Implications of an extensive salt water barrage on the distribution of black clam in a tropical estuarine system, Southwest coast of India

•Black clam larvae found throughout the year in Kochi Backwaters.•Mesohaline salinity is the optimum salinity for the larval production.•Salinity shift from limnohaline to oligohaline /mesohaline inducing larval production•Thannermukkom barrage alters the prevalence of the optimum saline conditions•Manmade hydrological barrier disturbs normal distribution of biological resources