Introduction

Increase in estuarine salinity due to upstream dam construction in the river for water diversion for various human wants such as irrigation, industry, domestic needs, etc. is a well-established fact. Such salinity increases due to human interventions can create changes in the composition of estuarine fauna (Chen, 2005; Dandekar, 2012; Estevez, 2002; Rodriguez et al., 2001; Seddon, 2000). Not much work, however exists on the salinity decrease in the estuary due to execution of hydroelectric projects in the upstream areas causing almost constant release of freshwater into the estuary following power generation. In a rather isolated study related to the second scenario Parada et al. (2012) reported the mortality of estuarine molluscan community on account of salinity decline in the estuary of Ulla River of Spain due to an upstream hydroelectric project. Salinity and substratum composition (for instance, sand-silt-clay mix) are the main factors influencing the estuarine molluscs (Harkantra, 1975a; Kanaya and Kikuchi, 2008; Mohan and Velayudhan, 1998; Parker, 1959; Sundaram and Shafee, 1989). In the case of Ulla estuary, salinity reduction directly or in synergy with other factors, caused the bivalve mortality when its level declined below the optimum or lasted for long periods (Parada et al., 2012).
In India, estuarine molluscan resources, especially clams and oysters, contribute to the livelihoods of many people (Boominathan et al., 2008; Rao et al., 1989; Rao and Rao, 1985). Overexploitation, shell and sand mining in the estuarine areas are widely considered as major causes for the depletion of molluscs. The estuary of the river Sharavathi was once known for good production of bivalves, (Alagarswami and Narasimham, 1973), a scarcity of which experienced in 1980, attributed to dam related salinity decline, although some clams were still available closer to the river mouth (Rao and Rao, 1985). By 1984, the clams had disappeared in the Sharavathi estuary, when salinity measured merely 1 ppt at 2 km upstream, even in February about 3 months after the rainy season (Rao et al., 1989). However, the euryhaline (tolerance of wide range salinity) species Polymesoda erosa, was reported as present in the estuary (Ingole et al., 2002).

Gangavali, yet another estuary of Uttara Kannada, also had extensive clam resources (Alagarswami and Narasimham, 1973). For no known reason clams were not found in the estuary in 1984. Minor harvests of M. casta and Paphia malabarica before 1980 was reported (Rao et al., 1989). However, in Gangavali estuary bivalve harvesting is an important livelihood activity these years. Hypothesizing that hydroelectric projects related salinity decline has adversely affected estuarine bivalves of Sharavathi in comparison with Gangavali unaffected by dams we undertook the current study.