Not many lifestyles that suck as much as being a fish that can’t tolerate cold water. Therapy bills must be enormous.
If it’s cold enough to drive away or kill what they eat, that could cause trouble. Also if the cold leads to a rise in the population of something unfriendly to fish.
Some delicate balance was likely disturbed in any case.
We can take heart, however...there are more fish down there than floating up here.