Staying warm in the winter is hard. Chickadees eat constantly in order to survive long, cold winter nights. Squirrels spend precious time and energy creating complex insulated nests. Deer browse on nutrient-poor twigs to get as many calories out of their surroundings as possible. Yet compared to fish and other aquatic organisms, terrestrial wildlife breathe easy – literally. As fish battle the cold through the long winter, they are steadily running out of oxygen.