Hello Mark..
It's not a silly question..
I understand what you are suggessting as regards to spawning cycles and migration but I don't think this is the case for the Norfolk Broads.
I would be fairly certain that the annual up-river migration is down to avoiding the salt tides.
There are many factors involved including the ones you've mentioned.
The Norfolk Broads are a unique UK wetland system which is completely govened by tidal cycles, perhaps only comparable to certain areas of the Dutch coast..
Its a lot to do with the the Broads geographical position and it's surrounding, flat topography.
The mouth of the entire Broads system is at Great Yarmouth and the aspect there is a very open one.
Occassionally in a worst case scenario, a strong tidal cycle happens to coincide with a strong low pressure sytem that moves down The Channel and out into the North Sea, producing strong north easterly winds which serve to drive the already high water right up into the heart of the Norfolk Broads. Salt tides have been known to push anything up to 15 miles+ inland.
The man-made 'Broads', boat dykes and such things can act as fish traps.
There have been in the past, fish kills running into the tens of thousands.
I've wintnessed a few in the past, including an occassion last year where I had to race home to get a landing net in order to transfer some very distressed fish from Upton Boat Dyke into the surrounding un-connected marshland dykes.
So since the medieval ages when the peat diggings started the fish have learned to minimise the risk by moving many miles up-river..
The fish in the Broads also spawn in certain places many of which are quite a way down river. For instance if you go down to Rockland broad which would be affected by the worst of salt tides in April you can find water weeds thick with fish spawn..
I would say that salt infiltration is a regular occurance up to say the 8-10 mile houses, with only the worst cases of 15+ miles being occasional..
Dan T