Brk Trt, jimp,
Thank you very much for your comments.

Alpinefly,
Very beautiful pictures and fish !!! I am curious :think: , with all the overcast (rain), do you have a species of Blue Winged Olive Mayfly that hatches there in Japan ????
I don't know much about insects. Besides, I haven't seen the mayflies you call "BWO" by my eyes. But I think the answer depends on the definition of BWO. If you call mayflies belong to the genus Ephemerella "BWO", we certainly have some BWOs hatching usually from spring to early summer, though I don't think they are the same species you have in your country.