Actually, they are coming up to get oxygen.. but it looks cool!

(Lakes and streams breathe the same as living things. When they are
covered with ice and snow they cannot get air and they much hold their
breath until the ice thaws. While they are holding their breath the
oxygen in the water is gradually used up by the living things sealed up
in it -- fish, plants "bugs", snails, and hosts of microscopic life. If the
ice lasts long enough, these living things die one after another as each
kind reaches the point where it cannot stand any further oxygen
starvation. Sometimes temporary relief is given by rains and melting
snow that bring fresh, serated water under the ice, but no method of
artificial respiration has been found that works. Sometimes, too, when
water plants get enough sunlight through clear ice they produce small
amounts of oxygen and delay the suffocation of the fish, etc.; but when
snow and cloudy ice cuts off the light this does not happen. [www.total-fishing.com])