Over the last 10 days, thousands of migratory birds have been found dead at Sambhar Lake, about 80 km southwest of Jaipur city. Officials have buried over 18,000 carcasses so far.

While there is no clarity yet on what has caused the deaths, investigations so far suggest avian botulism, a paralytic and frequently fatal disease caused by the ingestion of toxins.

Which birds have been found dead?

Sambhar Lake is India’s largest inland saltwater lake at 230 sq km, spread mostly across Jaipur and Nagaur districts and also a part of Ajmer. It has a catchment area of 5,700 square km, with the water depth fluctuating between 60 cm in the dry season to about 3 metres at the end of the monsoon.

Every year, the lake attracts thousands of migratory birds. A total 83 species of water birds have been recorded at the lake, the most abundant of which are little grebe, great crested grebe, great white pelican, little cormorant, black stork, and darter, apart from various species of plovers, egrets, herons, and geese.

Birds of about 25-30 species have now been found dead, including northern shoveller, Brahminy duck, pied avocet, Kentish plover and tufted duck.

Image shows the numbers of birds disposed in Rajasthan

How much is known so far about the cause of death?

The evidence points to avian botulism, but this has not been officially confirmed. The clinical signs exhibited by affected birds included dullness, depression, anorexia, flaccid paralysis in legs and wings, and neck touching the ground. The birds were unable to walk, swim, or take flight. There was no rise of body temperature, no nasal discharge, no respiratory distress or any other sign.

Is there a concern for human health?

Humans are primarily at risk from avian botulism only if they eat infected fish or birds.

What could be other possible reasons for the bird deaths at Sambhar Lake?

After a Division Bench of the Rajasthan High Court led by Chief Justice Indrajit Mahanty took cognisance of the deaths, the Rajasthan government listed likely reasons:

* Viral infection;

* Toxicity, as a new area has been filled up after almost 20 years, and there could be higher concentration of salts along the edges;

* Bacteriological infection; and

* Higher temperature and high-water levels due to a good monsoon. This might have led to an increase in competition for resources. The weaker individuals, exhausted from the long journey, perhaps were unable to compete, and may have succumbed to stress emanating from the shortage of food, susceptibility to disease/pollutants/toxins and other habitat-related factors in the wintering grounds, the government suggested. If that is the reason, the government said it is expected that with fall of temperature and lowering of water levels, incidence of such mortality will go down.

What are the reasons that make salt concentration a concern?

In a 2016 directive, the National Green Tribunal had noted the impact of the salt industry — including “unauthorised salt pans” — on the ecosystem of Sambhar Lake and asked the state government to cancel allotment of salt pans. Part of the lake has been leased to Sambhar Salts, a joint venture of Hindustan Salts Limited and the state government. Sambhar Salts produces 196,000 tonnes of clean salt every year, which is around 9 per cent of India’s salt production.

The lake was recognised as a wetland of international importance when it was designated as a UNESCO Ramsar Site in 1990. Today, as per NGO Wetlands International, it has the worst possible Wetland Health Score at E.

Source: The Indian Express

Relevant for GS Prelims & Mains Paper III; Environment