Its an interesting observation Colin.
in the last 2 weeks i reckon i've seen more fly life on the river than i've seen before, but it does seem to be localised to certain stretches of water.
Ie. over a 400yds stretch, the upper faster 100yds there are a few flies and some rising fish, over the middle 200yds there are a lot of flies and quite a few rising fish and the lower 100yds there are a few flies and almost no rising fish.
I have found you need to sit and watch for at least 15-30mins to spot any feeders and working out the flies can take over an hour.
Mid april we went through a lull in fly life and there wasn't much to be seen on any beats, but this has much improved in the last couple of weeks.
I know Inverurie quite well and do know that there are about 4 small stretches that will fish really well in April and May, because they will see the bulk of the hatching flies at this time of the year, this changes through the year as the hatches change.
Different flies inhabit different types of water, the Weed cover, the substrate or bottom type, the current all affect which insects live in which areas. This is one of the reasons i have always thought that despite the fact you don't need to know fly species in order to catch fish, it does help you to understand what is likely to be hatching in the water you are fishing and when. This is all part of the learning curve and goes hand in hand with reading the water to identify likely spots to watch in the first place (it is frustrating to spend hours watching random bits of water, much better to identify where you think the fish might be and then watch those bits) You can get a rough idea of this from talking to people and reading books, but it is time spent on the water that helps you to put all the aspects together.
Its great to see otters working the water, flushing big trout is a bonus. Just mark where he lives and work out why you think he is there, then you can apply that to other bits and have a leg up on finding others. Moments like that are opportunities to learn from
Hope i haven't tried to teach you to suck eggs
but hopefully that helps a little, PM me if you wish to discuss the beat and i can see, if i can give you any useful pointers
Cheers
Sandy