Goldfish make alcohol to survive the winter
It's not the cold temperatures, but the fact that when a river freezes over for months, the water doesn't get much new oxygen and becomes unbreathable for fish. When most animals run out of oxygen, they switch to anaerobic respiration which produces lactic acid and can only be sustained for a pretty short period (lactic acid is super acidic so it's okay when diluted but big problem in high concentrations).
But goldfish and crucian carp have a mutant enzyme that allows them to produce the much less dangerous ethanol as a waste product from anaerobic respiration, instead of lactic acid.
"During the extended periods of ice cover in northern Europe, blood alcohol concentrations in crucian carp can reach more than 50 mg per 100 millilitres, which is above the drink drive limit in these countries,” co-author, evolutionary physiologist at the University of Liverpool, says in the press release." More info @ Smithsonian Mag
------------------------------ For more like this, subscribe to my fortnightly newsletter, The Whippet.