Do Fish Hibernate or not? Truth or Myth? The Difference between Hibernation and Dormancy.

Hello, Readers!

December is finally here, and so is the end of Fall and beginning of Winter. With extreme cold temperatures coming (around freezing point), I thought it would be very interesting to bring you guys a topic on "fish hibernation," and finally be the first angler in the net to scientifically connect the sport with this term and unveil this interesting "secret" to you: freshwater fish do not hibernate.

The truth is that biologically speaking, they just can't do it. It's against science. Therefore, fish hibernation is a myth. Emphasizing, freshwater fishes do not hibernate (note that I classified them as "freshwater," meaning that there are a couple exceptions for saltwater fish -- more details later).
I am so lazy to usually explain the whole process of this myth to others that sometimes I just prefer to say that "they hibernate." Hahaha. The last person that asked me "what happens to Catfish in the Schuylkill River during Winter?" got this answer: "Oh...they hibernate."
But the scientific truth is: freshwater fishes enter a state of torpor during low water temperatures (dormancy), and by definition, "dormancy" is very different than "hibernation!" that I gave the statement, I guess you are waiting for the proof, right? Well...before I start explaining, these are some of the questions that you should keep in mind:
-- Where did this concept of fish hibernation come from?
-- Why don't they hibernate?
-- If they don't hibernate, what do they do?
Let's start with the first question! =)
1. Where did this concept of fish hibernation come from?
Just like many other rumors (from a Sociological perspective), most of the time nobody is really able to pinpoint the main source. In other words, it's impossible to tell who was the first person to use the "fish hibernation" term, which is totally incorrect.
We can, however, create a hypothesis about the creation of this myth! Actually, doesn't it make sense to say that this whole idea of fish hibernation during the Winter probably came from human observation? Across history, countless "myths" were created fully based on sensory experience, one of the basis steps of Empiricism.

According to science nowadays: the weather gets colder and the fishes bite less. However, science wasn't that advanced a couple centuries ago. In other words, what is obvious today certainly wasn't obvious back then. I mean...there was a point when we believed that the Earth was flat and the horizon was a cliff. There was a period of time in history when we did not understand how thunder and lightning worked (many myths in the ancient times associated this phenomenon with Gods: Thor and its Hammer, Zeus and its Spear, etc). And so on.   

The human knowledge was very limited back then, and people couldn't help themselves but to make up theories based on observations and experiences. As a matter of fact, this same "phenomenon" happens at any time in history: our future society will certainly be looking at us, maybe 20 years from now on, and be laughing at how much we lacked in terms of knowledge when it comes to certain fields. Science is beautiful because it's a never ending process packed with discoveries and advancements! 

Anyways...time for a story/example:
Imagine a new little village with a small lake full of fish on the countryside, somewhere in the colony of Pennsylvania. A group of refugees come from the city, expecting to find a place where they can live in freedom. The year is 1670, and it's currently Summer time. The Lake is fed and drained by a very small stream, trapping the fish within the lake. The villagers highly depend on the lake as a source of water and food, hence they are still slowly adapting to their new environment. Everyday, some of the ladies and their kids go to the lake to fill their buckets with water for domestic uses. While they are at the lake, the kids walk around and they can see small fishes swimming in the shallows, and sometimes, when the water is clearer, they can even see the big ones swimming around. The kids notice that there are two different types of big fish in the lake, since they look so different: one is darker and without scales while the other shines gold from its scales.

