No question lethal lower temps can occur both from a slow fall or a quick one. Some studies show the quick drops are a big factor alone even in the max lower temp is not reached. Much more stress with a fast fall as they can't acclimate fast enough. This results in a cellular chemical imbalance , muscle rigor and death.

Deep water refuge helps as the fish can move to deep water where the temps don't change as fast. That is why big water will support them where a pond may not.

I think local fish stock probably helped esshup with his results as over time the genetics have adapted to the area somewhat.

Will be interesting to see if the extra cold winter effects them this year.