Impact of hydroelectric projects on finfish diversity in the Sharavathi River estuary of Uttara Kannada District, central west coast of India

Results and Discussion

Altogether 29 species of finfishes from 24 families were collected during study period (Table-1). Family Lutjanidae was leading with 3 species, followed by Engraulidae, Gerridae and Mugilidae, each with 2 species. The rest had each single species only. Sillaginid fish Sillago sihama and mullets (Mugil cephalus, Liza parsia) were available throughout the year. Bulk catches of Gerrieds (Gerres filamentosus and Gerres limbatus) happened only during the monsoon season. Notable of marine euryhaline fishes (tolerant of wide ranges of salinity) included Stolephorus commersonnii, Stolephorus indicus and Arius arius. Whereas Stolephorus spp. was present year-round in station I nearest to the sea, Arius arius occurred throughout the estuary, mainly during pre-monsoon and post-monsoon.
Closeness to the sea seems an important factor for fish richness more than salinity factor which was uniformly low for the entire estuary. Thus Honavar station (2 km from river mouth), had all the 29 species recorded, followed by mid-estuarine Malkod (8.5 km away) with 23 species and the most upstream Balkur (14.km away) which had 19 species, lowest of the three (Fig-2).

Figure - 2: Station-wise fish species diversity in Sharavathi

Not many studies were made hitherto on estuarine fish diversity of the district. Naik (2003) reported seed resources of 46 species of fin fishes from Kali estuary while Roopa et al. (2011) listed 37 fish species from it. Shirodkar (2013) inventorised 46 species belonging to 39 genera from Gangavali estuary. However, recent studies by our group revealed 61 fish species of 50 genera from Kali estuary, 55 species of 48 genera from Gangavali estuary and 80 species of 64 genera from Aghanashini estuary (Ramachandra et al. 2013). Further south, in Kerala, Bijukumar and Sushama (2000) reported 52 basically marine species from Ponnani estuary.
It was quite intriguing how Sharavathi estuary (1336 ha) was quite impoverished in fish diversity in comparison to Gangavali an estuary much smaller (558ha) in water-spread. Higher salinity in Gangavali and for that matter in Aghanashini also (2842ha) may be considered the major environmental factor for higher diversity of fishes in them than in Sharavathi. What separates Sharavathi from the other two is mainly the functioning of two important hydel projects in it. The salinity conditions in the Sharavathi estuary was very low in all stations even during post monsoon times (Fig-3). It was indeed below 0.5 ppt indicating fresh water conditions. Way back in 1984, working on the clam resources, Rao et al. (1989) had already noticed that water salinity was down to 1 ppt in the Sharavathi estuary, as near as 2 kms from the bar mouth,  during February, even three months after the rainy season.

Figure -3: Salinity variation observed along the study sites of Sharavathi Estuary during 2011-2012.

The movement of many marine fishes into Indian estuaries has been linked to favourable hydro-biological conditions within them (Sinha et al. 1996; Bijukumar and Sushama 2000). In both Aghanashini and Gangavali, without hydel projects, salinity was down to zero ppt in peak rainy months of 2011, rising steadily thereafter to 30-34 ppt towards the river-mouth, decreasing gradually upstream. At least 7 fishes of Aghanashini and 2 of Gangavali were basically stenohaline marine fishes which entered these estuaries when salinity rose to nearly marine level (Ramachandra et al. 2013). Sharavathi estuary, overwhelmed by fresh water releases from hydroelectric dams, was poorest in fish species (29 only) and with conspicuous absence of stenohaline marine fishes. Most were euryhaline tolerant of wide range of salinity. Notable species were Sillago sihama, Lates calcarifer, Glossogobius giuris, Lutjanus argentimaculatus, Scatophagus argus, Mugil cephalus, Stolephorus indicus, Etroplus suratensis etc. A comparison has been attempted here of the primary habitats of fishes from five different estuaries of South Indian west coast, available from different studies: viz. Kali, Gangavali and Aghanashini in Uttara Kannada (Ramachandra et al. 2013), Ponnani (Bijukumar and Sushama, 2000) and Kodungallur Azhikode, (Nandan et al. 2012) in Kerala, of which good profiles of fishes are available for comparison with that of Sharavathi. It is noticeable that compared to other estuaries Sharavathi has been very adversely affected by too low salinity as good proportion of fishes are basically of marine-estuarine and of marine-estuarine-fresh water categories (figure 4). Some stenohaline species of very low salinity to fresh water conditions like Dawkinisia filamentosa, Hyporhampus xanthopterus also occurred in the estuary.     

M-Marine; M-E-Marine, Estuarine; E-Estuarine; E-F-Estuarine, Freshwater; M-E-F-Marine, Estuarine, Freshwater; F- Fresh water.
 Figure - 4: Primary habitat-wise fish community composition (%) in some south Indian   estuaries

(Based on Nandan B.S. et al. 2012 for Kodungallur-Azhikode; Ramachandra et al. (2013) for Kali Gangavali, Aghanashini; Bijukumar and Sushama 2000 for Ponnani).

A sand bar across the river mouth (Fig-5) leaving only a narrow outlet into the sea, probably also would be limiting the entry of sea water against the force of fresh water outflow. Dredging at the river mouth, aimed at removal of the sand bar at least partially, is likely to facilitate better mixing of fresh and salt water, and improve the estuarine fishery, involving also shrimps, bivalves and crabs, resources that are crucial for the livelihoods and nutrition of scores of coastal people, who have become unintentional victims of developmental interventions like hydro-electric projects (at Linganmakki and Gersoppa in Sharavathi River). The environmental impact assessment protocols on dam construction, anywhere in India, hardly touches upon impact on estuarine ecosystems and this study highlights the need for widening the scope of EIA on developmental projects to even far off ecosystems, if they are connected with project areas especially by water, as is depicted in the current study.

Figure - 5: Sandbar formation blocking the mouth of Sharavathi estuary (Google imagery)

Table - 1: List of fish species collected from the Sharavathi estuary (station wise).
Presence (+) Absence (-)