U.K. Fly Fishers Left in Knots by Mayfly Collapse

James Owen
for National Geographic News
January 21, 2003
A dramatic decline in aquatic fly life on the chalk rivers of England
has trout fishermen deeply worried about the health of their rivers and
the future of their sport.

Mayflies thrive in clear, spring-fed streams and are a critical component of chalk river ecosystems, which are found only in parts of England, France, and New Zealand. A recent survey of chalk rivers in southern England by the Environment Agency (EA) of England and Wales suggests a massive reduction in fly abundance.

"River fly life is one of the basic building blocks of the aquatic food chain, critical to the success of fish and many species of birds," said Allan Frake, a biologist with the EA. "The reasons for this serious decline need to be fully diagnosed and addressed."

The study confirms anglers' concerns.

There are more than 40 species of Ephemeroptera, or "upwinged flies," in the British Isles. Iron blue and large dark olive populations have declined by 66 and 65 percent respectively since the 1970s. The blue-winged olive and pale watery populations have suffered similar falls.

An independent survey of blue-winged olives on the River Test, in Hampshire County, England, indicated numbers had more than halved between 1995 and 2000.

Evolution of Fly Fishing

Victorian anglers revolutionized trout fishing by creating replicas of mayflies—known as "dry flies"—made from fur, feather, and silk tied to tiny hooks and floating them on the water to lure the fish.

The technique demands a high level of skill. To "match the hatch" anglers need to be amateur entomologists, able to identify the species the fish are feeding on, and the different stages of the adult insect's life cycle.

The first two stages —egg and nymph—of the mayfly life cycle occur under water. When the mayflies emerge from the water as winged adults, they're called "duns." In their final stage as "spinners" they shed their skins, become sexually mature, mate, and die, sometimes within hours.

Trout rise to the surface to feed first on the duns, and then again when the female spinners return to lay their eggs.

Popularized by English anglers like Frederick Halford and William Lunn in the 1880s, dry fly fishing for trout soon spread across the globe.

Lunn devised more than 40 dry fly patterns, including the Lunn's Particular and Houghton Ruby, well-known imitations of the medium olive and iron blue. His grandson, himself a respected chalk river angler, says few fishermen use them today.

"There's little point because the natural mayflies just aren't about anymore," said Mick Lunn. "Instead of a Lunn's Particular they use nothing in particular."

These sentiments are echoed by Warren Gilchrist, an amateur entomologist who has fished the River Test since boyhood.

"It's changed the whole pattern of fishing," he said. "Now it's a case of bombarding half-starved trout with something big and hairy in the hope it will entice them off the bottom."

Gilchrist added: "The art of matching the hatch has gone right out the window. And so has the fun of riverbank discussions about what mayfly the trout are taking."

Identifying Causes

The cause of the mayfly collapse has not yet been identified, but there are plenty of possibilities. Reduced stream levels caused by diverting water for commercial and domestic use, agricultural runoff, climate change, and pollution from sewage treatment plants are prime suspects.

Cyril Bennett, a freshwater invertebrate biologist, took a disturbing photograph of a large dark olive male laying eggs that may provide a clue.

An EA study published last year suggests up to half the male fish in lowland English rivers are developing feminine characteristics such as female reproductive ducts and eggs in their testes. Scientists suggest that synthetic estrogen used in contraceptive pills may be winding up in rivers via sewage treatment plants. It's possible, they say, that the chemical is having the same effect on aquatic insects that it's having on fish.

A more visible problem is predation by mute swans on water crowfoot, an important aquatic weed.

"They [mute swans] have completely ruined long stretches of river," said Lunn. "Water crowfoot is pulled out and gnawed down to the root—and this is the weed in which most of the bugs live."

More research is needed to get to the bottom of declining fly hatches, said Bennett, but the government-funded Environment Agency lacks the resources to do the work itself.

"River monitoring isn't nearly frequent enough," he warned. "It should be carried out each spring and autumn but at the moment it's done once every three years at sites on the River Test. By the time the Environment Agency finally decides a species is under threat, the chances are it will have already gone."

Bennett is encouraging fishermen to take matters into their own hands. If anglers can provide evidence based on scientific data collection methods, not anecdotal observation, government scientists will be forced to act, he said.

To achieve this goal, Bennett and others are offering courses to teach anglers mayfly identification and monitoring techniques. Bennett is also developing a method of "farming" mayflies artificially which could help replenish depleted stocks.

In the meantime, the reputation of England's chalk rivers as some of the world's premier trout waters continues to suffer.

"We would rather get back to the classical methods of chalk river fishing," said Jim Glasspool, who represents riparian owners of the rivers Test and Itchen. "But if the trout aren't rising to blue-winged olives there's little point in casting an exact imitation over them."

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