Academic Reading — Test 56

Reading passage

The Indian river dolphin, known across northern South Asia as the susu, is one of the few cetaceans on Earth to have adapted entirely to life in fresh water. It inhabits the murky channels of the Ganges, the Brahmaputra and their many tributaries, where it occupies a niche that no marine relative could fill. Unlike the familiar dolphins of the open sea, this animal almost never leaps clear of the surface; instead it moves slowly through the silt-laden current, surfacing only briefly to breathe with a soft exhalation from which its local name is thought to derive. For most observers, the species remains little more than a fleeting shadow in turbid water. The most striking feature of the dolphin's ecology is its near-total blindness. Generations of life in water so clouded with sediment that light barely penetrates have rendered its eyes almost useless; the animal lacks a crystalline lens altogether and can do no more than distinguish light from darkness. To compensate, it has come to depend overwhelmingly on echolocation. By emitting rapid pulses of high-frequency sound and interpreting the returning echoes, the dolphin builds a detailed acoustic map of its surroundings. This sense allows it to navigate cluttered channels, avoid obstacles and locate prey buried in mud, all without recourse to vision. A further peculiarity is its habit of swimming on one side, trailing a flipper along the riverbed, a posture that may help it detect food in shallow water. The susu is an opportunistic feeder. Its diet consists chiefly of fish and invertebrates, including catfish, carp, prawns and the occasional freshwater crustacean. Using its long, narrow snout, which is lined with sharp interlocking teeth, it seizes prey with a swift sideways snap. Feeding tends to intensify at dawn and dusk, although the animal hunts throughout the day and night. Because it sits at or near the top of the river food web, the dolphin is widely regarded by scientists as an indicator species: the health of its population is taken to reflect the broader condition of the river system it depends upon. Where dolphins thrive, the water is generally clean and the fish stocks plentiful; where they vanish, something has usually gone badly wrong. That something, in recent decades, has taken many forms. The construction of dams and barrages across the great rivers of the region has been particularly damaging. These structures fragment the dolphin's habitat, severing once-continuous populations into isolated groups that can no longer mix, and they alter the flow of water and sediment on which the whole ecosystem rests. Entanglement in fishing nets remains a constant threat, killing animals that surface to breathe but cannot free themselves. Pollution from agricultural run-off, untreated sewage and industrial discharge degrades the water still further, while the heavy traffic of motorised boats fills the river with noise that may interfere with the echolocation upon which the species so completely relies. Taken together, these pressures have driven the Indian river dolphin onto lists of endangered animals. Conservation efforts have nonetheless gathered pace. In 2009 the Indian government formally declared the Ganges river dolphin the country's national aquatic animal, a designation intended to raise public awareness and to direct resources towards its protection. Stretches of river have since been set aside as sanctuaries in which fishing and boat traffic are restricted, and surveys are now carried out at regular intervals to estimate how many dolphins remain. Researchers have also worked closely with riverside communities, encouraging fishers to adopt gear less likely to ensnare the animals and offering compensation for nets that are damaged in the process. Such measures recognise a simple truth: the people who share the river are essential partners in any attempt to save its most elusive inhabitant. Whether these initiatives will prove sufficient remains uncertain. The dolphin's slow rate of reproduction means that populations recover only gradually from decline, and the demands placed on the rivers by a growing human population show no sign of easing. Yet the very qualities that make the susu so vulnerable also make it valuable. As a creature exquisitely tuned to the rhythms of the river, it offers a living measure of the health of one of the world's most heavily used freshwater systems. To protect the dolphin, ultimately, is to protect the river itself, and the countless people whose lives are bound up with it.