Rajan R S Pandiya, Jyothibabu R, Arunpandi N, Parthasarathy S, Santhikrishnan S
CSIR National Institute of Oceanography, Regional Centre, Kochi, India.
Sci Rep. 2025 May 15;15(1):16895. doi: 10.1038/s41598-025-90573-4.
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.
黑蚬（Villorita cyprinoides Gray，1825年）是印度商业价值最高的蚬类，其上岸量的主要份额（约25000吨/年）来自科钦潟湖（KBW），这是印度西海岸最大的河口系统，约4000名渔民全年在此捕捞。本研究基于近期和历史数据集，了解了多种人为压力源如何影响黑蚬在科钦潟湖的分布。在第一部分，引入了一个近期数据集，该数据集来自在科钦潟湖中部和南部22个地点进行的广泛水文和沉积物采样，采样时间分别为季风前（3月）、西南季风（7月）和东北季风（12月），以划定黑蚬最适宜的盐度和沉积物质地条件。科钦潟湖的黑蚬偏好中盐度至低盐度条件且以沙子为主的底层的中游和上游区域，但它们目前的分布受到多种人为压力源的影响，特别是1975年为防止海水入侵用于水稻种植而建造的坦纳穆科姆大坝（TB）的影响。当前数据与历史数据相结合，并辅以文献，表明大坝对自然分布产生了各种压力源，导致科钦潟湖黑蚬数量减少。这包括：（a）它们最适宜的产卵盐度区（中盐度）从大坝以南（韦姆巴纳德）收缩并转移到大坝以北；（b）大坝导致韦姆巴纳德水流停滞，淤积增加，使得底层更细颗粒尤其是粘土的占比增加，而黑蚬不太喜欢粘土而更喜欢沙子；（c）大坝闸门的开合导致盐度冲击，造成大坝两侧大量黑蚬死亡。大坝的次生压力源受到以下因素影响：（a）水质差、富营养化以及水葫芦垫的大量蔓延，使得当地渔民难以开发黑蚬资源；（b）由于科钦潟湖适宜产卵环境的总面积缩小和转移，某些区域的黑蚬被过度捕捞。