Knowing about the presence of fish, the residents of this small village soon start to practice fishing. They use worms and other insects to fish, and they eventually succeed! The catches are good during the Summer, but the size of their catches is limited since their equipment's durability is also limited. Also, surprisingly, the caught ones were all of the same Species -- the darker ones. Families are happy, hence they have more than enough fish meat to consume. Soon, the residents start to hunt and plant as well, but they fail as they realize that they don't have the basic skills for those arts. They try their best and they accomplish little. 
Soon comes Winter. Their few crops die, and hunting becomes even harder. The fish that were once in the shallows are no longer there. In reality, they are no where to be found! The kids search for the fish, and sometimes they can spot the big golden ones. The darker ones are gone, though. When they talk to their parents about the presence of a second type of fish, their parents don't believe in their words. The main response is: "If there were other fishes in this Lake, we would have caught them already. The fish are probably sleeping somewhere -- after all, it's so cold outside. Who would like to move around? It's just natural for fish to sleep in the Winter." Soon, the kids start to realize through observation that the lake has truly become empty during Winter -- even the golden ones have disappeared. Some others start to get frustrated with the fact that they are no longer catching fish under cold temperatures, not to mention that their fishing spots are limited as the lake ices up. Even though they are still using the same techniques as before, there's just no fish on the other side of their lines. The kids stay home almost everyday. The ladies try their best to gather wood for fire, and the males go out in search for food.

With their current skills and technology, they are not able to survive the harsh Winter. Food runs out often, and they are forced to search for more. Part of the males die from hypothermia while trying to hunt or fish. Another part of the population die from winter starvation. And finally, another part leaves with their kids in search of aid. And so, the Winter of 1670 passes, leaving dozens of bodies in an abandoned little village, somewhere in the colony of Pennsylvania.
And time eventually, it's the year of  1710 -- forty years after the incident. A group of Swedish immigrants decide to establish themselves around the Lake in the Spring of 1710, once again giving life to that silent environment. They come from the city, expecting to have a calmer and pleasant life in the countryside. They bring with them not only their goods, but also the skills and technologies from their homeland. The women soon realize that the soil is good for agriculture: they are able to plant and cultivate many different kinds of crops. The men are able to hunt the local animals with their weapons and efficiently fish the lake with their poles. Now, the fishermen of the village are not only able to get both types of fish -- the darker ones (which turns out to be a Catfish), and the golden ones (which turns out to be the Common Carp), but they also find out a third type in the lake -- the Eels (American Eel). They use different baits for the fishes: insects and rotten meat (from the hunting) for the Catfish and the Eel, and some berries for the Carp. The golden ones fight much harder than the other types, but their equipment is able to withstand the pressure of the fish. 

The little village prospers in a short amount of time, and soon Winter arrives.
The ladies stay home, cooking for their families and sewing new clothing. They are able to make wonderful soups for their families using the last of the potato crops and the conserved salty meat of the hunted deers from early Fall. With the reinforced clothing and advanced weaponry, the males can safely hunt outside for deers and bring more food to the table. The kids watch and learn from their parents, and they often play around the lake. While they play around the lake in the dead of the Winter, they notice that a couple people are not only still fishing around the lake, but they are fishing on the ice. One of the kids approach a fisherman, and says: "I heard from my mother that the fishes sleep during Winter time." The fisherman eventually smiles, and replies: "Some of them do sleep during Winter time, but others still stay awake." The fisherman looks to his left, and soon the kids realize that he has a bucket full of golden fish in it. And so, the Winter of 1710 passes, giving birth to another wonderful Spring in the beautiful little village, somewhere in the colony of Pennsylvania.
It took me quite a while to write this example, but I think it was very worth it! It's from this perspective that rumors are born: empirical data -- observations and experiences. With those comes discovery as well, and the advancement of the limit of human knowledge and understanding. Interesting, huh?

If you are familiar with the history of Pennsylvania (or even the USA), you may be able to notice that this example is not totally fictional. I based this example on my knowledge of the history of this country, and I left many "ifs" and "hows" for the reader to think about.

