Hi John,
I fish the Irwell. Get in touch via mjduddy@yahoo.co.uk. or my blog www.manchesterfishingfiend.blogspot.com
There are no eels, or other migratory fish in the Irwell system (to the best of my knowledge) as they cant get past the hydro electric plant at Irlam Lock on the Ship Canal.
I have heard of sightings of salmon at the weir near Newbank Garden Centre, but when i was fishing the weirpool in october the only fish trying to get up over the weir were big brownies (possible case of mistaken identity by those who cant tell the difference between a big brownie in spawning colours and a salmon).
Your right about the fishing being very patchy, pollution and cormorants combine to make fishing very hit and miss.
However they have just completed work at Ringley Sewer works to reduce the amount of ammonia and phosphates that are released into the river. This should have the effect of reducing algae blooms in the summer thus increasing oxygen content. There are now plans afoot to do the same in Rochdale which should improve the roch aswell.
Despite this work which has cost in excess of 2.5mil there are hundreds of CSO's (combined sewage outflows) which deposit untreated **** everytime there is heavy rain. On the Irwell the majority of these CSO's are below Bury, which might explain why you rarely see canoeists below Burrs!
I rarely fish the river below Bury unless its been dry for a few days, as the amount of organics which flow by can at times be disgusting.
Mike