Mayflies dying out on legendary trout streams

The flies which anglers use as models for lures to catch trout are disappearing from some of England's most beautiful rivers, a new survey strongly suggests.

The flies which anglers use as models for lures to catch trout are disappearing from some of England's most beautiful rivers, a new survey strongly suggests.

Millions of life-like imitations of mayflies and other species of "upwing" flies – attractive aquatic insects similar to small butterflies on which trout feed voraciously – are produced by skilled fly-tiers annually. Made with materials such as feathers, hair and silk, anglers use the copies of the flies to deceive their quarry. However, the real specimens appear to have become substantially scarcer.

Fly numbers may have gone down by as much as two-thirds since the Second World War, the survey indicates, with the steepest falls over the last 20 years. The change, so difficult to detect that it has passed unnoticed except by observant anglers on the riverbank, is a serious signal that all is not well with our rivers. This is despite rivers and streams probably being cleaner than they have been since before the industrial revolution – a fact trumpeted loudly by the Government only a month ago.

Several causes may be involved but one serious candidate is intensive agriculture, known to be responsible for massive declines in bird, flower and insect life on farmland in recent decades.

For, while industrial and sewage discharges into watercourses may have been curbed, agricultural run-off of fertilisers, pesticides and other chemicals has been steadily rising. This run-off has been increasingly damaging to the ecosystems of rivers, not least because the continuing abstraction of water by water companies means pollution is becoming more concentrated.

The survey, carried out for the Environment Agency and the Wiltshire Fishery Association and due to be published shortly, is based on the observations and records of anglers over many years. Some cases go back to before the Second World War.

The data comes from 365 experienced fishermen on the chalk streams of southern England. These classic trout rivers have long been noted for their very pure water (filtered by the surrounding chalk), their physical beauty and their big fish. The latter have always grown fat on prolific numbers of flies.

So prolific, in fact, have the simultaneous hatches of fly been in the past that, emerging from the depths in which they have spent their lives up to then as unwinged larvae, or nymphs, the winged insects have sometimes appeared to cover large expanses of water.

In particular, the mass hatching of the adult mayfly – the largest and most beautiful of the 20 or so upwing species – is not only a remarkable natural spectacle, but can drive the fish into a feeding frenzy.

The main period of their emergence, from late May to early June, is known to aficionados of fly fishing as "duffer's fortnight" because this is the time when trout can supposedly be caught by even the most incompetent of anglers.

However, this picture of mass hatches is changing dramatically, the survey indicates. In many places, how anglers gauge the size of a hatch has changed from counting the flies per square metre of river surface to counting the square metres that have any flies in them at all.

The study, organised by Peter Hayes, a passionate fly fisherman and market researcher, and Allan Frake of the Environment Agency's south-west Wessex region, is wide-ranging. It is based on a questionnaire sent to hundreds of anglers, riverkeepers and fishery owners on 32 chalk streams in Hampshire, Wiltshire and Dorset. They included the most famous trout rivers such as the Test, the Itchen and the Hampshire Avon.

Anglers were asked to gauge the abundance of fly life on their rivers over recent years and recent decades, according to one of six levels. These ranged from "good hatches frequently" to "absent". Each level was ascribed an "abundance score".

The anglers who responded spent an average of 17 days each on the riverbank each year. When professionally analysed, their figures showed a picture of general fly abundance in free-fall: it had plummeted to 34 compared to a maximum score of 100 in the decade before the war.

Individual fly species showed steep declines in the scale of abundance: the mayfly down to 43.3 from before the war; the large dark olive to 25; the iron blue to 20.4; the blue-winged olive on some rivers to below 20; the grannom or caddis fly to below 20 and to a complete absence on some trout streams.

"Although memories may be biased and selective, contemporaneous, written recording of hatches has often been extremely detailed," the survey report notes. "In addition, the coverage both of rivers and reaches, and of days and seasons, afforded by gathering data from those who spend far more time on the river than scientists can, is many thousands of times greater."

The report's authors believe that its conclusions are ominous. "We did the survey because we had a growing but definite impression that fly life had seriously declined, but there was no evidence," Peter Hayes said. "Now there is. I believe that what has happened is part of the same process which has devastated farmland birds and butterflies."

"It's a bit like an aquatic version of the miner's canary." said Allan Frake. "The survey results are telling us something, and they are telling us that all is not well with our rivers."

* Report on the Millennium Chalk Streams Fly Trends Study (unpublished).