Behind the pannier market in Tavistock, one can take a break from shopping and peer over the riverside wall to view the River Tavy. The rain-fed flow that rises on Dartmoor is as unpredictable as the weather and prone to seasonal mood swings. After heavy downpours the river will race by in a rage. But this dry summer has left it lean and listless today, in little hurry to reach the sea, dawdling through rocky gullies and unwinding in languid glides.

Low water and bright sunlight mean that trout are clearly visible in a shallow pool. Beneath them I notice what at first resembles a half-metre section of discarded cable – until it moves, snaking over the stony riverbed in front of me: an eel.

Just an eel, some might shrug. And at one time they were so plentiful they were taken for granted. No longer. Overfishing, disease and dams that impede migration have caused European eel numbers to plummet over recent decades, to such an extent that the species is now classified as “critically endangered” – a higher level of extinction threat than that of the tiger and giant panda.

The dark brown individual I am watching twists and probes snout-first into nooks and crannies in its sinuous search for food. As fluid as the river it inhabits, it appears and disappears amid the eddies, like a flexing length of muddy water: part solid, part liquid.

There is something prehistoric and unfathomable about these long-lived bottom dwellers. Slippery with secrets, much of their lives remains shrouded in mystery. While Atlantic salmon famously battle upstream to breed, mature eels head in the opposite direction, meandering downriver and navigating the Atlantic to spawn somewhere in the Sargasso Sea. The Gulf Stream currents transport the drifting offspring to our shores where they seek out fresh water, growing over years in rivers, lakes and canals.

For now, this Tavy eel looks content to bide its time. But some day, a wanderlust will stir – an urge to return to the distant place of its birth to breed. From a small pool behind a town launderette, the vast ocean beckons for one of nature’s great travellers.