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 onethe 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."
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.
In the early 19th century, allis shad made up a third of fish catches in the Severn (Britain's longest river). Fetching more at market than Atlantic salmon, many thousands could be caught in a night.
Aprahamian said: "Their decline began in the 1850s when weirs were constructed to aid navigation. This stopped them getting any further up the river than the city of Worcester."
The two-year study found this pattern repeated in other rivers where shad were once found. Because they have become so elusive, hundreds of "WANTED!" posters were issued, asking the public for "information concerning the whereabouts of this rare fish." Commercial fishermen were also recruited by scientists in an effort to track them down.
Douglas Herdson, from the National Marine Aquarium in Plymouth, who helped to coordinate the search, said: "A lot of data came from salmon netsmen fishing in the estuaries because shad migrate upstream at the same time."
The allis and twaite shad are now considered scarce across their entire range. EA biodiversity specialist Paul Smith said: "There has been a considerable decline of these species across Europe, to the point where they are now listed under both the Bern Convention and the European Commission Habitats Directive."
"It's a similar situation with other shad species in Asia and North America," added Aprahamian.
In the U.S. and France, where shad migrate up to 500 kilometers (310 miles) inland, special lift facilities have been laid on for them at hydro-power dams.
"The fish are attracted into the lift, the door closes and they're raised above the dam before being dumped onto a chute and allowed to continue on their journey," said Aprahamian.
Trucks and barges have also been used to ferry the fish upstream, while restocking programs are proving highly successful in the U.S.
Scientists now want to see similar conservation efforts made in Britain. And for his part, Aprahamian dreams of the day when shoals of silver shad again flood the Severn and Thames.
He said: "First, we have to make sure they have access to their historic spawning grounds. Finance is needed to provide the facilities for them to bypass obstructions. We are also looking at reintroductions using artificial rearing which is being pioneered at one of our fish farms."
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