Insects that are not of aquatic origins. Commonly blown into the water with the wind, or when startled. Others fall in from branches above. There are many different taxa in various sizes. Population numbers vary through spring, summer, and autumn, often peaking during a species-specific time period. Look around before you cast.
Various species, commonly black or brown in color, some winged, with sizes ranging from 12-28. Important, especially in summer and early autumn. Winged ants are likely Monomorium sp.
Please note, when I fed the pet trout in my springhouse, (shop) the trout would race each other to get them. They loved them.