The sweetfish of Lake Biwa do not grow. Something about the available food stops them at around seven centimetres, and so they are called ko-ayu, little ayu, as though smallness were the species rather than the circumstance.

Move them into a river and they eat the algae off the stones and reach twenty centimetres, which is the normal size for the species. The same fish. They stay small only while they are in the lake. Because of this, fry from Biwa have been shipped all over the country for generations and released into rivers, where they grow to full size for somebody else.

For this dish they are simmered whole with sansho pepper or ginger, which covers the muddiness that freshwater fish carry and that no amount of soy sauce will hide on its own. Starch syrup goes in, and the pan is never turned or stirred, so that the little bodies do not break up.

In 1998 the prefecture listed the tsukudani of its lake fish among its designated intangible food traditions, alongside the other things it makes from what the water gives it. Which means what was formally protected is the practice of eating, at seven centimetres, a fish that would have reached twenty anywhere else.