No Hibernation in fishes
Hibernation is a state of constant hypothermia (low body temperature) and low metabolism. This can often take a long period. The evolutionary advantage of hibernation is that a non-migratory mammal can survive during the winter without having to spend energy searching for food, which is then difficult to find. There are facultative hibernators entering hibernation only when either cold-stressed, food-deprived, or both, and obligate hibernators, who enter hibernation regardless of ambient temperature and access to food.
Fishes and reptiles Body temperature in fresh- and saltwater fishes including larger predators like sharks reflects the temperature of their watery environment: they are ectothermic. They do not hibernate in the strict sense because they cannot actively down-regulate their body temperature or their metabolic rate. However, some species experience decreased metabolic rates called dormancy, associated with colder environments and/or low oxygen availability (hypoxia). Water also makes a good shelter for freshwater fishes as well as reptiles such as frogs and turtles. When the weather gets cold, they move to the bottom of lakes and ponds. There, they hide under rocks, logs or fallen leaves or may even bury themselves in the mud where they become dormant. Cold water holds more oxygen than warm water, and the frogs and turtles can breathe by absorbing it through their skin. The same holds for the common goldfish (Carassius auratus) in domestic ponds that are able to survive in temperatures below 10 degrees Celsius, even when the pond is covered with ice, as long there is some oxygen available. With European winters becoming increasingly mild, goldfish may delay or even skip their annual period of a dormant state as long a there remains some food available from the surface.
Sea mammals A different situation holds for sea mammals. Whales are warm-blooded (endothermic) and will keep a high body temperature that does not change in the colder water. In order for whales to keep warm in cold/polar climates they have developed a thick layer of insulating blubber, which protects against freezing winds and icy water. Whales also profit from migration to colder oceans where food like krill is more abundant. Manatees, however, that lack the protective blubber do migrate from the sea to warmer water in winter, often found in inshore freshwater springs, or even power plants along the shore.
Climate change and hibernation To what extent do rising winter temperatures affect hibernation and animal's chances of survival? If the bees, hedgehogs or bats get out of their winter rest too early due to high winter temperatures, then there is far too little food (e.g. insects) available. When two periods of frost are separated by one warm week, the hedgehog is in trouble. When it awakes, it is hard for the animal to get back to sleep. And a hedgehog that is awake but unable to find food will not survive in the cold. Even the state of dormancy, accompanied by minimal use of the body resources and slowing down of physiological functions (think of our goldfish) could be essential, not so much to overcome a temporary shortage of food or lower body temperature, but as a rest period allowing recuperation and recovery of metabolic functions.