The source is from "The Records of the Three Qins of the Xin Family": "One is Longmenkou and the other is Yumenkou in the northeast of Hancheng City, Shaanxi. The locals call it Longguan. Yu carved the mountain to open the door, which is more than a mile wide. The Yellow River flows down from the middle, but its banks are closed to traffic. Every spring, yellow carp swim upstream, and those who cross it turn into dragons.
Example: As the saying goes, a fish leaps over the dragon's gate and turns into a dragon when it passes by, but this may be the case for carp. (Piya Shiyu by Lu Dian in the Song Dynasty) The Han myth and legend of the carp leaping over the dragon gate was officially recorded in "The Records of the Three Qins" written by Xin of the Han Dynasty. The book has been lost. According to the citations of the book in later works such as "Yi Wen Lei Ju" and "Taiping Guang Ji", "San Qin Ji" mentioned the legend of "Fish Leaping over the Dragon Gate" in many places. For example, "Hejin is called a dragon gate. The water is impassable and no fish or turtle can get up there. Thousands of big fish from the rivers and seas gather under the dragon gate and cannot get up. If they go up, they are dragons." Another example is "Longmen Mountain is on the east side of the river... Every day In late spring, a yellow carp swims upstream, and the one who catches it turns into a dragon." And so on.
In fact, modern science can solve the mystery of "carp leaping over the dragon's gate": the "carp" mentioned here is actually "tuna", or "sunfish", also known as "anchovy" or "Yellow croaker", also known as sturgeon. Sturgeon is a migratory fish in rivers and seas, with a body length of about two meters and a maximum length of more than 5 meters. my country’s rare animals, Chinese sturgeon, Northeastern sturgeon, and Yangtze sturgeon, are such fish. Since the big carp in ancient times was also called "鳣", the ancients mixed "tuna" with the big carp, and it was said that it was called "carp leaping over the dragon's gate".
Sturgeons come to Longmen not to "leap into dragons", but to reproduce. The book "Research on the Biology and Artificial Reproduction of Yangtze Sturgeon" written by the Yangtze River Aquatic Resources Survey Group of Sichuan Province states that sturgeons spawn "mostly in the upper reaches of rivers, where the water temperature is low, the flow speed is high, the flow pattern is complex, and the river channels are wide and narrow. And a rapids area with gravel bottom." The Longmen area at the junction of Shan and Shaanxi provinces has the above landform characteristics, making it an ideal place for sturgeons to gather to spawn.
The "Longmen Red River" phenomenon is due to the male and female sturgeons chasing each other before spawning, often jumping out of the water. Every spring, large numbers of sturgeons swim back to Longmen Cave to gather and jump frequently in the two or three days before spawning. When the sturgeon jumped out of the water, its engorged and red fins also emerged from the water. For a time, thousands of big fish were flipping on the river surface, and a red light appeared in the distance, so the scene of "Red River" appeared. Because there are so many fish, they can be several miles long and can last for several days, forming the special phenomena of "Three Days in the Red River" and "Three Miles in the Red River".
"Bamboo Book Chronicles" is the work of Wei historians at the end of the Warring States Period. The record of "Red River at Longmen" in the book shows that the myth of "Carp Jumping over Dragon Gate" had not yet been formed at that time. Most of the records of "fish turning into dragon" come from the classics of the Han Dynasty, so the mythical story was formed in the early Western Han Dynasty, and may be related to the strengthening of the concept of dragon worship in the early Han Dynasty.