Pure Indian carp are among the most sought fish in the world, and an entire fishing market has been built around them for many centuries now. For the most part, they are only attainable from the Halda River, which is a large yet singular source of natural carp breeding based in Bangladesh, and all of South Asia has suffered its losses as of late to the tune of some 26 different fish species in general — all wiped out in seven years' time by water pollution. India has been suffering similar water pollution issues itself and has concluded that linking certain rivers may be the best way to deal with the problem; however, new research indicates that this will have adverse effects on Indian ecosystems.
In Bangladesh, the construction of dikes and dams on either side of the Halda has contributed to the proliferation of water pollution. A sizeable contingent of that pollution stems from the infirmed biotic community in key areas. The river itself, mind you, is exceedingly long (98 km) with a rather aggressive tributary known as the Dhurung River that merges with Purba Dhalai some 48 km downstream. Halda stretches from the Badnatali Hill Ranges in the Chittagong Hill Tracts down through Fatikchhari upazila, Bhujppur Thana, Hathazari upazila, followed by Raozan and then Chandgaon Thana. It finally spills into the Karnaphuli River, and all of these mouths, so to speak, indicate to anyone who knows rivers that the Halda River is a fairly massive body of water feeding an intricate network of other rivers.
Last year, the Impact Assessment on Upstream Water Withdrawal to Conserve Natural Breeding Habitat of Major Carps in the River Halda — a public study commissioned by multiple academic institutions — completed and published its analysis. It found several contributing factors to the eradication of the 26 fish species, and one was the residue flushed by tobacco farmers, factories, siltation and sand extractors. Bangladesh Agricultural University, Chittagong University and Bangladesh University of Engineering Technology all contributed researchers to the team responsible for the study, and the interdisciplinary team of experts researched the issue for a full year.
About 76 river species were counted in total based on a previous 2009 study according to Dr. Manjururl Kibria, a zoology professor at Chittagong University. He now says there are only 50 species remaining. Between 1976 and 2011, so many dams have been built that these constructs now present ecological upheaval within the food chain and on the riverbank. This has greatly diminished the amount of plankton and benthos present; both of which serve as critical food sources for fish.
Similar problems have prompted India to try all sorts of things to clean and protect certain rivers, but India's concerns are also a bit different. They're informed by the need for better water distribution in general as certain areas go dry while others don't. The Union government now wants to interlink rivers via the new Interlinking of Rivers program. The idea is that linkages will yield water distribution equity so that, at the very least, all regions will have the same amount of water available. They've already started working toward this goal, in fact, for the Betwa and Ken rivers.
Environmentalists in India say that this presents a whole new problem, as they've been arguing from the very beginning. They assert that linkages will change the natural flows of rivers and thereby impinge on the stability of ecosystems that rely on these rivers for a variety of important things on daily, seasonal and annual bases. Some such environmentalists published an article on it in Nature Ecology and Evolution. They argue that the natural flow already changes over time via naturally occurring patterns, and repeat cycles of drought and flood have caused this facet of river ecology to become increasingly dynamic after generations of unimpeded cycles. The man-made variation in river flows, according to the paper, is a direct threat to the general health of all fish species and, therefore, their diversity as well.
Canals, dikes, dams and other infrastructure have already made indelible changes to these ecosystems as it is, and that plight's only expected to get worse as a result of the Interlinking of Rivers program, not to mention climate change. Global warming has already been said to be intensifying the changes in both distribution of inhabiting species and the flows of rivers by making droughts more frequent in the first place. Plant species grow collectively in what are referred to as plant guilds and, over time, have biologically adapted to help make each other more productive. These guilds populate specific sub-ecosystems.
The new research used the plant guilds in the river ecosystem of the Western US for comparisons and contrasts. That river ecosystem feeds off melting snow every year, and climate change scientists have already noted the likelihood that this process in the Western US will change climate in the future. The new study suggests that those observations and the river ecosystem in the Western US actually augur poorly for the futures of dry-land regions worldwide. They used network theory and population modeling to analyze the interaction between guilds and gain a bigger picture. They found that one of the best markers of sound ecological health is the quality of the interconnections between plant guilds.
The problem is that river flow alterations cause external stress on these plant guilds. Basically, guilds were more stably connected at points where river flow was most natural and less stably connected in areas where flows were more affected by human intervention. Better connections were found to yield stronger resistance to that external stress.