Nature Studies: Marvel at the mayfly hatch – one of Britain’s natural wonders

Restoring mayflies to London’s own trout stream would be a big achievement


The mayfly is one of our loveliest insects. It is the largest of the upwing river flies, or Ephemeroptera, and with its pale translucent wings and long, slender, creamy-yellow body it is bigger than some of our butterflies. But Ephemera danica is notable not only for its beauty but also for its life cycle, spending two years in the water as a larva or nymph, then hatching in thousands, hundreds of thousands, sometimes millions, for a fleeting – indeed, ephemeral, hence the name – day or so, of mating, egg-laying and death.

The mayfly hatch is one of Britain’s few natural spectacles and I think it contributes to the glamour, if that’s not too strong a word, which attaches to the creature: I feel its presence graces any watercourse, and I am always elated to see it, usually in the fortnight in mid-May when the hatch takes place. Any river with mayflies on it is a special river for me, which is why I was fascinated and delighted to hear last week that attempts are being made to establish a mayfly population on the River Wandle – central London’s trout stream.

The story of the Wandle is remarkable in itself. Nine miles long, the river flows from the foot of the North Downs at Croydon to the Thames at Wandsworth, not far from Clapham Junction, through some of south London’s most industrialised areas, and for hundreds of years, because of its rapid flow, it was used as a power source: at one stage its banks were lined with 90 mills, producing goods from dyed cotton and leather to gunpowder and snuff.

Naturally, the pollution was enormous, and the river ended up stinking and dead. But in its pristine state, the Wandle is a chalk stream, the type of river which is the richest of all in wildlife, and over the last 15 years it has been cleaned up considerably, to the extent that brown trout (as well as other fish) are not only now flourishing there, but being caught by fly-fishermen.

The idea of fly fishing for trout in central London has captured myriad imaginations and has been told around the world; I wrote about it myself 10 years ago this month when I first walked the river and saw how it had been transformed. Trout. Yes. Great. But I never imagined that the Wandle might host a mayfly hatch.

Cyril Bennett has imagined that. Dr Bennett is Britain’s leading expert on river flies, and his speciality has been to breed mayflies and restore them to rivers where they have been lost, often through catastrophic pollution incidents. With the help of the local fishing club, the Wandle Piscators, he has just introduced several million mayfly eggs into the river, in Carshalton not far from its source, and now all concerned are crossing their fingers that in two years’ time there may be a hatch of the splendid adults, which will lay eggs of their own and so begin a breeding population.

“We are very keen to restore the biodiversity of the river and this is an iconic, pollution-sensitive insect,” said William Tall, vice-president of the Piscators.

Theo Pike, the author who is the chairman of the Wandle Trust, used the Wandle as an example of how some of Britain’s filthiest urban rivers are being reborn, in his intriguing book Trout in Dirty Places, published two years ago. “As urban rivers go, the Wandle is really good, and this is a really exciting project,” he said.

I agree wholeheartedly. Some people dream of restoring wolves to the Scottish Highlands, but I think restoring mayflies to London’s own trout stream would be just as big an achievement. And it has a much bigger chance of happening.


Invertebrate life is an index of the health of rivers

Dr Bennett, who runs the Centre for Riverfly Conservation, is very concerned that invertebrate life is tumbling on Britain’s rivers, not least because of agricultural pollution from phosphate fertilisers.

Last week I witnessed a small step taken in the right direction: the Salmon and Trout Association and Southampton University have placed monitoring equipment on the River Itchen in Hampshire, after watercress firms using the river were given limits on the amount of phosphate pollution they could emit, for the first time ever.

Visiting the monitoring site, I saw not a single fish in half and hour, and not a single aquatic insect either. Just chance, perhaps? Maybe. But it didn’t exactly fill me with confidence.

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