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The impact of a permanent weir (bund) construction on the ecology and fish species diversity of Muriyad wetland is studied. As this wetland area remains 0.5-1 meter below the mean sea level, rice cultivation is carried out after dewatering the fields, and construction of temporary bunds. Permanent weirs are constructed for mitigating the risk of crop loss and reduce the recurring expenditure for annual bund construction. It is carried out with a view to increase the rice production by growing an additional crop during the summer season. However, these interventions have drastically altered the natural hydrological cycle of the wetland.

Muriyad wetland also supported a good number of economically important fish species. The present study indicates that the species richness of the fish has come down. Some of the once abundant species of fishes like W allago attu and Heteropneustes fossilis have become rare. Increased irrigation facilities allowed the farmers to use high yielding varieties of rice. The concomitant use of large quantities of fertilisers and pesticides resulted in the contamination of water. Even though rice production has increased significantly, the developmental activities have diminished several other major functions and values of this natural wetland.

The imposing presence of Western Ghats as the eastern boundary of Kerala with its western slopes merging with the midland plains and the zigzag boundary of the western sea coast provide an ideal topographic feature for the development of myriad forms of wetlands in the state . Recently Nair and Sankar (2002) mapped the wetland systems of Kerala using IRS satellite data and showed that the state has 217 wetland units of which there are only 157 units greater than 56.25ha. The total wetland area is 127930.07ha-comprising 34199.57ha of ‘inland wetlands' and 93730.5 ha of ‘coastal wetlands.' The majority of the wetlands of Kerala are brackish, however, a few fresh water wetlands also are seen here. Muriyad wetland is one among the eight fresh water wetlands of Kerala others being Vellyani, Sasthankotta, Pookot, Enamackkal, Manakkodi, Eravikulam, Devikulam and Elephant lake (Nayar and Nayar, 1999).

Muriyad wetland forms a part of the ‘Kole lands', which is spread into Thrissur and Malappuram districts of Kerala. This is said to be the rice granary of these districts at par with Kuttanad the ‘Rice Bowl' of Kerala. Rice cultivation in Kole lands is said to have started way back in the eighteenth century. Kole lands lie between Bharathapuzha in the north and Chalakudy River in the south. It is located between 10 o 20' and 10 o 40' north latitudes and 75 o 58' and 76 o 11' east longitudes (Johnkutty and Venugopal 1993).

The Muriyad wetland is situated 8 km northeast of Irinjalakuda town of Trissur district. Kurumali-Karuvannur river is the northern boundary. The total field area is 1,215 ha. The water spread is a narrow central strip running north to south from Karuvannur to Thommana. The Nedumthode (Thamaravalayam canal) running along the centre of the wetland is the major opening which functions for the discharge of floodwater and letting irrigation water into the fields. M.M. canal (Muriyad - Moorkanad Canal) is the only outlet for floodwater.

The area for intensive study was determined based on the watershed boundaries of the locality. The study area falls between 10 o 25' 32'', 10 o 18' 47'' N latitude 76 o 17' 19”, 76 o 12' 48”E. longitude. About 70000 people belonging to different walks of life depend on the system directly or indirectly for meeting their various needs.

The area is mapped from Landsat imageries (1:250,000 scale) and aerial photo mosaics (1:50,000 scale) supplemented with topographic sheets of Survey of India (1:50,000 scale). Field checks were carried out using maps of Kerala Land Use Board (1:10,000 scale) and appropriate corrections were made. The drainage pattern and micro watershed was traced after preparing a slope and relief map of the area. Developmental activities in the wetland and other human interventions were studied by direct observation and photographic documentation.

A preliminary survey was conducted covering the entire area of Muriyad wetland and five collection sites were identified based on the availability of fishes. For the detailed study of the species diversity of fishes of the wetland, extensive collection was done from the five selected sites through regular fortnightly visits. Fishes were collected either personally or with the help of fishermen using indigenous fishing methods or were purchased from the fishermen on the spot.

The information on the occurrence, abundance, previous history and economic value of the fishes was gathered from the local fishermen as well as from other available literature. All the specimens collected were preserved in 10% formaldehyde and brought to the laboratory for further studies. Using standard taxonomic keys viz. Fishes of India (Day 1889); Inland fishes of India (Vol. 1 & 2) Talwar and Jhingran (1991), The fresh water fishes of the Indian region (Jayram 1999), the species status of the collected fish was identified. By comparing the research reports (published and unpublished) on the fish fauna of Trissur region, specifically that of Karivannur River, the present status and trend of species composition of Muriyad wetland were determined.

