Introduction
The estuaries, in general, are places of high productivity and biodiversity, being rich in nutrients, inputs from the land and from upwelling in the marine areas brought in by the tides. The mangroves and marshes with reeds and grasses provide additional nutrients and habitats for a variety of fishes, most of them moving in from the sea, some resident in the estuary itself or sharing even fresh water habitats. The estuaries thus are ideal for fishery development, as important areas for spawning, nurseries and feeding. They are essential habitats in the life cycle of many organisms, in particular fish. Estuaries are important seed sources for many shrimps and fishes for culturing. Fishes and many invertebrates being valuable food resources calls for integrated management of the coastal zone of which estuaries are integral parts (Laegdsgaard and Johnson, 1995; Blaber et al. 2000; Brinda et al. 2010; McLusky and Elliott, 2004; Breine et al. 2011). Despite rising awareness on their importance estuaries are among the most modified and threatened environments today. Estuarine fish community is an indicator of its health and reflects the ecological dynamics within the estuary (Thiel et al. 1995; Marshall and Elliott, 1998; Paperno and Brodie, 2004; Gutierrez-Estrada, et al. 2008).
Composition of fish community provides a first approach to the health level of the estuarine ecosystem. Of the physico-chemical characteristics decisive on estuarine fish diversity salinity is a crucial one (Barletta et al. 2005). Gutierrez Estrada et al. (2008). Attempts at comprehensive inventorisation estuarine fish diversity, in relation to salinity factor, are practically new to the south-west Indian State of Karnataka, characterized by over a dozen rivers flowing westward into the Arabian Sea from the Western Ghats, one of the global biodiversity hotspots.
The coast of Uttarakannada district of Karnataka (longitude 74o and 78o East and latitude 11o and 18o North) has played important role in Indian fishery scenario, through especially marine and estuarine fishery. Nevertheless, comprehensive studies on estuarine fish diversity are practically lacking barring isolated works, such as of Naik (2003) on fish seed resources of Kali estuary, Roopa et al. (2011) on fin fishes of the same estuary and Shirodkar (2013) on the fish diversity of Gangavali estuary. Except some allusions hardly any notable work exists for Sharavathi River estuary. A fishery collapse in the estuary was widely known locally and the fishermen link it to the execution of hydro-electric projects in the Sharavathi River. The first major hydroelectric dam in the river at Linganmakki in Shimoga district became operative from 1964 and the second at Gersoppa in Uttara Kannada district from1999. The fishery collapse made us undertake a study on fish inventorisation in the estuary in relation to salinity, a factor that got altered in a big way since the hydroelectric dams.
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