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Name: Brooke
Status: student
Grade: 6-8
Location: MS
Date: January 2008

Question:
Do fish breathe better in hot or cold water and why?



Replies:
If a fish can get enough oxygen to live, then it can breathe well enough. Having "more" oxygen isn't necessarily "better." There are fishes that live in hot water, cold water, and everything in between. Some need a lot of oxygen, some don't. Cold water can carry more oxygen, but that doesn't mean it's better for fishes.

Even though "more" oxygen isn't necessarily "better", there are some factors that affect how much oxygen is available to a fish, so I'll share a few more tidbits:

First, is the maximum amount of oxygen that can dissolve in the water. For both salt and fresh water, oxygen solubility (solubility is the word we use for the maximum amount that can dissolve) increases in colder water. Hotter water cannot hold as much oxygen as colder water. Having more oxygen in the water may help a fish to breathe more. As a side note, this is considered 'weird' -- usually, hotter things dissolve better, but the opposite is true for gases. Gases tend to dissolve more easily in liquids when they're cold, but solids tend to dissolve more easily in liquids when they're hot.

Second, is the amount of oxygen that's actually in the water. At the surface, air can exchange into water easily, and the water right at the surface will be at (or close) to the maximum. However, as you go deeper, the amount of oxygen decreases below the maximum. This is because fish or chemical reactions in the water use the oxygen, or currents in the water carry oxygen-rich water away and leave oxygen-poor water. All of these factors can make a fish unable to get as much oxygen as it might need.

Third, are various biological factors such as how well-suited for various temperatures a given fish is. Some fishes prefer cold water, while others prefer warm. Some fishes actually do better in oxygen-poor water, while others prefer oxygen-rich water. Also, various impurities in water can prevent fishes from breathing -- for example, some kinds of chemicals bind to gills and stop them from working. I am very much not a fish biology expert, so I won't spend much time on this, but all of these can influence a fish's ability to get as much oxygen as it needs.

So overall, the answer depends on the fish and where it is, but cold water can hold more oxygen than hot water can.

Hope this helps,

Burr



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