Environment

How Irrigation and Fishing Pushed Dolphins in Nepal Into an Ecological Trap

Nepal can ill-afford further decline in what is an already small dolphin population, and only about 50 individuals of a river species survive in all of Nepal’s rivers.

An illustration of the Ganges river dolphin from 1894. Credit: pd-old/Wikimedia Commons

An illustration of the Ganges river dolphin from 1894. Credit: pd-old/Wikimedia Commons

Dolphins are among the most easily recognisable animals in the world. They can be found in freshwater habitats as well as oceanic waters. They are often mistaken to be kind of a fish but are in fact mammals, giving birth to calves underwater.

One such species, the Ganges river dolphins (Platanista gangetica gangetica), are found in the Saptakoshi, Narayani and Karnali rivers in Nepal. In India, these dolphins are found in the Ganga and the Brahmaputra; in Bangladesh, they occur in the Karnaphuli-Sangu river system.

Most of these rivers are prone to natural seasonal flooding. In a recently published paper, Gopal Khanal, a young researcher with the River Dolphin Trust, along with his colleagues from the Department of National Park and Wildlife Conservation of Government of Nepal, showed how flood-induced changes and subsequent increase in irrigation demands may escalate the risk for the dolphins of Karnali river. Speaking to The Wire, Khanal said, “The main finding of the study is that even in unregulated rivers, if you extract more water from a river, the depth reduces – increasing the chances of dolphins getting entangled in fishing nets.”

The Karnali is one of the few undammed rivers in Nepal. It originates in Tibet and flows through the Himalayas in Nepal before joining the Ghaghra in India. After it comes down the craggy mountain slopes, it passes through Chisapani. A few kilometers downstream from Chisapani, the river bifurcates into two channels: the Geruwa and the Karnali, spreading across the plains. Chisapani also forms the upstream limit of the dolphins’ range.

Where the Geruwa passes through the protected area of Bardia National Park, fishing by local fishermen is restricted. Until 2010, the researchers note, a small population of about 11 dolphins were known to inhabit the Geruwa channel. However, fishing is permitted in the Karnali channel that lies outside the park’s remit. In 2010, after a major natural flood in the Karnali, the active flow shifted from the Geruwa to the Karnali channel. As a result, the water level in the Geruwa channel dropped from about 3 m to 1.5 m. The water level in the Karnali channel increased from around 1.5 m to 2.5 m.

Dolphins are evolutionarily clued to gauge safe depths of rivers for them to occupy. Despite greater fishing pressures in the Karnali channel, the reduction in water levels of the Geruwa channel was a driving force for the dolphins to move to the Karnali, which now had a deeper river habitat. Such a situation, like the one the dolphins now found themselves in, is called an ecological trap – where animals, due to their evolutionary adaptations, find themselves in risky situations despite following cues on which they regularly depend for their survival.

In 2012, the Rani Jamara Kulariya Irrigation Project was operationalised at Chisapani. The interfluve area between the two channels of Karnali and Geruwa is a highly populated region with around 210 persons per sq. km. The majority of the population practices agriculture and for which the project diverts water into the area. This diversion has increased substantially since 2012, leading to a decline in the depth of the Karnali channel, from 2.5 to 2 m, and it continues to decline further.

“In Nepal, the major problem is the diversion of water for irrigation purposes due to which, slowly, the water depth is reducing,” says Khanal of the nationwide situation. The research team monitored the impact of this decline in river depth on the dolphin population.

They found out trends in annual rainfall and river discharge volumes. Enumerating this data, the researchers attributed the decline in the river’s depth directly to the rise in irrigation demands. They also observed significant depth reductions where the river earlier used to have deep pools of water, which make for productive fishing grounds for both fishermen and dolphins. They also act as connecting links for dolphins to move through. Now, however, the diversion of river water and excessive groundwater extraction for irrigation are endangering the connectivity between the pools.

Incidentally, the researchers didn’t record a single live dolphin between 2012 and 2015 in the Geruwa channel. Two dolphins have died – one in the Karnali in 2012 and another in Geruwa in 2013 – due to accidental entanglement in fishing nets. The researchers noted that these deaths corresponded with a sudden decrease in river depth due to diversion of water for irrigation. The overall dolphin numbers have reduced from 11 in 2012 to six in 2015. So although fishing pressures may adversely impact dolphins and other river species, plummeting river depths might prove to be the final nail in the coffin for those creatures dependent on rivers. Addressing one without arresting the other could prove to be knotty for the dolphins of Karnali.

Nepal can ill-afford further decline in what is an already small dolphin population. Only about 50 individuals of this charismatic river species survive in all of Nepal’s rivers. In light of this precarious existence, conserving small populations like the one found in Karnali is of vital importance.

The country has a beautiful system of community participation in managing and monitoring the use of natural resources. Such platforms can be leveraged to facilitate dialogues between fishing and agriculture communities for co-management of water resources. Alternatively, Khanal believes that restoring water levels to the 2-m-mark in the Geruwa – essential for dolphins to reoccupy the channel – should be considered. He and his team hope that their study will have policy-level implications for dolphin conservation.

If the mammals are to survive in their new habitat in the Karnali channel, fishing will have to be restricted even as river depths of at least 2.5 m will have to be maintained. “Our study suggest that multiple stakeholders can sit together and discuss amount of water allocation for agriculture while maintaining the ecological flow of the river for the fisherfolk as well as dolphins,” Khanal said.

On the Indian side, the National Waterways (NW) Act, passed in March 2016, proposes to ‘develop’ 111 rivers into national waterways for cargo transport. The longest of these, NW 1, will be on the Ganga. Preliminary surveys indicate impacts of river dredging could prove to be potentially serious for the dolphins. One can only hope that the writing isn’t already on the wall.

Vrushal Pendharkar is a writer at IndiaBioScience.

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