Dams Spawn a Crisis for U.K. Shad, Study Says

James Owen in England
for National Geographic News
September 8, 2003

Scientists say a herring with a salmon's instinct to run up rivers is vanishing from British waters. The warning follows a two-year investigation into the country's shrinking shad shoals.

The study suggests shad are being denied access to traditional breeding grounds because of man-made obstacles blocking their way. Its findings mirror concerns over the shad's plight worldwide.

The allis (Alosa alosa) and twaite shad (Alosa fallax) now breed in just a handful of U.K. rivers. Besides other human impacts such as pollution, government scientists identified weirs and dams as the main cause of the shad's decline.

Miran Aprahamian, principal fisheries scientist at the U.K. Environment Agency (EA), said: "The barriers stop migrants moving upstream and various species of shad can end up using communal spawning areas which results in hybridization. Shad congregating below the barriers are easily fished and this can lead to their exploitation. Poor water quality and river engineering works also impact on shad numbers."

The joint investigation by the EA and English Nature, both government agencies, found that twaite shad now breed in just four rivers, while the allis shad is confined to one—the Tamar, in southwest England.

Both members of the herring family, the fish have forked tails, large eyes with fleshy eyelids, and big black spots down each flank. They are found from Iceland to northern Africa and return to freshwater to spawn in spring and early summer. Unlike salmon, however, they lack the power to jump weirs and waterfalls.

Aprahamian says existing fish passes, devices that allow migratory fish to bypass barriers via a succession of small pools, were built with salmon in mind and are unsuitable for shad.

He added: "Salmon are strong swimmers and have a great leaping ability. But the poor old shad just aren't up to it. They don't like turbulent water, either, so they get battered around quite a bit."

Food Fish

Before the days when Britain's rivers were littered with such obstacles, shad were part of the national diet.

An important commercial fish on the Thames, huge shoals once swam through the heart of London. In 1697 a special local law was introduced to regulate all those who used "the Art, Mistery or Craft of a fisherman to take Shadds in Shadding-time." Netsmen without a proper license faced hefty fines.

During the main spawning season, when females shed their eggs while thrashing the surface with their tails, they congregated in such numbers that on a calm evening it's said they could be heard for miles. Today, shad are extinct in the Thames.

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