Anthropogenic stressors of black clam distribution in Kochi backwaters on the Indian west coast.

Abstract

The black clam (Villorita cyprinoides Gray, 1825) is the most commercially important clam in India and the major share of its landing (around 25,000 tonnes/year) comes from Kochi backwaters (KBW), the largest estuarine system on the west coast of India, where approximately 4000 fishermen harvest them year-round. This study based on recent and historical data sets, comprehended how multiple anthropogenic stressors impact the black clam distribution in the KBW. In the first part, a recent data set from an extensive hydrographic and sediment sampling from 22 locations in the central and southern sections of the KBW during the Pre-Monsoon (March), Southwest Monsoon (July), and Northeast Monsoon (December) was introduced to demarcate the most conducive salinity and sediment textural conditions of black clam. Black clam in the KBW prefer midstream and upstream regions with mesohaline to oligohaline conditions and sand-dominant substratum, but their current distribution is shaped by multiple anthropogenic stressors, notably the consequence of the installation of the Thannermukkom Barrage (TB) in 1975 to prevent saltwater intrusion for paddy cultivation. The combination of current and historical data, supplemented with literature, demonstrates that TB generated various stressors on the natural distribution, resulting in a decrease in the abundance of black clam in the KBW. This includes (a) shrinkage and relocation of their most preferred salinity zones (mesohaline) for spawning from the south of TB (Vembanad) to the north of TB, (b) the increased siltation due to stagnancy in the Vembanad caused by TB increased the contribution of finer particles especially clay in the bottom substratum, which is less preferred over sand by black clam and (c) the opening and closing of the TB shutters cause salt shock causing vast mortality of black clam on both sides of TB. Secondary stressors of TB are affected by (a) poor water quality, eutrophication, and massive spread of hyacinth mats, making it difficult for local fishermen to exploit the black clam resource, and (b) overexploitation of the black clam in certain areas due to shrinkage in the total area and relocation of the conducive spawning environment in KBW.