During 1990-2001, the area received a mean rainfall of 2950mm, which is far below the mean annual rainfall obtained in the state. During the South West monsoon period, floodwater level was up to 5.5meter. Five micro watersheds demarcated within the area are Parappukkara (MW1), Anandapuram (MW2), Pullur (MW3), Avittathur (MW4) and Thazhekkad (MW5). The area under each micro watershed is presented below (Table 1)

All of the 5 micro watersheds together cover an area of 57.75 km 2 . Permanent water bodies are more in the MW1 and MW2. MW1 is relatively flat whereas all other micro watershed areas have a gentle slope.

The extent of changes that is occurring in the area is very obvious even for a casual observer. Encroachment is taking place from all sides of the wetland. Construction of roads, mining of clay and sand, conversion of paddy fields into brick kilns and reclamation of paddy fields are taking place at an alarming rate. This unscientific land use practices has made the geo-environment increasingly fragile. Very deep mining may lower the ground water table level and facilitate the intrusion of seawater, altering the whole ecology of the area.

Large-scale clay and sand mining is restricted at present to MW1 and specifically indiscriminate sand mining is taking place in Madappura, Konthilapuram, and Nedumpal area. The depth of excavation site has reached up to a 7.5m. Recently it has been observed that sand mining has started in Madayikkonam in MW2, indicating that it can spread into other regions in the near future. Earlier, manual removal of sand in a small scale was practised in this region. Recently sand along with water is being sucked out using high power diesel engine. Hence the quantity of sand removed from the wetland per day has increased enormously. The lumpsum money that the sand miner offers the farmer is far more attractive than the annual income from the paddy cultivation. The dwindling price of rice and increasing expenditure of farming together with the non availability of agricultural labourers force the small scale farmers to lease their land for sand mining. One or two individuals who resist such a situation and insist on keeping their farmland fail to withstand the temptation in the long run. As the level of the surrounding area lowers, the possibility of irrigating the paddy fields becomes an uphill task. ‘The River bank protection and sand mining control act of 2001 of Kerala' restricts illegal sand mining from the rivers. However, to the best of our knowledge there are no legal measures to prevent sand mining from the wetlands or dry lands of Kerala. As a result the District collector and revenue authorities are in a quandary. Despite the awareness of the ill effects of sand mining on the ecology and environment of the area, the people in authority remain as silent spectators augmenting the process of degradation with the passage of time.

A thick sand horizon below the clay zone serves as a potential aquifer of the area and its removal will lower the water storage capacity of the wetland significantly. Recently the reclamation and conversion of paddy fields for mixed crop cultivation has accelerated considerably. These in turn can adversely affect the ground water recharge.

Earlier the bunds were constructed using materials like earth, bamboo, trunk and leaves of coconut and other locally available materials. During floods, these temporary structures were washed off permitting the deposition of fertile alluvium transported by the river. The construction of permanent bunds has dissected water spread in to numerous smaller areas and the free flow of nutrient rich flood water from the Karuvannur river stopped completely. Moreover, the alluvial deposits brought down by the river were trapped in dams constructed in the upper reaches of the river. This prevented the annual enrichment process of the paddy fields. In addition, the bunds and dams constructed in the area for improving agricultural crops hindered the migratory activity of the fishes.

Reclamation of the water body also adversely affects the natural recharge of ground water. If this practice is continued, the fringe areas will experience water shortage in the near future itself. The conflict between the agriculturists and fishermen could have been avoided if developmental projects were carried out with proper environmental impact assessment studies.

Very little information is available at present regarding the diversity of fish fauna of Muriyad wetland. The aquatic birds that frequent Kole wetlands have been well documented (Birds of Kole Wetlands, 1992; 1993). In the present study, about 28 species of fishes belonging to 21 genera were identified from the wetland. Khadar (1993) has extensively surveyed the fish fauna of the Karivannur river and reported about 23 species of fishes belonging to 15 genera. However, he has reported only three species of fishes from Muriyad wetlands. Since Muriyad wetland is situated in the flood plains of the Karuvannur river similarity in the species diversity of wetland fishes with that of the fishes seen in the river is not very surprising. The present study indicates that Muriyad wetland harbours a rich diversity of fish species. Only further investigation on the habitat utilisation and feeding relations of the fish will reveal the contribution of the wetland in supporting the economically valuable fishes of the area.

