Atop Mt. Snowpeak Summit there is an ice covered lake with a hole in it, the Coldwater Tuna can be seen swimming around inside. Easily noticeable by its large icy shape that pokes out of the icy waters.
Sometimes the Coldwater Tuna does not spawn in the icy water and instead minor fish will appear. The player might need to enter and exit the map or sleep in a bed to get the Coldwater Tuna to appear.