Scientists have long wondered how goldfish and crucian carp can survive in ice covered lakes with little oxygen and then recover with no apparent ill effects when the weather warms up.
What they have discovered is that in low temperatures they can turn lactic acid in their bodies into alcohol. In the absence of oxygen, eating carbohydrates generates lactic acid which will kill them in high concentration, so the fish have a set of proteins that are activated in cold weather to produce the alcohol which is then excreted via the gills.
In normal times a different set of proteins send the carbohydrates to the mitochondria, where they are used to generate energy.
The longer the fish are in freezing airless conditions the higher the alcohol levels become. It can be above the legal drink drive limit of many countries. However you still very unlikely to get drunk eating them.