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

Abstract

Based on a monthly field sampling over a year in the Kochi backwaters (KBW), this study presents the larval ecology of black clam and discusses how an extensive saltwater barrage [Thannermukkom barrage (TB)] impacted the natural black clam resource distribution. Spatial variations in salinity were found minimal during the Southwest Monsoon (June-September) due to the predominance of the freshwater associated with heavy monsoonal rainfall. Conversely, significant spatial changes in salinity were evident during the Pre-Southwest Monsoon (March-May) and Post-Southwest Monsoon (October-February). Monthly sampling exercises revealed that the black clam stock in the KBW breeds throughout the year, as their larvae were found (8 indiv. m-3-494 indiv. m-3) in all the locations. This observation is the modification of the traditional belief that black clam in the KBW breeds only twice a year. Mesohaline condition (salinity 5-18) is the most conducive for peak spawning and larval production. There were two peaks of larval production in the KBW over a year, mainly associated with the prevalence of the optimum salinity conditions on different spatial scales. The closing of the TB after the Southwest Monsoon (September) causes shrinkage of the area of the oligohaline and mesohaline conditions, the most conducive environment for the peak spawning and larval production of black clam in the KBW. This study presents a clear case of how human alterations of the natural environment impact valuable biological resources, which may apply to many similar aquatic ecosystems across the globe.