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.

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