In the case of the Aghanashini River untampered to this day by hydroelectric projects, only minor interventions happened with the diversion of some amount of river water for a drinking water project for the coastal towns of Kumta and Honavar. All our sampling stations in Aghanashini estuary, including the most upstream station Hondad Hakkal located about 23 km were also in estuarine condition, showing more than 0.5 salinity. The situation was not different earlier as the Gazetteer of Uttara Kannada District (Kamath 1985) also had reported that the marine tides had travelled upstream for over 23 km. High tide salinity conditions of Aghanashini at comparable distances from the river mouth were substantially higher in Aghanashini than that of Kali. The present salinity conditions of Aghanashini estuary are comparable to the pre-dam estuarine situation of Kali (Fig. 2).

Commercial bivalves

The dam constructions for electricity generation in Kali and the fall in salinity on account of continuous release of freshwater after electricity generation, however, has not eliminated the bivalves from the estuary. But what is glaring is major shifts in the occupation zones (in km from the river mouth), and shrinkage in occupied areas of bivalves, after the dams getting commissioned. Tegillarca granosa, a bivalve reported to be associated with salinity in the range of 13.69 - 34.40 (Narasimham 1988) was restricted to the extreme river mouth at the Kodibag in Kali, where alone this medium to high range of salinity conditions prevailed after the execution of dams upstream. Even in the Aghanashini estuary, its preferred zone was closer to the river mouth.
Meretrix meretrix, one of the most abundantly used commercial bivalve from Kali estuary in 1978, had a wider distribution zone from 2-12 km in the salinity range of 30.82 to 8.76 (Nair et al. 1984). It may be assumed from Rao et al. (1989) that with the commissioning of the first hydroelectric project at Nagajhari, in 1980-1984, the species got confined to a narrower zone of 1-4 km, indicating the influx of freshwater driving the species towards a more seaward position. The situation worsened with still more hydel projects as the habitation zone of the species during the current study was just 1-3 km (Kodibag to Sunkeri) (Fig. 3a) where the salinity prevailed ranged from 11.75 to 6.40 . The decline of this popular commercial clam in the Kali has affected badly both the fisher-folks and the consumers. In the Aghanashini estuary, M. meretrix which had an earlier (in 1984) reported distribution zone from 1-3 km upstream from the river mouth (Rao et al. 1989), continues unchanged to this day. The mudflat of this zone accounted for the highest production of bivalves from the estuary (Boominathan et al. 2008).

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Fig. 3. Distribution of (a) Meretrix meretrix and (b) Meretrix casta in Kali and Aghanashini estuaries. K
– Kali estuary, A – Aghanashini estuary, K - 1978 (Nair et al. 1984), K - 1984 and A - 1984 (Rao et al.
1989), K - 2012 and A - 2012 (Current study). Note: K – 1978 pre-dam Kali, K – 1984 and K – 2012 post-dam
Kali

