Title : Bengal estuarine mangroves and shelf biodiversity degraded by trawl seeking conservation
Abstract:
Hooghly estuarine mouth and continental shelf area of the Bay of Bengal offshore in West Bengal develops huge phyto-planktons reserve serving as the broad baseline of the Largest Mangrove Food Web, ‘Sundarbans’ (World Heritage Site, 1989), an excellent combination of aquatic food chain occupying continental shelf and estuaries as well as mangroves forest food chain in the coastal wetland. It exhibits numerous marine species ranging from the base level microscopic phyto-planktons and mangroves up to those of higher trophic levels, pivoted by Sundarban’s Royal Bengal Tiger in the forest and large oceanic mammals under ocean.
Non-selective trawl nets dragged through ocean bottom scoops out sea-floor biodiversity, also destroying under-sea habitat of phyto-planktons at the baseline of the Food Web. Additionally, massive trawl destruction of estuarine mangroves and heavy metal pollution in trawl resting spots also damaged the Food-web baseline and triggered severe survival crisis for all apex species. Continuous destruction of non-commercial trawl discards is causing higher trawling mortality which is also triggered by trawl heavy metal pollution in deep sea fishing fields. This massive baseline species destruction already started collapsing the whole marine food pyramid, endangering all top consumers and worse affecting the fishing-dependant coastal population. Intensive trawl survey into the Tajpur-Shankarpur-Digha-Petua fishing zone has proved biodiversity richness inversely related to offshore distance, resulting in decreasing marine biodiversity loss with increasing depth and starting distance of trawling. So, shallower continental shelf with maximum benthic nutrients deposition shows higher trawl mortality indicating higher biodiversity loss.
Ultimate huge mangroves loss, absence of prawn seedlings, sea conches and finally, extinction of a special fish species (Chandana Hilsa) seen before the last forty years, are evidenced. Suggested conservation strategy highlights mangroves restoration, no night fishing, instant release of trawl discards alive into ocean and above all, Trammel net application utilising its poorer selectivity with minimum abrasion on continental shelf keeping undersea baseline shelf biodiversity intact. Trammel net enjoys poorer selectivity because of the tangling nature of three layers of netting instead of single layer of the commonly used gill net. So the fisherman can easily minimise by- catch shooting his net only in those areas obtaining maximum target species. Moreover, sea bed contact with trammel net is very limited only through footrope and also minimal contact through small anchors at each end. Though the anchors penetrate sea bed, this will have minimum ploughing effect on sea bed only during hauling. As not towed over sea bed, very little abrasion takes place and the thick undersea bed layer of marine biodiversity, specially the phyto-planktons remains mostly intact.
This is considered just as a blessing to Conservative Eco-friendly Bengal offshore and Estuarine fishing ensuring prosperous Mangrove Food Web. As the richest biodiversity zone of Bengal, continental shelf still remains exposed to prolonged rampant anthropogenic trawl overuse as before, proper ecological restoration specially through the introduction of Trammel net must be made mandatory here continuously for a pretty long time to witness its miraculous impact on the World’s largest Mangrove Food Web of the Sundarbans
