Assessment of bio-accumulation of bacteria in oysters from shellfish growing waters in Ashtamudi Lake (Kerala, India): A RAMSAR wetland

Oysters are commercially cultured from the Ashtamudi Lake in India, and support 12,000 tonnes of bivalve fisheries/year. Oysters and oyster growing waters were sampled from July 2012 to June 2013 for analysis of total coliforms (TC), faecal coliforms (FC), Escherichia coli, faecal Streptococci (FS) and total plate counts (TPC). E. coli MPN values in oyster growing waters were below the threshold limits set by the USFDA and EU during the months of December to April. Seasonally, the highest MPN values for E. coli were obtained during the monsoon season (June–September), and this trend gradually decreased during the post-monsoon (October–January) and pre-monsoon (February–May) periods. E. coli displayed a significant (p < 0.01) variation in accumulation during different seasons. A strong negative correlation (R2 = −0.70, p < 0.05) between temperature and E. coli numbers in oysters was observed, while rainfall and E. coli were positively correlated (R2 = 0.695, p < 0.05). Hence, we strongly recommend depuration and proper cooking of oysters before consumption during the monsoon season