Construction of permanent barrages in an estuary significantly alters morphology, hydrography and biodiversity, eventually leading to alteration of ecosystem structure and species distribution. Present study analyses the spatio-temporal variations in fish diversity, distribution and assemblage structure, across a permanently-built saltwater barrage in Vembanad Lake, a tropical estuary along the south-west coast of India. A barrage constructed in the middle part of the Lake in 1975, and its unscientific management over the last 45 years has hydrologically transformed the estuary into three distinct zones with distinct salinity regimes (an upper freshwater estuary, middle estuary on either side of the barrage, and a predominantly saline lower estuary). Monthly sampling was carried out in each zone from June 2015 to May 2016, representing three seasons, i.e. monsoon, post-monsoon and pre-monsoon. Fish diversity in the estuary is currently represented by 90 species belonging to 17 orders and 48 families, a 40% reduction since the 1980s. Two distinct fish assemblage were recorded on either side of the barrage indicating a lack of, or reduced habitat connectivity, with greater species diversity observed in the middle estuary. The spatial and temporal distribution of fish in the estuary was influenced primarily by salinity. Most marine species recorded prior to construction of the barrage has now disappeared and are replaced by freshwater and estuarine species. Since saline stretch of the estuary has reduced considerably due to hydrological modification, most migratory species are trapped near the barrage and caught in local fisheries. Adaptive management strategies to reduce impact of the barrage is urgently required to ensure a sustainable future for the biodiversity of this Ramsar site.

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