When the water surface is completely frozen, the oxygen content in the water will decrease, because oxygen is difficult to dissolve in ice water. Fish need enough oxygen to sustain life, so in this case, fish can easily suffocate.
After making a hole in the ice, the oxygen in the air will enter the water through this hole, thus increasing the oxygen content in the water. In addition, these pores can also be used as channels for the exchange of oxygen and carbon dioxide in water, thus maintaining the acid-base balance in water and maintaining the health of fish.
Therefore, in the cold winter, there are always many holes in the ice of fish ponds in the north to ensure the living environment of fish.