I read that the sturgeon was common over 200 years ago in large UK rivers including the Severn, Avon, Ouse, some Scottish rivers and the Thames, with remnants of sturgeon found in the medieval remains of Westminster Abbey.
I think they came up our rivers to spawn, and then returned to sea.
I also recollect reading about Sturgeon being sold at Billingsgate Wharf in London back in times gone by and that they reached weights of around 250 to 300lbs in uk rivers.
When I get time I will search my book cases to try to find the books where I read this info from.
Keith
NB: I just found this info on the web about sturgeon in our rivers in the past which contains a map showing the locations on the Severn and Wye where they have been recorded in the past.
Sturgeon Acipenser sturio Page 2
It interesting to read that The Common or Baltic sturgeon, (Acipenser sturio) which inhabited the major UK river systems and coastal regions (as well as all other major European river systems) reached a length of 4.6 metres and a weight of 600 kg.
It makes even the largest Wels Catfish look like a tadpole LOL.
Keith