A £20 million project was announced in 2016 to put that right and work is now about to start.
The fish could soon return to the the River Severn north of Worcester as part of work to help the species navigate weirs.
This week, results of a research project, which used some of the latest technology to examine the lifecycle of the fish, were announced.
Monitoring work during the spring and summer found that around 15,000 shad can make it above Upper Lode weir, near Tewkesbury, before being halted by Diglis Weir in Worcester.
These monitoring results are significant because they indicate the current levels of twaite shad in the river which once supported millions of this species.
The research was conducted by the Severn Rivers Trust, Environment Agency and Canal & River Trust as part of the multi-million pound Unlocking the Severn project, which is supported by the Heritage Lottery Fund and EU Life.
The information gathered will be vital for the project which aims to restore the shad’s access to 155 miles of the River Severn, north of Worcester, by providing fish passage solutions at a series of weirs that currently the fish cannot swim over or around.
Environment Agency fisheries monitoring specialist, Charles Crundwell said: "We had no idea how many shad we’d find – we thought a few thousand, but in fact results suggest we could have as many as 15,000 in the lower reaches of the river. This shows great promise that by unlocking the river there’s scope for a really thriving population.
"Plus the work to help the shad will open up the river for all fish species, so helping the shad will help everything else for the benefit of everyone – wildlife, residents, tourists and anglers.”
In order to learn more about the remaining small population of shad, particularly the conditions they need to prosper, volunteers and staff from the Severn Rivers Trust, the Environment Agency and Canal & River Trust spent many hours watching and counting twaite shad swimming over Upper Lode weir, during April, May and June.
In addition, a suite of remote monitoring techniques enabled monitoring all day, every day.
This included cameras, counter plates triggered when a shad passed upstream and even the use of an acoustic beam giving an image similar to the ultrasound you get of a baby in the womb.
Acoustic tracking tags fixed to 25 shad – another first for the UK, under licence from the Home Office – showed how they migrate up the river, what habitats they use, and how barriers delay them.
An suite of underwater camera equipment was installed at the spawning sites to understand this behaviour.
In addition to the twaite shad, the monitoring also recorded the rarer allis shad.
Mr Crundwell added: "Historically the allis shad were even more prized as a food fish and would certainly have been an important component of the catch prior to the navigation weirs being built.
"This is the first photographic proof that a tiny run of these fish still hold on in the Severn, which is really exciting and means that the natural restoration of this species is also likely to occur if we are able to provide fish passage solutions at the weirs further up the river.”
The salmon and eel are also set to benefit from the project, which started last year and is expected to take five years to complete.
The scheme is the largest of its kind ever attempted in Europe and was developed jointly by the Severn Rivers Trust, the Canal and River Trust, the Environment Agency and Natural England.
Funding includes £10.8m from the Heritage Lottery Fund and £6m from the European Commission.