The fish species richness of Vembanad lake has fallen by 40 per cent since the construction of the Thanneermukkam barrage, according to a group of researchers who assessed the post and pre-barrage period fish data of the waterbody.
The fish diversity in the estuary is currently restricted to 90 species belonging to 17 orders and 48 families, which is 40 per cent lower that that of the 1980s. Most of the marine species that recorded prior to construction of the barrage has now disappeared. They have been replaced by freshwater and estuarine species, according to a group of researchers from the Kerala University of Fisheries and Ocean Studies (KUFOS), Kochi.
The research paper was jointly published by K. Roshni, C. R. Renjithkumar, Rajeev Raghavan, K. Ranjeet of the university.
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The barrage, which was constructed in the middle of the lake in 1975, has a length of 1,402 metre.
It prevents seawater from entering the low-lying paddy fields in the southern part of the estuary, enabling double cropping of rice and ensuring drinking water to nearby areas.
Every year, the barrage is closed for three months from mid-December to mid-March. At times, the closure period will be extended up to May to prevent saline water intrusion.
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The unscientific management of the barrage over the past 45 years has hydrologically transformed the estuary into three distinct zones with distinct salinity regime and diverse physiography and hydrography, they noted.
Fish sampling was carried out in each zone from June 2015 to May 2016, during monsoon, post-monsoon and pre-monsoon periods. Since the saline stretch of the estuary had been reduced considerably due to hydrological modifications, most migratory species are trapped near the barrage and caught in local fisheries, concluded the researchers.