Removal of sewage and other suspended solids has made the water clearer and less rich in organic matter, thus making small fish more vulnerable to predators while having less to feed on. The result has been the disappearance of much aquatic life, especially the spectacular shoals of roach for which the Trent was renowned. In its heyday match fishermen made catches of up to 100lb at a time.
Competitive match fishing has been vital for the angling clubs that line the river, providing the income they need to pay their rent. But now it is virtually unviable and anglers are deserting it.
At one English national championship match last year, more than 700 of the 984 fishermen lining 20 miles of river were unable to catch 1kg of fish (2.2lb) each during the day.
The debacle prompted the Angling Times headline "Saturday July 18; the date the Trent finally died?" and led to the scrapping of the national championship match on the Trent in 1999 - the first time there will have been no championship fishing match on the river for 17 years.
The problem for the coarse fishing on the Trent is as serious in its own way as the problems of the great salmon rivers such as the Severn and the Wye, which have received much more publicity.
The National Federation of Anglers is launching the biggest survey of coarse fishermen to be done in Britain to find out if, as is suspected, the Trent's difficulties are also being encountered in other rivers, such as the Witham in Lincolnshire.
The theory of cleanliness leading to fish disappearance, unlikely as it may seem, is not only suspected by anglers, but confirmed by the Environment Agency. "It is the case that the clean-up has made the river poorer in fish in some ways," said Martin Stark, the agency's Midlands regional fisheries manager.
Like many British rivers, the Trent has been getting steadily cleaner for the past decade, not least because of a pounds 20m improvement scheme by the Severn Trent water company at its sewage works at Stoke Bardolph below Nottingham.
And there is no denying that the fish are fewer.
"We've seen a decline in the smaller fish over the last five years or so," said Tim Aplin, who runs a tackle shop in West Bridgford, Nottingham's cross-river suburb. There were big roach shoals, and they were the attraction for the match fishermen, who would travel to the middle Trent from all over the country. But now they are going.
"People don't go down to the river now, even locals, because they might not catch anything. They go to commercial fisheries instead.
"The roach do spawn, and you do get fry, but there isn't a lot of natural food for them to eat now. And there is a lot of predation, especially by cormorants. The water is a lot clearer and that makes it easier for the cormorants to get them.
"But what do we do about it?" asked Mr Aplin. "We don't want to make the river dirty again, do we? It's a bit of a Catch-22 situation."Reuse content