EDIBLE BIVALVES OF UTTARA KANNADA ESTUARIES

There are six edible clams, viz. Anadara granosa, Meretrix casta, Meretrix meretrix, Paphia malabarica, Polymesoda erosa, Villorita cyprinoides and some oysters present in all the estuaries of Uttara Kannada district (table 1), except in Sharavathi estuary where only one clam Polymesoda erosa occurs today and the oysters can be seen on rocks close to the river mouth with higher salinity. Among these edible bivalves the genus Meretrix, Paphia, and Villorita, and oysters contributes to the livelihoods of many peoples (Rao and Rao, 1985; Rao et al., 1989; Boominathan et al., 2008).

Table 1: Edible Bivales of Uttara Kannada estuaries. P = Present, A = Absent

Anadara granosa is present in all the estuaries except, Sharavathi. In Kali, Gangavali, and Aghanashini the distribution of A. granosa is restricted to one kilometer range from river-mouth (Table 2.1). It prefers soft intertidal muds bordering mangrove swamp forest (Pathansali, 1966) and salinity range of 13.69 – 34.40 ppt (Narasimham, 1988) hence, A. granosa present only close to the river-mouth where the salinity is usually high. This species was previously reported from Kali (Boominathan et al., 2012), Aghanashini (Boominathan et al., 2008, 2012), and Venkatapur (Rao and Rao, 1985) estuaries of Uttara Kannada District.

Table 2: Current distribution of bivalves in the Kali, Gangavali, Aghanashini, and Sharavathi estuaries. P = Present.

Meretrix casta is distributed in Aghanashini and Gangavali (without any dams) estuaries from the river-mouth to six km interior. Compared to this, M. casta is distributed in Kali (with dam) only for three km range from the river-mouth (table 2.2), its occupational area reduced on account of the influx of fresh water releases from the hydel projects upstream. M. casta is a euryhaline species (adapted to a wide range of salinity) (Rao et al., 1989) and it has a greater degree of physiological adaptation in the salinity range of 25.00 to 56.00 ppt (Durve, 1963). Since, the salinity of Kali estuary is very low M. casta is distributed only upto three kilometer distance from river-mouth. Whereas in Sharavathi estuary M. casta is absent, probably because of extremly low salinity due to dam water releases. M. casta was reported by various authors from Kali, Gangavali, Aghanashini, Sharavathi, and Venkatapur estuaries (Alagarswami and Narasimham, 1973; Harkantra, 1975a, 1975b; Rao and Rao, 1985; Rao et al., 1989; Bhat, 2003; Boominathan et al., 2008, 2012).

Whereas the distribution of Meretrix meretrix in the undammed Aghanashini and Gangavali estuaries range from river-mouth to three kms inside, in Kali (with dams) M. meretrix has only a one km range from river-mouth (table 2.3). M. meretrix prefers high salinity (Rao et al., 1989) hence, its presence closer to the river mouth can be justified. In the Sharavathi estuary M. meretrix was present earlier (Alagarswami and Narasimham, 1973; Rao and Rao, 1985), but seems have vanished today, obviously because of decline in salinity caused by release of fresh water from hydel projects. M. meretrix is present to this day in all the other estuaries (Alagarswami and Narasimham, 1973; Rao and Rao, 1985; Rao et al., 1989; Bhat, 2003; Boominathan et al., 2008, 2012).

Paphia malabarica occurs closer to the river mouths (Rao et al., 1989) with salinities of 20 to 30 ppt (Mohan and Velayudhan, 1998). It occurs to this day in the high salinity regions of Kali, Gangavali, and Aghanashini estuaries (table 2.4). However, in Sharavathi estuary P. malabarica was not reported earlier nor it occurs currently. The species occurs in all the other estuaries viz. Kali Gangavali, Aghanashini, and Venkatapur (Alagarswami and Narasimham, 1973; Harkantra, 1975a; Rao and Rao, 1985; Rao et al., 1989; Bhat, 2003; Boominathan et al., 2008, 2012).

Polymesoda erosa prefers salinity of 7 to 22 ppt (Modassir, 2000). It is present in all the four estuaries (table 2.5). Even though, it is present in Kali, Gangavali, Aghanashini, and Sharavathi estuaries, the population is high in Sharavathi estuary than the other estuaries and also it is the only species of edible clam present to this day. P. erosa was earlier reported by Ingole et al., (2002) from Sharavathi estuary where it is still present. Boominathan et al., (2012), for the first time, reported its occurrence in Kali and Aghanashini estuaries.

Villorita cyprinoides associated with medium salinity conditions is known to withstand freshwater conditions (Nair et al., 1984; Rao et al., 1989; Boominathan et al., 2012). It was reported from Kali (Rao et al., 1989; Boominathan et al., 2012), Aghanashini (Rao et al., 1989; Bhat, 2003; Boominathan et al., 2008, 2012), and Venkatapur (Alagarswami and Narasimham, 1973) estuaries. Extremely low salinity of Sharavathi estuary might have caused its present elimination from here where according to elderly fisher-folks the species was present earlier. In Kali estuary, which has more salt water ingress, despite the dams, it is found in 6-12 km range. It occurs in 5-16 km zone in Gangavali, and 9-23 km zone in Aghanashini respectively (table 2.6).

Oysters are present in all the estuaries (table 2.7), usually it occurs in moderate to high salinity regions in the estuary. They were also previously reported from Kali (Rao, 1974; Boominathan et al., 2012), Gangavali (Rao, 1974), Aghanashini (Rao, 1974; Boominathan et al., 2008, 2012), Sharavathi (Rao, 1974; Rao and Rao, 1985), and Venkatapur (Rao and Rao, 1985) estuaries.