By now, you should have a better idea of where myths come from. =)  
2. Why don't they hibernate?
The answer is plain: because they can't! They simply can't! Science wasn't very advanced during 1670 or 1710, but it's nowadays. Thankfully, I took plenty of Biology classes in college; therefore, let's review some Biology.
When it comes to the regulation of body temperature of any animal species on Earth, there are two classifications for it: Endothermic ("inside") and Ectothermic (outside). 
-- An Endothermic living organism produces its own body temperature from within. In other words, the temperature of its body is generated by its own body system. I know that it may be a little bit confusing at this point, but can you think of an example of an Endothermic animal? Heh. In reality, all of us (human beings), including you, are endothermic. Our body temperatures are created from our metabolic breakdowns (i.e. breakdown of food). (For Biology people: remember that nearly 60% of all energy contained in food is actually turned into heat during the process of catabolism).
- An Ectothermic living organism, on the other hand, obtains its body temperature from the environment. Its body temperature needs to adapt to the environment's temperature. Fish are Ectothermic creatures -- the temperature of the water directly influences the temperature of their bodies. The colder the water, colder the fish. Warmer the water, warmer the fish (note that some big Species of Saltwater fish are Endothermic - i.e. Tuna). Have you ever caught a Catfish or Carp in the middle of Winter? Or any other Species of fish? If so, I'm sure you know of how cold they can be. =)
And, of course, it doesn't end here! There are two subclassifications for Endothermic and Ectothermic living organisms: Homeothermic and Poikilothermic.
- Homeothermic beings are the ones that must maintain a certain body temperature range at all time, with slight variations, and no choice of temperature change. Our body temperature should stay at the range of 35-37 degrees Celsius, isn't it? Slightly variations will change the whole scenery -- 38 degrees means that we have a fever already, and 34 degrees means extreme hypothermia. 32 is already critical state, and 27 degrees Celsius is death! Have you ever wondered how a thermometer can actually indicate if you are ill or not? Or how different human beings have different body temperature ranges? That's all because we are homeothermic.    
- Poikilothermic beings have the choice to vary their internal body temperatures, meaning that they can "control" their metabolism at their will! Bears and birds are good examples. Bears can, indeed, hibernate! That's when they lower their body temperature, hence Poikilothermic, to save energy during the Winter.  
Human beings are Endothermic, Homeothermic.
Tropical reptiles (most) are Ectothermic, Homeothermic.
Birds (most) are Endothermic, Poikilothermic.
Freshwater fish are Ectothermic, Poikilothermic.
Okay...after all these definitions, I can finally tell you why fish cannot hibernate. The reason is because hibernation, by definition, is a state of inactivity, when the metabolism drops dramatically, and ONLY ENDOTHERMIC beings are able to hibernate. A bear, for example, does nothing during Winter, hence inactivity, dropping his heart rates up to 95%. Some common grounds between hibernating beings and fishes is the fact that both of them save a big layer of fat by consuming a lot of food before lowering their metabolisms (remember, they are both Poikilothermic), and that their goals are to save energy during Winter. A freshwater fish, however, will NEVER stay inactive during the whole Winter, meaning that it will never stay in one specific position, doing nothing, during a whole Winter Season.
Although freshwater fishes do not hibernate, they do bite much less. Sometimes they don't even eat! And that's the topic of next session...
3. If they don't hibernate, what do they do?

It's not totally wrong to say that fishes do  "sleep" more during Winter time. Although they don't enter a state of hibernation, they do enter a state of dormancy during low water temperatures. Fish often go into a state of torpor -- "a state of physical or mental inactivity; lethargy;" However, that doesn't last more than a couple hours a day. For example, Northern Snakeheads enter a state of torpor for about 8-9 hours a day during Winter time. During those hours, they are totally unaware of their surroundings and they do not move at all. As a matter of fact, torpid fishes barely respond to any stimuli! 

Therefore, it's not wrong to say that fishes are lethargic during Winter time; however, it's wrong to say that they are hibernating. Scientifically speaking, hibernation is basically an extended form of torpor (dormancy); however, dormancy and hibernation are triggered very differently: dormancy is triggered by ambient temperature and food availability whereas hibernation is triggered by the length of the day and hormonal changes in the living organism.   

