The simplist answer, to save further conjecture about river mouth/estuarinal fishing, is to write to the Clerk of the appropriate District Salmon Fishery Board and ask the question........ :z17 They should be able to provide you with a written answer explaining what the limits etc are.
Relevant legislation like the Salmon and Freshwater Fisheries (Consolidation) (Scotland) Act 2003 and the Estuary Limits (1986 Act - Fixing of estuary limits) have a bearing on the options for estuary fishing and in light of the increasing number of us who like to have a cast in the salt (every now and again) and ask questions about where one can fish legally for fish other than salmonds in estuaries and by what or which methods.
To recap though - for a definative answer ask the competent body and that would be, in most cases, the District Salmon Fishery Board.
The difficult thing is proving you're fishing for something other than salmonids when you're fly fishing for Bass etc. That's why it's difficult getting a straight answer out of anyone about where you can and cannot fish. Sadly the view most riparian owners will take is likely to be negative to such an approach, more due to the lack of demand for such fishing than through any intolerance.
******* I have edited my post in light of a previous post being removed by its author *******