach must be needed. It should include habitat protection; prevent the unfriendly activities in the ecosystem avoiding further degradation. So establishment of protected areas has been considered as the most effective means to mitigate the impacts of overfishing and arrest further deterioration of coastal ecosystems [44] . Apart from establishment of protected areas, reducing the number of fishers through the economic assistance and other livelihood programs, strengthening law enforcement against destructive and unsustainable fishing practices seem- ed to be more urgent. The fishing pressure could be alleviated by promoting transition of fishers into alternative livelihood [45] . Capacity building and livelihood programs must be followed to increase chances of fishers to shift from fishing to alternative occupations. Ultimately ecosystem modification, stock assessment, harvest ma- nagement, declaration of closed fishing season, closed area provision and ban on negative fishing methods and a strong evaluation measures are needed for the better fishery management.

The authors are grateful to the Head, Department of Marine Biology, Microbiology and Biochemistry, Cochin University of Science and Technology for providing necessary facilities to carry out the work. This study forms a part of the research project “Ecosystem Based Monitoring and Modelling of the Vembanad Kol Wetland in Kuttanad, Kerala, India” funded by Ministry of Environment and Forests, Government of India and the authors are thankful for the financial support.