Almost certainly a chub I would guess, barbs being very thin on the ground in the Mersey. I would be careful of saying that its death was due to the outflow though. As you know Mark, you and I were watching a trout rise yesterday, and that survived the more intense mess that came out of the outflow a few days ago. And there were some other fish present. I don't expect that there have been any fish casualties due to the outflow.
Remember also that this was the first weekend of the season, there were anglers on the Gosjacs platforms and they may well have injured the odd fish.
However retrieval of the fish could still be useful, for if analysed, it could give an indication of recent water quality, provided it was resident in the affected stretch, and did not float downstream from the Goyt or Tame, or simply higher up the Mersey. i suspect you would need to see a few dead fish before the outflow could be blamed.

The manhole covers are on the other bank of course, higher up, and I doubt that they are in any way linked to the sewage farm. Nice to know what they ARE of course. I suspect simple surface drainage fro the industrial estate.