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Q & A: Density of a fish

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Most recent answer: 01/25/2011
Q:
What is the approximate density of a fish, compared with the density of the water in which it swims?
- Mary
Austin Tx
A:
Hi Mary,

  A fish should have a very similar density to the water it swims in.  Archimedes' principle states that the buoyant force on an object is equal to the weight of the water it displaces.  The total vertical force on a fish is that of gravity, the buoyant force, and any reaction force generated by swimming ("dynamical buoyancy").  If the fish were much denser than water (or saltwater, as the case may be), the weight of the displaced water would be much less than the fish's weight, and the fish would sink.  If the fish were much less dense, then the fish would float to the surface.  To keep at a desired depth, perhaps with a little help from swimming, the fish should be about as dense as the water it swims in.

 Many fish have a swim bladder (also called a "gas bladder") which fills with a gas (usually oxygen) and which helps regulate the fish's average density.  The rest of the fish then is a bit denser on average than water.

  When a fish dies, bacteria decompose it and generate gasses as byproducts, which often swell the gut.  The dead fish then floats to the surface, belly up.

Tom

(published on 10/22/2007)

Follow-Up #1: Calculating Density

Q:
what is the exact density of catfish
- gabriel (age 25)
lagos Nigeria
A:
Hi Gabriel,

All fish have densities near the density of water, but just like people have different weights and volumes, so do different fish. Therefore, it's hard to describe an "exact" density of a specific fish. However, if you have a specific fish that you want to find the density of, you can easily calculate its exact density in a fun do-it-yourself experiment. You just need a balance, some water (and containers to hold it), and a catfish.

Density is hard to measure exactly, but density is equal to the mass of an object divided by its volume. If you have a scale or a balance, you can measure the mass of your catfish easily. If the catfish is alive, measure the catfish with its water and container, then take the catfish out of its container and remeasure its mass: the difference will be the mass of the catfish. The volume is also easy to measure. Put the catfish in a container and fill up the container until the water just reaches the top. Then take out the catfish and fill up the container the rest of the way. The volume of water you had to put in to replace the catfish is exactly equal to the volume of the catfish. This is called displacement, and it's said that the fish displaces that volume of water. As long as you can accurately measure the mass of the catfish and the volume of water displaced, then by dividing the mass by the volume you have an estimate for the exact density of your catfish!

Water has a density of 1 grams per cubic centimeter, and ice is about 8 percent less dense- 0.92 grams/cc. You should find your catfish has a density right around these numbers.

Thanks for the question!
Ben M.

(published on 01/25/2011)

Follow-up on this answer.