Discussion

Along the coast of Uttara Kannada district harvest and sale of estuarine bivalves have been an important livelihood activity. The commercial bivalves played a key role in nutritional security of thousands of families and also generated a reasonable income, especially for women bivalve harvesters, who have otherwise very little role in the regular marine or estuarine fisheries using boats and nets (Boominathan et al., 2008). Unfortunately, this scenario has been changing rapidly in the recent years due to various threats like unrestricted shell and sand mining, shrimp aquaculture and over-exploitation of bivalves to meet growing demands from far away urban markets. Practically very little was known earlier about what the impact would be on bivalves from execution of large hydroelectric projects in the upstream areas of west flowing rivers from the Western Ghats. Major dams for power generation have been established in Sharavathi and Kali rivers of Uttara Kannada with practically very little knowledge about the likely repercussions of such gross developmental interventions on estuarine ecosystems.

The power projects in Sharavathi River have a total installed capacity of 1469.20 MW. The hydroelectric power generation history in Sharavathi began in 1948 with the 120 MW Mahatma Gandhi Power House, now with 139.20 MW capacity. The commissioning of 1964 Linganmakki dam with a 55 MW power house would not have made any perceptible impact on estuarine ecosystem as Alagarswami and Narasimham, (1973) mentioned about the existence of “extensive clam beds” dominated by Meretrix meretrix, a high salinity tolerant species, providing regular employment for about 100 clam collectors. As clams collected in those days from several estuaries of the region were mainly used for local consumption (Boominathan, 2008) the exploitation would have been within sustainable limits. The scenario change happened with the use of Linganmakki dam waters for power production in the Sharavathi Generating Station, with installed capacity of 1035 MW. The maximum use of water for power production from Linganmakki dam, with a storage capacity of 4417.51 Mcum, was a major developmental intervention in a relatively small river like Sharavathi.

By 1980's the “extensive clam beds” would have declined substantially as Rao and Rao (1985) observed M. meretrix and M. casta (both high salinity tolerant) as “sparsely distributed” over an area of 11 ha near the river mouth. Further upstream, 5 km interior near Mavinkurve islet, the “standing stock was negligible”. Four years later, during February 1984, when Rao et al. (1989) surveyed 50 sample plots covering a distance of about 17 km from the river mouth, the clams occurred no more, and the local collectors admitted that clam fishery in Sharavathi had virtually ended about a decade earlier itself. The addition of yet another hydroelectric dam, further downstream at Gerusoppa in 1999, with a storage capacity of 166 Mcum, and installed capacity of 240 MW of power, spelt nemesis for the clam community of Sharavathi estuary. The cumulative impact of year long water releases from upstream dams with a total installed power generation of 1469.20 MW, obviously caused a sharp fall in salinity conditions in the estuary which was between 0.67 to 13.80 ppt at the time our study.

During our survey only Polymesoda erosa, whose known range of salinity tolerance is from 7 to 22 ppt (Modassir, 2000), occurred in the estuary. It was also sparse and its harvest in Sharavathi was limited to meeting domestic needs of a few families. Clemente and Ingole (2011), Gimin et al. (2004), Ingole et al. (1994; 2002), and Modassir (2000) associated the species with muddy substratum which was true for Honavar and Kasarkod stations of Sharavathi. Its upstream distribution range was from 2.5 to 4.5 km from the river mouth (Table 1). In Gangavali estuary also P. erosa (not reported earlier) was associated with muddy locations, especially in mangrove areas.

Unaffected by any dams, Gangavali continues to have M. meretrix and P. malabarica, as reported earlier (Alagarswami and Narasimham, 1973; Rao et al., 1989). In addition M. casta, P. erosa, Tegillarca granosa, and Villorita cyprinoides also occurred as Gangavali estuary continues to provide natural habitats, specifically in relation to salinity gradients, for various clams. In this regard it resembles the neighbouring Aghanashini estuary, without any dams, where 8 species of commercial bivalves were found (Boominathan et al., 2008).

