Phil, another canal that has a self-sustaining chub population (though not one I'm that well acquainted with) is the Kennet and Avon canal.
I don't know if the canals I've mentioned meet the criteria you say must be met for successful chub spawnings but they are self-sustaining.
I imagine the lock overflow channels and inlet streams provide a fairly suitable, although limited habitat.
Certainly in the case of the Oxford canal, there isn't much of a predation burden, that obviously helps their cause.
"why they “struggle to reproduce”
in most cases"
I like the inclusion of "in most cases" - did you say that in your original post re chub spawnings in stillwaters and canals??
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Ade, I think post #97 goes some way to summing up TBO:
http://www.fishingmagic.com/forums/...ishing-generally-declined-10.html#post1325462