Here's a summary of Hibernation Vs. Dormancy, in general (not only restricted to fishes, which are ectothermic, poikilothermic):

Dormancy/Torpor (in general):

-- Living organisms go into the state of torpor once they are in their natural habitats (refuges).
-- Living organisms in a state of torpor barely respond to external stimuli, not to mention that they cannot perform coordinated movements.
-- The metabolic rate of torpid organisms is very low; therefore, they are able to save a lot of energy in that state.
-- The time to awake from a state of torpor varies from organism to organism, sometimes taking even 1-2 hours to get out of it.

Hibernation (in general):

-- Living organisms go into the state of hibernation once they select a special "hibernacula" (i.e. nest).
-- Body temperatures drop a lot lower for hibernation than for torpor; therefore, external stimuli is non-existent.
So, now we know that the mental picture of a Catfish staying under a log during a whole Winter season without moving is certainly a fake. The fish is not really "hibernating." For most of the time, the fish knows exactly what is going on around it, but the same is restricted to stay put, swim less, and even eat less for some of the day. Why? Because of what we already said above: the fish needs to save energy. Why the fish needs to save energy? Because the fish cannot maintain its body temperature -- as water gets cold, its metabolic system slows down. Why fish cannot maintain their body temperature? Because they are Ectothermic, Poikilothermic. 
There we go! We have broken the questions to its core. From this Biological perspective, we understand now why Fall fishing is so good -- because many different types of fish feed aggressively in order to stock on their fat layers to save energy for another year. It's a harsh situation for them, isn't it? Some think that they are unfortunate for not being Endothermic, Homeothermic like us or other mammals, but can you imagine how things would turn out if they were? They would obviously freeze and die during the Winters because of the cold water temperature, unless the characteristics of what we know as water were different. I guess it is good to think that nature has made things very interesting for everyone, folks...

Therefore, the main reason why they "stop biting" during Winter time is the fact that the fish's metabolic system goes down. We talked about it above -- the state of torpor. In order to adapt to the cold water temperatures, fish's metabolism drops down dramatically at certain periods of the day, in order to save energy for the season. The difference between the fish and the bear is in the fact that the bear can vary its body temperatures at its own will, while the fish has no choice at all -- once the water temperature drops, his metabolic system MUST drop (remember: bear is Endothermic, fish is Ectothermic).
When the water temperature drops, meaning that the temperature of the fish drops, there are changes in the chemistry of the fish. For example, there are significant increase rates in mitochondrial production in fish during Winter, as well as capillaries in skeletal muscle. For those who are not familiar with biology, Mitochondria is an organelle inside the cell (click to be redirected to Wikipedia), or even simpler: it's a component of living cells. They are super important to all of us because they are the ones providing oxygen for our cells, and also producing ATP (Adenosine triphosphate), which is the energy that cells need in order to function (once again, it goes down to Krebs Cycle). 
Evidently, during Winter, the fish's body adapt in a way that it needs to produce more mitochondria to produce more energy, since the body works slower due to cold temperature. Different Species of fish have different resistances to cold; therefore, the metabolism slows more for certain types of fish than others.

It's within these guidelines that fishing slows down during Winter time:
First, depending on the Species of fish, it's not beneficial for them to chase targets at all. Under cold water conditions, most fish lose a little bit of their ability to swim. In other words, they swim slower. Considering that they are also saving energy, it's better for them to just stay put and wait for a big banquet next Spring than chasing your lures/baits around. Under a state of torpor, they won't even respond to your bait/lure, even if you cast it right next to it.
Second, the ability for the fish to digest the food drops dramatically during Winter. All the energy focuses on certain parts of their bodies, meaning that their digestive system is definitely not a priority. Most of the times, the fish will vomit what it ate because the body cannot digest the food. Other times, when the fish is more resistant to cold, the food will be digested, but the components of the food will not be totally absorbed by the fish's body because the metabolism of the same is too low.

So, fishing during Winter time is truly tougher than other seasons. It's at this point that a fish temperature chart turns out to be pretty handy, guys!
Chart 1 - Game Fish Temperature Preferences
Hope you all learned something new today!
Best luck for all of us,
Long Days and Pleasant Nights,
Leo S.