Huazi fish, or Leuciscus waleckii, in Dali Lake in Chifeng, Inner Mongolia autonomous region, swim against the current to lay their eggs in fresh water upstream from mid-April to early May every year. Large masses of fish flow in the water to form a quick-moving black shadow that can stretch up to 5 kilometers at peak times. Such movement is dangerous for the fish, as they must overcome obstructions, such as rocks, as well as threats from birds, which see them as food.

[Video by Meng Zhigang]