It's frozen all over, but not dead. The ice would keep it fresh anyway.
Restores 120 HP when consumed.
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.
Tips for spawning Coldwater Tuna:
Talk to Pam and move home to Log Cabin.
Exit and go right to take the plane to Mt Snowpeak.
Talk to soldier to go to Royal Cabin.
Run north. If Coldwater Tuna is not there, repeat.