MONITORING BIVALVE HABITATS IN NETRAVATHI ESTUARY

Bivalve aquaculture, in particular oyster, clam, scallop and mussel culture, is a globally increasing activity. Researchers show that razor clams may be on the cutting edge of oil spill cleanup. Results of previous studies indicated that some marine mollusc possess detoxification enzymes, which can biotransform PAHs. Nevertheless, bivalves usually have been reported to be a media as retaining xenobiotic contaminants.

In open waters where, bivalve farming is carried out the anthropogenic activities including oil & grease will affect the survival of the bivalves. Oil residuals from workshops, automobiles, refineries, oil terminals, depots, merchant ships, fishmeal plants and tankers get discharged into the rivers and contaminate the estuaries. These eventually reach the coastal environment. Oil pollution may directly or indirectly cause impact to the marine ecosystem such as coastal wetland, mangroves and also human health as the pollutant enters the food chain. Spilled oil & grease floats on the surface of water bodies limiting gaseous exchange, entangles and kills surface organisms and coats the gills of fishes. It also retards phytoplankton photosynthesis, respiration and growth, causing development abnormalities in zooplankton and the young stages of many aquatic organisms.

Estuaries provide ideal conditions for farming bivalves in optimum densities in pens, where transplantations are possible from denser spat settlement or nursery grounds. Natural clam beds in Netravathi Estuary are being monitored for bivalve biomass and sediment characteristics regularly. Considering the proximity of the clam beds to 13 fishmeal plants located near the bar-mouth (Ullala) the extent of oil & grease in the waters were also studied from the bar-mouth, to upstream stations located at Ullala (R1), Ullala Hoige (R2), Permannur (R3), Jeppinamogaru (R4) and Bajal (R5). Monthly water samples were collected during March 2016 to January 2019 and analysed according to pre-monsoon (PRM) (February-May), monsoon (MON) (June-September) and post-monsoon (POM) (October-January) seasons. Oil & grease in the water samples were quantified gravimetrically.

In pre-monsoon period, oil & grease ranged between <0.001 mg/L and 0.176 mg/L with the lowest in Jeppinamogaru station and highest in Ullala Hoige. In monsoon period the range was between 0.0013 mg/L to 0.007 mg/L with the lowest in stations near Ullala Hoige and highest in bar-mouth. In post-monsoon period the level was between 0.002 mg/L and 0.2228 mg/L with the lowest in Jeppinamogaru and Bajal stations and highest in Ullala station. These levels were below the maximum permissible limit of oil & grease in waterbody of 10 mg/L (Environment Protection Act 2002). During 2016, a Division Bench of the National Green Tribunal, Southern Zone, had imposed fine on the 13 fish meal units operating in Ullala after finding them guilty of polluting estuarine waters by discharging untreated effluents and ordered the closure of one fish meal unit that had been operating illegally in violation of the Coastal Regulatory Zone notification of 1991. Consequently, the oil & grease levels in the water reduced over the next 2 years. The level that was ranging from 0.0023 mg/L to 0.2228 mg/L in 2016 has come down to <0.001 mg/L to 0.0093 mg/L since 2017. The healthier water conditions were also reflected in the clam spat settlement in the Netravathi Estuary. With the treatment of effluents, the environment became conducive for the clams to recuperate and hence such monitoring studies are important for bivalve site selection for the farming activities.