I was wondering why different fish in cooler temperate waters react differently to cold.

Some such as yellow perch and chain pickerel from what I see seem relatively unaffected.

Others such as bass become much more sluggish.

I heard Cusk (burbot) are the opposite being much more active under ice.

I have heard of bass and bullhead acting almost in semi hibernation in the cold. But have also seen both caught through the ice by ice fisherman (though winter catches of bullhead are very very rare, just found a video of one on youtube here though )

So what makes some fish species more active in cold than others? what keeps many fish from shutting down in the cold like cold blooded tetrapods do? Do fresh water fish use muscle heating and heat exchange veind like some large ocean tuna and sharks do? Do they have special sugars in their blood to serve as antifreeze like some polar saltwater species do?

I think I may have asked a similar question (but perch specific) a year ago and got the lame answer of "they adapted to the cold as they evolved in that environment" which really did nothing to answer my question as to HOW they adapted.

I know saltwater fish have an "anti-freeze" protein that allows them to survive in very cold water. Maybe some freshwater fish have the same protein.

http://www.msnbc.msn...e/#.TubYAPLNnog

I think it has to do with metabolism. Fish that prefer warmer temperatures (bass, catfish, gar, bowfin) are often sluggish (very low metabolism, don't move and don't need to feed often) in very cold water, while coolwater (perch, walleye, pike, muskie) and coldwater fish (trout, burbot, lake herring) will roam to a degree, with coldwater fish being the most active.

The variability in thermal regulation goes well beyond a simple distinction between ectotherms and endotherms. We generally think of ectotherms as cold blooded, that their body temps are dependent of environmental temps. While we are endotherms maintaining a very constant internal temperature. Yet both ectotherms and endotherms can also be poikilotherms, which we are not since we maintain thermal homeostasis. Some species have an endothermic thermostat similar to ours, but do not maintain a constant temperature. Rather it still allows their body temperature vary over a much wider range, and environmental temperatures play a huge role in supporting sufficient body temperatures. When this fails the responses range from hibernation to estivation or death.

Sharks in particular are bradymetabolic. Meaning that they maintain metabolic temperatures through activity rather than an internal metabolic thermostat like us. They rest to lower body temperatures and become active to increase it. Hence they tend to be most active in cooler temperatures and require more food to maintain that activity. They can also maintain higher temperatures, through circulatory control, in their brains and eyes so they can hunt better in the cold. I understand that most fish are ectotherms, yet also mostly homeotherms. Hence they require a very constant body temperature. I doubt a catfish is a homeotherm given the capacity for some of them to revive from being frozen in a block of ice.

The range of these thermitic control strategies and the degree to which each strategy can be employed varies greatly from species to species. Even when two species share the same set of strategies they still differ in the extent to which each strategy is effective. I do not know enough about different thermitic control strategies in fish to say much about individual cases but dividing into separate groups of either ectotherms or endotherms, hibernation or not, antifreeze agents or not, is invalid. Science does not even use the term warm or cold blooded anymore due to most animals not falling strictly into one group or the other. More like a continuum. Even we have a very limited capacity for hibernation, which the early stages tends to come at the cost of sacrificing our extremities. Once in the later stages people can sometimes survive for extended periods (many days) in a catatonic state, which sometimes saves their lives. Like a progressive shutdown system which conserves our lives longer till the very end of the shutdown sequence.

Thinking of these strategies in terms of sets of strategies as well as the varying extent of effectiveness of each strategy for a given species easily explains the differing reactions to cold among fish. I do not think there is a lot of data classifying the range of strategy sets and effectiveness for many fish species. Why exactly fish can swim between temperature differences in seconds that would kill them if we put them in water with a much smaller temperature difference is a bit of a mystery itself. Possibly due to a constant adaptive regulation of blood flow near their epidermal layer to regulate internal temperatures. Catch this regulatory system by surprise and it likely results in a core temperature gain/loss before the blood flow can adapt to it in such small organism or over large skin surface area to radiate. We are still learning how it works in people, which can also vary significantly in some respects.