Meretrix casta in the Aghanashini estuary had a distribution range from near the river mouth to Mirjan, 10 km upstream (Rao et al. 1989). In the current study it occurred up to seven km upstream. We do not rule out its sporadic occurrence up to Mirjan itself, despite the instability conditions in the estuary due to both natural (flooding from excess rainfall) and anthropogenic causes (mainly intensified sand extraction from its habitat). Whereas M. casta was earlier reported in Kali in the mid-estuary zone of 7-11 km interior (Rao et al. 1989) in the current study its range was found restricted more seaward to a narrower zone at 1-3 km (Kodibag to Sunkeri) (Fig. 3b), obviously on account of dam related salinity reduction in the estuary.
Paphia malabarica, known as a high salinity zone estuarine bivalve, requiring 20 to 30 for survival (Mohan and Velayudhan 1998) had its reported occurrence in Kali in a two km range from the river mouth to Nandangadda (Nair et al. 1984). The species apparently underwent local extinction in Kali, after the execution of the first hydroelectric project itself, as it was not reported in the study by Rao et al. (1989), who testified to its presence until 1982. It's near decimation from the estuary can be attributed to lower salinity in the estuary. In contrast, in the Aghanashini estuary, it is one of the most abundant commercial bivalves, occurring in the river mouth region itself, from where supplies are made even to Karwar and Goa.
Polymesoda erosa, another clam, confined mostly to mangrove areas, has a reported salinity tolerance range of 7 to 22 (Modassir 2000). Though the species was hardly reported earlier from both the estuaries, it occurred sporadically in Kali estuary in some semi-stagnant mangrove swamps, in pockets of salinity from 10-12, between 1-6 km distance from the river mouth. Its occurrence in Aghanashini was, however, in higher salinities (23.80 to 28.55 ) from the river mouth (1-8 km zone). Based on the rarity of the species, it is difficult to make any pronouncement on the species and its preferred distribution ranges in both the estuaries.
Villorita cyprinoides is a low salinity preferring estuarine bivalve, able to withstand prolonged freshwater conditions (Nair et al. 1984; Rao et al. 1989), although the latter conditions are not favourable for its growth (Arun 2009; Laxmilatha et al. 2005). In the pre-dam Kali, its presence was mainly in 11-26 km zone (between Kinnar and Mallapur) within salinities of 8.76 to 2.43 (Nair et al. 1984). It got shifted to 7-24 km after the commissioning of the first hydroelectric project (Rao et al. 1989). The current study reveals a further narrowing of its zone from 6 to 12 km (Ambejug to Kinnar) (Fig. 4), within the salinities of 1.71 to 0.06 . This pronounced habitat shift towards downstream, was obviously necessitated by a steep decline in salinity in Kali. In Aghanashini, V. cyprinoides occurred in a wider zone of 8-23 km (Masur to Hondad Hakkal) within the salinity ranging from 23.80 to 4.61, as measured during February 2012. Rao et al. (1989) reported V. cyprinoides distribution from 8-15 km only, probably not prospecting for the species in more upstream areas.

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  Figure 4

 : Distribution of Villorita cyprinoides in Kali and Aghanashini estuaries. K – Kali estuary, A –
Aghanashini estuary, K - 1978 (Nair et al. 1984), K - 1984 and A - 1984 (Rao et al. 1989), K - 2012 and A
- 2012 (Current study). Note: K – 1978 pre-dam Kali, K – 1984 and K – 2012 post-dam Kali.

Oysters as a group of bivalves belonging to several species are known to inhabit coastal waters of higher salinities. In Aghanshini estuary, its occurrence was from the river mouth to seven km upstream, in the high tide salinity range of 28.55 to 26.35 . In Kali estuary the oysters were once distributed up to eight km upstream, and their high density occurrence was found at 30.82 (Sunkeri at 3 km from seafront) in 1978 (Nair et al. 1984). In the current study on Kali, for reasons explicit, its distribution zone was found to limit to three km upstream only from the earlier eight km. The salinity range in its current distribution zone was from 11.75 in the river mouth area (at Kodibag) to 6.40, at Sunkeri where the pre-dam salinity was as high as 30.82. The case of oysters also underscores the crucial role of salinity in the distribution of estuarine bivalves.

 

 

 

 

Citation :M. Boominathan,  G. Ravikumar,  M.D. Subash Chandran and T.V. Ramachandra, 2014. Impact of Hydroelectric Projects on Commercial Bivalves in a South Indian West Coast Estuary, J Biodiversity, 5(1,2): 1-9

* Corresponding Author :
	Dr. T.V. Ramachandra Energy & Wetlands Research Group, Centre for Ecological Sciences, Indian Institute of Science, Bangalore – 560 012, INDIA.		Tel : 91-80-23600985 / 22932506 / 22933099, Fax : 91-80-23601428 / 23600085 / 23600683 [CES-TVR] E-mail : cestvr@ces.iisc.ernet.in, energy@ces.iisc.ernet.in, Web : http://wgbis.ces.iisc.ernet.in/energy