The day​ after Brexit, in need of distraction, I joined nine other volunteers at a pub on the bank of the River Lea in East London to count eels. The European eel is critically endangered, and annual counts have taken place on London’s rivers since 2005. We were met by representatives of the Canal and River Trust and an ichthyologist from London Zoo named Joe. Across the river there was a weir and a few old industrial buildings – what the Canal and River Trust people called ‘heritage structures’ – by the side of which had been built an eel trap. This was a long pipe lined with what looked like the plastic turf you used to see in butchers’ windows, down which water flowed. It was an attempt to recreate a mossy crevasse by the side of a waterfall, Joe said: eels wriggle up the bristles in search of a way past the weir and then get trapped in the box at the top.

We made our awkward introductions. There was a couple from Stoke Newington, trailing a herd of whippets, who said that they enjoyed being outside and wanted to volunteer for something but not anything ‘too political’. There was an angler from Romford who spent his weekends fishing for specimen coarse fish on the Lea and had always been fascinated by eels. There was a man who’d just got back from a trip around Britain to collect the seeds of native trees for preservation in a doomsday vault in Svalbard in case of ‘apocalyptic extinction events’, as he called them.

John was wearing a plaid shirt and a poacher’s jacket; his moustache drooped and his teeth were furred with plaque. He said he was a hunter and forager. I asked him what this involved and he told me he collected nuts and berries on Walthamstow marshes and ate roadkill he found on the verges of the A12. To earn money he led corporate foraging sessions: teaching bankers how to dig up pignuts in parks, brew elderflower champagne and trap squirrels. I asked him what he had eaten for dinner the night before and he said he’d had a pot noodle: ‘They’re easy to catch.’ He must have used the gag before. I had volunteered for the project because I’d spent the last six months stuck in my house reading novels and I wanted to get out. I’d been interested in eels since reading Graham Swift’s Waterland, in which he describes the curious life-cycle of these fish and I was intrigued by what the river on my doorstep might contain.

As a boy I was an obsessive angler, exploring what were then the neglected waterways of north-east London with my fibreglass rod and a box of maggots. I fished the murky water of the Regent’s Canal and the dark ponds of Hampstead Heath. Back then you were as likely to hook an old boot or a nicked car stereo as you were a fish, but every now and then I would catch something alive: a small, brilliantly coloured perch, with its armoured cladding and crest of spines angrily raised; a common roach, with silvered flanks and red eyes. Once or twice, usually when dusk was falling and they were beginning to move out from their lairs under the bridges to feed, I caught eels. There was something unnerving about them, with their serpentine bodies, their deep-set eyes and the ruinous slime they’d ooze over your tackle as you unhooked them.

Back on the riverbank, Joe told us that all European eels are born in the Sargasso Sea, a placid oceanic gyre in the North Atlantic, off the coast of Mexico. There they hatch amid the trailing fronds of Sargassum weed before developing into leptocephali: transparent leaf-shaped fry that were once thought to be a distinct species of fish. As leptocephali the baby eels drift and feed in the current for a few months before heading east, riding the Gulf Stream to Europe.

On the journey their bodies change dramatically. When they get to the coastal areas of Europe they become glass eels – still transparent, but skinnier now. Glass eels are a delicacy in Spain and Japan, something that is largely responsible for their decline. As they move on through estuaries and begin to run up freshwater rivers in the spring they darken and become elvers. Over time their bellies turn yellow, then dark green. They live in freshwater for up to a decade before at some point returning – no one really knows why – to the Sargasso Sea to breed. If, in the intervening years, they get stuck in a canal or a lake, Joe told us, they can climb out of the water and slither overland until they find a river again. They’re sometimes found on dewy nights writhing in the middle of fields, trying to find a way to get home.

When they return to salt water, their bodies change again: they take on a silvery sheen, their eyes grow, and they lose their stomach and intestines and never eat another meal. Then they start swimming. The journey is long: around four thousand miles, and they complete it in six months. Arriving back at the Sargasso Sea they breed and finally die. Unlike salmon and migratory sea trout, who spend their middle years at sea before returning to the rivers they were born in to breed, eels have no memory of the rivers they return to year after year. Quite how they do it is still unclear. Is it a kind of embodied recall? A smell? Joe said he didn’t know, but suspected it was a simple process, a combination of tidal forces and the eels’ aversion to areas of high population density.

Until one hundred years ago the life-cycle of the eel was a mystery. Some in the 17th century thought, according to Izaak Walton, ‘that as Pearls are made of glutinous dewdrops, which are condensed by the Suns heat in those countries, so Eels are bred of a particular dew falling in the months of May or June on the banks of some particular Ponds or Rivers, apted by nature for that end; which in a few dayes are by the Suns heat, turned into Eels, and some of the Ancients have called the Eels that are thus bred, The Off-Spring of Jove.’ Before he became a neurologist, Sigmund Freud spent many months dissecting adult eels in the hopes of finding their sexual organs and thus proving once and for all that they bred by sexual reproduction, rather than being spontaneously created by the action of the sun on glutinous dewdrops. He never managed it, and the story of the European eel was only fully explained in 1922, when the Norwegian scientist Johannes Schmidt finally proved that leptocephali were indeed juvenile eels.

The eels we were monitoring had survived the first part of that journey, but the genus isn’t doing well. Since the 1980s, Joe said, there has been a 95 per cent decline in the number of eels returning to London’s rivers. John the forager said he thought it was due to cormorants, whose population has increased in the same period. Joe thought it was probably caused by climate change and overfishing at sea.

There were no eels in the trap that day, but a week later I returned with another eel monitor, a student of molecular biology at UCL. It had been raining the previous few days, and the water was high when we got to the weir. There had been 300 eels in the box two days before, and we were expecting a few more to have made it up the bristles since then. Once we’d checked that the water was still flowing along the pipe and pulled back the lid, we could see them: dozens of eels, dark ribbons swimming in frantic circles, their shovel-shaped heads madly weaving through the mass of bodies, knotting and unknotting themselves. Every now and then one would try to slither up the walls of the trap before dropping back down into the water. We decanted them into a plastic bucket before scooping them out one by one, putting each eel into a sandwich bag and measuring it against a ruler screwed to a plastic chopping board. We recorded their lengths, which ranged from 6 to 18 centimetres. There were 34 in all: a decent haul. When we’d finished we released them back into the river above the flow of the weir and watched them swim away into the weeds.

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