Introduction

The Molluscs are soft bodied invertebrates with or without an external protective shell. The Molluscs appeared for the first time towards the end of the Precambrian period, about 550 million years ago (Sturm et al. 2006). It is the second largest phylum of the invertebrates comprising more than 100,000 species worldwide, of which, 5070 species occur in India (Venkataraman and Wafar 2005). They have been exploited worldwide for food, ornamentation, pearls, lime and medicine (Nayar and Rao 1985). Geologic evidence from South Africa indicates that systematic human exploitation of marine resources, including molluscs, started about 70,000 to 60,000 years ago (Volman 1978). Molluscs usually inhabit the marine, estuarine and freshwater ecosystems, although many are also terrestrial, often found associated with moist shaded lands. The class Bivalvia includes clams, mussels, and oysters which contribute to the livelihoods of scores of people in India. Bivalves such as Crassostrea gryphoides Schlotheim, C. madrasensis Preston, C. rivularis Gould, Marcia opima Gmelin, Meretrix casta Chemnitz, M. meretrix Linnaeus, Paphia malabarica Chemnitz, Perna perna Linnaeus, P. viridis Linnaeus, Pinctada imbricata fucata Gould, P. margaritifera Linnaeus, Placuna placenta Linnaeus, Polymesoda bengalensis Lamarck, Saccostrea cucullata Born, Tegillarca granosa Linnaeus, Tridacna crocea Lamarck, T. maxima Roding, and Villorita cyprinoides Gray were cited as utilised in India for food, industrial or ornamental purposes (Appukuttan 2004). In the estuarine villages of Karnataka State, India, molluscan fisheries involving mainly three clams Meretrix casta, Paphia malabarica, and Villorita cyprinoides and oyster Crassostrea madrasensis, sustain the livelihood of a good number of coastal people (Boominathan et al. 2008; Rao and Rao 1985; Rao et al. 1989).
Recent times have witnessed anthropogenic pressures of varied kinds affecting estuarine ecology, including bivalve fishery and diversity. Such pressures are ongoing with scant concern shown especially towards the impact on bivalves. Dam construction is known as one of the major forms of human interventions in the rivers that has consequences on downstream ecology, especially in estuaries. Dams in rivers obviously create alterations in the freshwater flow and input of nutrients into the downstream estuaries (Baisre and Arboleya 2006; Kennish 2002). Salinity has been considered one of the important among the various physicochemical properties of the estuary influencing organisms (Harkantra 1975; Kanaya and Kikuchi 2008; Mohan and Velayudhan 1998; Sundaram and Shafee 1989). The dams for diverting water for irrigation or other human wants can reduce the input of freshwater into the estuaries and increase estuarine salinity is quite obvious from several studies (Bernacsek 2001; Chen 2005; Estevez 2002; Rodriguez et al. 2001; Seddon 2000). In the Volta River, for example, regulation of inflow through Akasombo and Kpong dams led to the disappearance of a clam industry due to increased salinity (Chen 2005). The construction of a series of dams on the Colorado River and diversion of freshwater for human uses led to increased salinity in the estuary downstream caused a serious decline in the population of low salinity preferring bivalve Mulinia coloradoensis Dall (Rodriguez et al. 2001). A review on the impacts of large dams in rivers for diversion of freshwater caused 40 to 80% decline in downstream molluscan diversity in the Wolf Creek, Caney Fork and Tombigbee River also due to increased salinity (Seddon 2000).
Although studies abound on the impact of dams for freshwater diversion for irrigation, etc. only isolated works deal with on the impact of hydroelectric dams on the estuarine bivalves through decreased salinity conditions. In the Ulla River estuary of Spain Parada et al. (2012) observed salinity changes related mortality of bivalves due to the execution of hydroelectric projects upstream. Salinities of < 15 for a mean period of three consecutive days were enough to cause moderate mortalities and salinity of < 5 for an average of four consecutive days led to near-total mortalities in bivalves Cerastodermaedule Linnaeus and Venerupissenegalensis Gmelin. And also, salinity of < 15 for 9 days caused severe mortality of bivalves Tapesdecussatus Linnaeus and Tapes philippinarum Adams & Reeve (Parada et al. 2012). Likewise, mass mortality of Soletellina alba Lamarck occurred when the salinity level became ≤ 1 (Matthews and Fairweather 2004).
The recent decades, particularly after Indian independence in 1947, witnessed the construction of a number of dams in many of the rivers, both for irrigation and electricity generation. Whereas environmental impact assessment was hardly carried out on these projects prior to 1970’s, such assessments thereafter hardly ever referred to impact on estuarine biodiversity consequent on especially the execution of hydroelectric projects. In the Sharavathi estuary of Uttara Kannada district of South-west India, Rao and Rao (1985) and Rao et al. (1989) had already alluded to a drastic decline of bivalves such as Meretrix meretrix, Meretrix casta and Crassostrea madrasensis because of continuous freshwater inflows from the hydroelectric projects executed upstream. Despite these studies not elaborating further on this aspect they make clear that these bivalves did occur in the Sharavathi estuary about three decades ago and some local people were involved in their fishery. The fisher-folks of the estuary also attributed the recent decline in fish diversity and collapse of bivalve fishery in Sharavathi estuary due to the damming of the river for power generation (Boominathan et al. 2014). Except for sporadic references there has hardly been any efforts to portray what would be the impact of execution of upstream hydroelectric projects on estuarine bivalve fishery. This situation prompted us to undertake the current investigation in the Uttara Kannada district of Karnataka State. Of the four notable west flowing rivers of the district, two (Kali and Sharavathi) have been harnessed for hydroelectric power and the others (Aghanashini and Gangavali) are unaffected. For the current study, estuaries of the river Kali (dammed) and Aghanashini (undammed) were chosen. In a pre-dam period study on Kali by Nair et al. (1984) and the earlier situation of commercial bivalves in the Aghanashini estuary is known from Rao et al. (1989).