We all take the fish in our rivers for granted! "They have always been there, haven't they?"
Well, actually they haven’t. A little over 8000 years ago most of the UK was still covered by a thick sheet of ice. As the ice sheet started to thaw, large river systems formed and our newly created Yorkshire rivers flowed down through the Humber and into Doggerland (Now underwater). Doggerland was a huge expanse of land, similar in size to the modern day UK. It was an incredibly diverse and rich ecosystem where red deer and many other large herbivores and their predators thrived. Humans also lived in Doggerland and it acted as a land bridge to Continental Europe. There is good evidence to show that the rivers that flowed across Doggerland were rich in fish and these fish would have moved upstream into the UK as the ice age came to an end; just as humans did!
Some salmon and sea trout wander and it is likely that sea going Arctic char would also have been present; along with many other coarse fish species, whitefish and species like shad. The coarse fish, brown trout, grayling, migratory trout, salmon and shad are still here but the migratory char (anadromous char) have now vanished and char are only present now in the deepest and coldest UK Lakes and Lochs. Species like the Burbot died out as late as the 20th century with records of burbot being caught in angling matches on the River Ouse during the 1950's and 60's. It may be possible to re-introduce burbot at some point but coastal and riverine water temperatures are now too high to support riverine or migratory char.
The River Ouse System would have once been the top of an enormous river delta that discharged into the Norwegian Trench. It is still thought that our young salmon cross the North Sea and travel up the coast of Norway to reach their feeding grounds in the North Atlantic; as they would have done thousands of years ago.
Doggerland was slowly inundated as sea levels rose and finally disappeared about 6000 years ago after a Tsunami caused by an undersea landslip off the coast off Norway. It is likely that during the gradual inundation of Doggerland, the Humber may have flowed into the River Rhine for a time. The Thames certainly did and the resulting enormous river, discharged through, what is now the English Channel and into the Atlantic. This certainly fits in with the species of fish that were recorded as present in the Yorkshire Ouse from Roman times and earlier. Iconic Rhine species like barbel, that were originally absent (until man intervened), from rivers north of the Humber.
Another species that used to frequent the system was the European sturgeon (Acipenser sturio). A fish was recorded from the Tidal Ouse in 1900 and was reputed to have measured over eight feet in length. They seem to have been relatively common in the Ouse/Humber System throughout the medieval period.