The Aghanashini River, originating from two sources, at Manjguni and Donihalla, both in the Sirsi Taluk of Uttara Kannada, runs its 121 km course winding through deep gorges and valleys towards the Arabian Sea, also through richly wooded landscape of the Western Ghats. The river joins the sea between the villages Tadri in the north and Aghanashini in the south about 10 km towards the north of Kumta town. The high tides from the Arabian Sea reach up to Uppinapattana at about 25 km upstream in the river, conferring estuarine status to about 21 km2 of water spread area. As Aghanashini has no hydel projects the estuary is expected to retain its original salinity regimes. The study of Aghanashini bivalves is expected to portray a situation unaffected by hydel projects, which could have been similar to that of its counterpart Kali of pre-dam period. The study area map (Fig. 1), covering both the estuaries, was created by using QGIS version 1.7.4 (Quantum GIS Development Team 2011).

Salinity measurements

In both the estuaries, surface water salinity was measured in February 2012 during high tide using EXTECH EC400 salinity meter. After the subsidence of the torrential monsoon rains, normally in October, salinity starts rising in the estuary, and attain peak salinity conditions by February. Salinity levels that prevailed in the Kali estuary in the pre-hydel power project period were obtained from Nair et al. (1984). Bar graph to showing the salinity difference was produced using statistical software R version 3.0.1 (R Core Team 2013).

Bivalve data collection

The presence of different commercial bivalves in both the estuaries was obtained from Alagarswami and Narasimham (1973) and Rao et al. (1989). Nair et al. (1984) had provided data on distribution range of bivalves in the pre-dam Kali estuary. A later study by Rao et al. (1989) on bivalve diversity in Kali is also quite informative. Live and dead bivalves, the latter represented by shells, were collected randomly from different parts of both the Aghanashini and Kali estuaries, and from persons involved in bivalve fishery. A questionnaire was also used to gather information from the fisher-folks of Kali estuarine villages on bivalve diversity and distribution in the pre-dam period and current times. About 50 members of the fisher-folks were interviewed for this purpose. The data gathered is covered the occurrences of commercial bivalves Tegillarca granosa, Meretrix casta, M. meretrix,Paphiamalabarica, Polymesodaerosa Solander, Villoritacyprinoides and estuarine oysters. As sand mining from both the estuaries was taking place at different localities, searches were also made in the excavated sands for bivalve occurrences. The bivalves collected were identified using diagnostic characters given by Apte (1998), Dey (2006), Morton (1984), Rao (1989), and Rao et al. (1989).