Longmen (mythology)

The legend states that while many carp swim upstream against the river's strong current, few are capable or brave enough for the final leap over the waterfall.

Thus, all the carp competed at a yearly competition to leap the Longmen falls; those who succeeded were immediately transformed into dragons and flew off into the sky.

[3] Pictures of carp attempting to leap the Longmen falls have been enduringly popular in China and other parts of Asia such as Japan (known as Tōryūmon).

There are other Dragon Gates (Longmen) in China's rivers, typically with steep narrows, and the mythological geography does not depend upon an actual location.

[4][5] The fish's jumping feature is set in such a proverbial idiom as "Liyu (Carp) jumps over the Dragon Gate" (鲤跃龙门) an idiom that conveys a vivid image symbolizing a sudden uplifting in one's social status, as when one ascends into the upper society or has found favor with the royal or a noble family, perhaps through marriage, but in particular through success in the imperial examination.

Jumping the falls print, folio from the Fang shi mo pu (方氏墨譜). It is a Chinese symbol of an examination.
Two carps in a cascade ukiyo-e woodcut print by Katsushika Hokusai , Edo period