In the south of the North Sea many fish of the species Zoarces viviparus died during the summer, attributed to water heating. For this fact has been investigated in Bremerhaven (Germany), where they have seen that as the temperature increases the fish is increasingly difficult to breathe and that from a temperature dies (about 25 degrees).
It seems that heat accelerates fish metabolism, needs more oxygen (and hot water has less oxygen) and for this the heart pumps blood faster. When the situation reaches the extreme, the body of the fish does not support and dies drowned.
Researchers believe that something similar can happen to other species, that is, they die drowned above the normal temperature range.