Fish cluster at an ice hole, covered with thin pieces of ice, struggling for air in a pond in a park in St. Petersburg, Russia. As the long Russian winter drags on, fish in the ponds of St. Petersburg become increasingly desperate for oxygen, clustering in vast, thrashing masses at shrinking holes in the ice. Not only does ice block oxygen that could be diffused into the water from the air, but it also impedes sunlight from reaching oxygen-generating plants and algae in the water.