Plenty of those bent backs coming out of the Goyt (nicknamed boomerangs).
The Mersey, starting in the centre of Stockport is fed from both the rivers Goyt and Tame, it initially receives water from both and otherwise remains un-stocked by the EA of Barbel, interestingly it contains a few escapees that have inadvertently moved over various weirs from the Goyt since it was originally stocked in the mid 90's and become a self sustaining population helped and supplemented genetically from the population found in the river Tame, both the Goyt and the Tame were illegally stocked with Barbel from the river Severn, prior to getting full EA backing and being legally stocked of Barbel.
While its not totally impossible for these fish to make there way back into the Goyt (as the last two weirs there are often drowned out by late winter/early spring rains) they are still able to navigate upto the last weir on the Tame to spawn, I would assume spending much of their time moving from the Tame into the Mersey and vice versa, more interesting still is the lack of bent backs reportedly coming from either the Tame or the Mersey (I don't know of any), all of which contained in an area of only a few miles in all three directions..
It was explained to me by one of the bailiffs on the Goyt a while back that it was some sort of virus infection that would show during the juvenile stage of the young Barbels life but was unfortunately carried into adulthood, not really effecting it after the damage had already been done so living with the deformity and being otherwise healthy, I never asked at the time whether this was suffered prior to being released or was a result of something happening in the farming of the fish.
Afterthought.....
I had wondered for a while if this might be caused by the beaks of predatory birds on juvenile fish, I have caught Chub/Grayling and other silvers that have the scars and beak point still to show from escaping these birds but none have got deformed spines, so I could at least count that out as one possibility!