One significant feature observed in the present study is the absence of Wallago attu and Heteropenustes fossilis in the present collections. Information collected from the traditional fishermen of the area reveals that Wallago attu and Heteropenustes fossilis were abundant up to a few years back. It is said that in the deepest regions of the wetland, Wallago of about six feet in length was common. A sudden decline in the availability of two economically important species requires detailed investigation and urgent attention from the part of investigators as well as from the local authorities. It is relevant to note that both these species are bottom feeders and hence accumulation of aquatic pollutants like pesticides in the bottom sediments affecting the normal physiology of the fish species cannot be ruled out. It is well documented that sub-lethal level of pesticides can interfere with the several vital behavioural patterns of fishes and thus lower the chances of reproduction or abundance of a species (Pathiratne and George, 1998; Smitha, 2002).

Another interesting phenomenon noted in the present study is that Muriyad wetland provides habitat for two important migratory fishes from the estuaries. Caranx carangus (Bloch) and Lutjanus argentimaculatus (Forsskal) are known to be residents of estuaries, which are available in plenty during the post monsoon periods. However, whether the migration of these fishes is for spawning purpose or for feeding is not clear. The present study is not exhaustive and hence further detailed investigations are needed for understanding the impact of permanent bunds on the migratory species and resident species that utilise the wetland. It is clear that even after the completion of the permanent bunds, rice cultivation is on the decline in this region for various other socio-economic reasons. Thus the possibility that a developmental activity ignoring the multifaceted functions of a natural wetland can turn into a futile exercise is evident in the present study.

Name of the fish Present work Abdul Khadar (1993)
1. Anguilla bicolor	+	_
2. Puntius chola	+	+
3. P.vittatus	+	+
4. P. parrah	+	_
5. P. filamentosus	+	+
6. P. amphibius	_	+
7. P. sarana	+	+
8. Rasbora daniconius	+	+
10. Esomus danricus	+	+
11.Chela labuca	_	+
12.Mystus vitatus	_	+
13. Mystus malabaricus	+	+
14. Mystus oculatus	+	+
15. Xenentodon cancila	+	_
17. Aplocheilus dayi	+	_
18. A. lineatus	_	+
19. Parrambassis thomassi	+	_
20.Caranx carangus*	+	_
22. Nandus nandus	+	+
23. Etroplus maculatus	+	_
24. Etroplus suratensis	+	_
25. Glossogobius giuris	+	+
27.Wallago attu	_	+
28.Heteropenustes fossilis	_	+
29. Anabas testudineus	+	+
30. Macropodus cupanus	_	+
31. Channa striatus (Bloch)	+	+
32. Channa maurulis	+	+
33. Mastacembelus armatus	+	_
34. Macrognathus guntheri	+	+
35. Tetradon travancoricus	+	_

We gratefully acknowledge the financial assistance provided by Kerala Research Programme for local level development, CDS, Thiruvananthapuram. We also express our gratitude to the Management and Principal of Christ College, Irinjalakuda, for providing necessary facilities for carrying out the research programme. Our sincere thanks are due to Mr. Raju Varghese who helped us in the preparation of a new land use map of the area.

Birds of Kole Wetlands: Survey Report. (1992, 1993). Nature education Society, Trichur (NEST).

Day, F. (1889). Fauna of British India Fish. London, Taylor and Francis Red Lion Court Flat Street.

Jayram K.C. (1999). The fresh water fishes of the Indian region. Narendra Publishing House, Delhi, Pp. 551.

Johnkutty I. and V. K. Venugopal (1993). Kole lands of Kerala. Kerala Agricultural University, Vellanikara, Trissur.

Kadhar, A.P.B. (1993). Studies on the fish and fisheries of inland waters of Trichur District. Ph.D. Thesis. University of Calicut. 392Pp.

Nair A. S. K and G. Sankar (2002) Wetlands of Kerala. In, Wetland conservation and Management in Kerala. (Proceedings of the theme topic of fourteenth Kerala Science Congress. Pp 27-36.

Nayar S. and N.M. Nayar (1999) Wetlands In: The Natural resources of Kerala. K.B. Thampi, N.M. Nayarand C.S. Nayar (Ed.). WWF State office, Trivandrum.

Pathiratne A. and S.G. George (1998) Toxicity of Malathion to Nile tilapia, Orechromis niloticus and Modulation by contaminants. Aquatic Toxicology. 43: 261-271.

Smitha, M., (2002) Influence of Methyl parathion (Metacid) on the behaviour of the Climbing perch, Anabas testudineus. M. Sc. Dissertation

Talwar P.K. and A.G. Jhingran (1991). Inland Fishes Vol. 1&2 Oxford and IBH.ublishing Company. Pvt. Ltd., New Delhi., Bombay, Calcutta.

*Department of Zoology, Christ College;
** Department of Geology Christ College;
Irinjalakkuda,
Kerala.
Phone: 0488-828711.
E-mail: kurishinkal@yahoo.com