i know its a TOTALLY different scale but you can get hatches of mayfly on loch ruthven up here.
A few years ago a chum and myself had hired a boat in June and there was a big whitish coloured mayfly in the boat - but we could not see anything hatching. It was about 25 celcius and fat calm. We roasted in the boat and only got one or two hits all morning
After a lunchtime nap i put on a whacking great mayfly imitation and my pal called me a disgrace and said that no self respecting trout would look at my fly.....
Suffice to say i got a fish on my forst cast and then it was a take a cast from there on !
Turned out that all the trout were subsurface waiting on the mayfly nymphs to rise to the surface. Not a ripple to be seen on the surface but i had an amazing afternoon. My boat partner not so much ( as he only had wee flees !)