A fast relaxing winter for fish
By Martha Krohn
Fortunately for fish, ice floats. The water below, which shelters the fish, is a relatively balmy 4_C. Yet, we would only live a few minutes in water at this temperature. So how can fish survive?
Cool off and slow down
Mammals, such as ourselves, use energy to maintain constant body temperatures that deviate by less than 1_C. Fish are ectothermic; they do not control their body temperatures. If you stuck a thermometer into a typical fish, you would find the fish was within 1_C of the water around it. Fish physiology is flexible, and the biochemical processes that keep them alive can work over a range of 15_C. Everything continues to function in cold water, but much more slowly - - for every drop of 10_C, the fish's metabolic rate (the speed at which machinery of the body operates) is cot in half. For example, a cod at the freezing point has a heart rate off seven beats per minute, compared to 25 beats per minute at 10_C. (Compare this to our resting heart rate of 70 beats per minute.)
The downside of a slow metabolism is that it limits what fish can do. For instance, the are much slower to escape danger. On the plus side, a reduced metabolic rate in cold water can be a blessing, because it reduces the energy required to maintain the body. This is an advantage when there is less food around, and therefore little energy to fuel their activities and to keep their bodies working smoothly.
Fish in the slow lane and fast lane
There is more to the story than simply slowing down; fish use two main strategies to cope with the winter.
Fish is the slow lane store fat over the summer and fall, and stop feeding in the winter. This often makes sense, since there is little food available anyway. Taking advantage of their reduced need for energy, they hide out for the winter. Bass, carp and eels do just this. In fact, eels will even go so far as to bury themselves in the mud!
Salmon, trout, pike and perch, on the other hand -- as any ice fishermen will tell you -- keep feeding throughout the cold season. These fish have evolved the ability to raise their body temperatures in spite of the cold water, and this allows them to swim and to digest food (eating takes energy). Of course, the have to keep feeding on whatever they can find to finance the more expensive life style in the fast lane.
Challenges of salt and fresh water
Sea water does not freeze until it hits -1_C. Since fish blood freezes at a somewhat warmer threshold, marine fish can freeze even when there is no ice. To counter that, fish such as northern cod and flounder produce an antifreeze, just as herptofauna do. They can therefore live in water at sub-zero temperatures. They still choose to overwinter in the deeper, warmer waters on the slopes of the continental shelves.
Although freshwater fish do not have as great a risk of freezing to death, they have to overcome another problem - it's hard to breathe! Fish, even though they live in the water, still need oxygen. Once winter comers and lakes are covered with ice and snow, not much light gets through, so plants produce very little oxygen. Furthermore, the decay of dead plants and animals, which uses up oxygen, continues. To add to the challenges, water covered in ice is not mixed by wind, and the lake becomes stratified, or layered, with the oxygen-poor zone at the bottom. As oxygen becomes depleted through the winter, fish literally migrate up the water column toward the surface, where oxygen levels are highest. Fortunately for the chilled fish, they need less oxygen, and their blood is better able to absorb it.
Oxygen can sometimes become so limited that large numbers of fish die. The phenomenon of winterkill is most common in shallow lakes, bodies of water that are frozen over for months, and lakes with rich, oxygen-consuming sediments. Game fish such as bass, pike and perch are especially sensitive to winterkill, while carp and eels are more resistant.
We "warm-blooded" creatures have a bard time imagining spending the whole winter in freezing water. but there may be something to letting the heart rate drop and sitting out the cold - if time is measured by heart beats our four month winter would only feel like one month to the relaxing fish.
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