Ayu are sweet smelt or freshwater trout. They are sometimes caught by cormorants and regurgitated. We attended a kaiseki ryori in Kyoto one May, the right season for these fish. One of the courses consisted of ayu. They were lightly fried and served whole, unlike most of the things which appeared in front of us that night, including raw sea cucumber and something unspeakable called junsai. The late season ayu is known as ochiayu and is considered the best.