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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