Mahseer diversity - a hidden crisis
By Steve Lockett - MT Officer (Education and Outreach)
Just as I was putting the finishing touches to my presentation for IMC2, an email dropped with some startling information. Following a 40-year study of the entire fish assemblage of Gandaki River in Nepal, a tributary of the Ganges, it could be shown that what is commonly called Tor tor, the giant red-finned fish of the Himalayan region, had suffered a catastrophic population collapse. Other species had also disappeared, but Tor tor had been the most populous large cyprinid in the river. No longer.
For mahseer lovers, it was not all bad news, as the data confirmed that Tor putitora had replaced the other three mahseer species and taken the place as the most populous large cyprinid and, indeed, the only remaining species of mahseer. The question left was: what had happened to the other three mahseer species? (In the surveys, Naziritor chelynoides was reported as Tor chelynoides. Given the taxonomic revision of this species into this new genus of ‘lesser mahseers’, I completed the relative abundance graph using names currently valid).
There are two worrying implications: one is that stockings of Tor putitora across many Ganges tributaries by multiple agencies in India have overwhelmed the ‘normal’ relative population balance in a neighbouring country; the second is that widespread dam building over the same period has either, blocked migration routes for some species, while others have adapted, or, more likely, the corresponding water impoundments have added to climate change pressures in increasing water temperatures beyond those that some species can tolerate, or both. It seems highly unlikely that destructive fishing methods, one of the most regularly sited reasons for mahseer population decline, was responsible for a selective loss of large fish species.
I suspect the answer lies somewhere in a combination of all of the above. As ever, until we can fund long-term, wide-ranging studies of the impacts of various human interventions upon native biodiversity and freshwater habitats, then we continue to battle in the dark. All too often, agencies are attempting things that make matters worse, very often without putting into place the required monitoring protocols that ensure we understand the ‘hows and whys’ of our actions.
These actions, whether building a dam, removing sand and gravel, or stocking fish, have huge ramifications for river health, both in the immediate neighbourhood of the action, and - usually ignored by those planning river interventions - throughout the entire river basin, and on into the sea.
Every stage of an ecosystem process has to function for the whole river to be healthy. If we stop the water, we also stop sediment transfer and the movement of many organisms, both upstream and downstream. If we stock fish, either native, or non-native, we create population pressures throughout the entire food web. Those pressures can include over-predation, as well as removing pest controls, and forcing changes to river habitat.
Luckily, I have been asked to bring forward proposals for better protection of native biodiversity, for consideration by the Convention on Biological Diversity, whose next full session is in Yunnan, China, in October 2020. This area of China is home to mahseer, so I hope that my proposal, that all mahseer range countries have to take steps to control unregulated fish stocking, both for their own and for their neighbour’s biosecurity, will be taken seriously.
For further reading on this subject, follow this link to my recent article in India’s conservation bi-weekly magazine, Down To Earth.