People blame the shut down of the powerstations, but for every theory, there seems to be an exception to the rule. Winter matches were a lottery, and often limited to one lucky/persistent angler managing a couple of chub. Others pointed to the increasing clarity, sewage etc. The only thing I can identify is that the downturn in sport coincided (roughly) with the privatisation of the water industry.
Don't underestimate the impact this had on the river. The power stations made the river anything from 2 to 4C warmer than it should be.
Come the spring this helped the silver fish stocks spawn consistently every year due to the artificial temperatures. In turn keeping the stocks artificially high.
As the phased closure of the stations kicked in, which started in the mid 80s, the river temperature returned to what it should be by the time the last closure took place. Also remembers the Trent's true river temperature is on the cold side because of where its whole catchment rises. The high Pennines.
The return to the true temperature then left those silver fish spawnings to the vagaries of the weather, as rivers are when in a normal state of water temperature.
It's wrong to assume that riverine stocks spawn consistently and in the same quantities every year because they don't. Weather plays a big part in how successful each years spawnings will be. Cold springs do cause very poor spawnings and with some species no spawning at all.
The process of returning to a natural river stocking on the Trent took quite a few years to manifest itself and probably wasn't helped by the predation of the cormorant explosion that started in the late 80s.
Interestingly, Neville Fickling in one of his articles at the time predicted this would happen on the Trent. And for those that don't know NF isn't just a tackleshop owner and pike angler. He's a PhD in in Fishery Management and worked for the Trent Region of the NRA in the 80s.