Villorita cyprinoides, although not recorded in earlier studies from Sharavathi, nor from the current study, was also present in the estuary until some years ago, according to local fisher-folks, who showed the shells of the species. As V. cyprinoides is common to the estuaries of Karnataka (Rao et al., 1989), and because of its presence in other estuaries of Uttara Kannada like Aghanashini and Kali (Boominathan et al., 2008; 2012) and Gangavali, according to the current study (Table 1), the species, in all probability, was present in Sharavathi until it was eliminated due to the dam impact. V. cyprinoides is a species that could persist in freshwater conditions for some period (Nair et al., 1984; Rao et al., 1989). However, low salinity/freshwater conditions were reported to have adverse consequences on its growth and survival (Arun, 2009; Laxmilatha et al., 2005; Pillai, 1991). Nair (1975) found growth retardation of V. cyprinoides var cochinensis in low salinity conditions. It attained larger dimensions in its optimal salinity range from 4.73 ppt to 27.11 ppt, whereas, small sized individuals tolerated more extreme range from 0.87 ppt to 29.85 ppt salinity. In Kelagin-Idgunji station, about 8 km upstream in Sharavathi estuary, according to local fishermen, V. cyprinoides used to be present some years ago. Obviously dam related low salinity (0.02 ppt recorded in December 2011) would have caused its elimination. In comparison the species occurred in Ulware station, 8 km upstream in Gangavali, where December 2011 salinity was 18.05 ppt.

Meretrix meretrix is a high salinity species (Rao et al., 1989). In the laboratory conditions it could not withstand salinity lower to 13 ppt (Sundaram and Shafee, 1989). However, in the natural conditions, M. meretrix survived in 10.5 ppt in Kalbadevi estuary (Ranade and Kulkarni, 1973), and 7.45 ppt in Kali estuary (Nair et al., 1984). The Gangavali estuary study, confirmed its presence in high salinity stations Gangavali (28.85 ppt) and Agragone (28.5 ppt). In the Sharavathi estuary, it was present almost until 5-6 years ago according to clam collectors. The commissioning of Gerusoppa hydel project in 1999, would have caused its extinction in a span of a few years, although we do not rule out its recurrence during drought years in the future.
M. casta is a high salinity tolerant species upto 56.00 ppt (Durve, 1963). Its growth gets arrested in low salinity conditions (Harkantra, 1975b). Abraham (1953) and Sreenivasan (1983) reported growth retardation of M. casta at 15.65 ppt and 20 ppt respectively. Paradoxically Jayawickrema and Wijeyaratne (2009) observed that M. casta lived well between 3 and 40 ppt salinity. In Gangavali estuary, M. casta occupies a salinity zone from 28.85 to 22.55 ppt in December 2011. According to local fisher-folks M. casta had also persisted in the Sharavathi estuary in small numbers until a few years ago. Here again, drastic decline of salinity related to freshwater release would have caused the species elimination.

Although Tegillarca granosa prefers salinity range of 13.69 to 34.40 ppt (Narasimham, 1988) it functioned better in salinities above 23 ppt, and more efficiently between 26 and 31 ppt (Pathansali, 1963; Broom, 1985). T. granosa occurred near a mangrove-mud flat area, its preferred habitat (Pathansali, 1966), in the Gangavali estuary, in 28.85 ppt salinity. Other estuaries of the district Aghanashini and Kali, have the species to this day. In all probability it would have vanished from Sharavathi due to inundation with freshwater. Paphia malabarica, also a high salinity species, present in Gangavali and Aghanashini was not found in Sharavathi. Thus sequential interpretation of historical data on Sharavathi, based on available literature and survey undertaken during the current study, including interviews with erstwhile clam collectors, as well as comparison of clam diversity situation in the Gangavali estuary of Uttara Kannada, and the earlier study in Aghanashini (Boominathan et al., 2008) suffice to establish the fact that the cumulative impact of a series of hydroelectric projects have far reaching consequences on the estuarine clams mainly because of dilution of salinity conditions.

Estuary is a place where unique environmental conditions are created by the mingling of freshwater from the river and salt water from the ocean. Mangroves are unique formations of these brackish water habitats. Several marine fishes use estuaries for spawning or as nurseries for their juveniles. Even adults of many marine fishes move into the estuary during times when salinity conditions are favourable for them. The current study, focusing on commercial clams shows how major incursions by humans, like execution of hydroelectric projects, can be adversely affecting the species diversity of clams with devastating effects on the livelihoods of scores of people who traditionally depended on the estuarine fishery for livelihood and family nutritional security. More concerted works are required for unraveling ecosystem changes in estuaries as the aftermath of dam constructions, rampantly happening in the Western Ghats.

CONFLICT OF INTEREST

The authors confirm that this article content has no conflicts of interest.