I have heard that spring fish derive from slow growing parr/smolts that are hatched in higher streams with less food availability. Hence, the slower growth rate. Perhaps there could be some truth in this - i.e. slower growth in the river, more time spent at sea feeding ???? This would mean that returning fish would seek out the same spawning streams and in turn produce more spring fish. Perhaps somebody else can provide clarification on this point.
As for killing springers; only if we have a sustainable run. I disagree with blanket catch and release but surely if an animal is endangered we should not be killing it for the pot. However, on rivers where they still allow the nets to kill fish(North and South Esk), is it fair to impose a ban on killing fish with rod and line.
Unfortunately, some fisheries managers/riparian owners/DSFB's have had to impose bans on killing of fish because a minority of anglers cannot be trusted to reap only the minimum that they require.
Would you be happy killing one springer if you caught three or four or more in a week, or all spring?
I usually kill one salmon a year for a special occassion, and return all the rest. If I caught a springer, unless on a river where they are in abundance, I think I would be returning it and waiting until summer for a grilse for the pot.