How much would the oceans drop if all the fish, etc. vanished?

Short answer: A few microns.

Long answer: Let’s talk about biomass again. Different people give different estimates for the amount of animal mass on earth, but we can probably get this right to an order of magnitude.

For reference, humans – all seven billion of us – put together have about 100 million tons of dry biomass, which is basically a measure of our weight without water, or approximately our carbon content.

Global estimates say there is about 560 billion tons of dry biomass on earth, but only about 1-2% of that is the oceans. That’s crazy considering that oceans occupy twice as much of the earth as land – the reason for the discrepancy is that a lot of the land biomass is locked up in plants and trees and soil.

The most successful species on earth, in biomass alone, is probably the antarctic krill, with estimates of their mass being 125-750 million tonnes.



Have you ever wondered how the largest whales on earth can survive off eating crustaceans smaller than your finger? It’s because there’s lots of them.


Anyway, this paper estimates there is about 1 to 2 billion tons in fish, so let’s go with that.  Assuming the fish has an average density of 1 ton/m3, which is approximately the density of water and a good estimate for nearly every biological material, then the total volume of fish is:

    V = (1 billion tons)/(1 ton/m^3) = 1 billion m^3 

If the oceans cover 70% of the earth, then the area of the oceans is:

    A = 4 pi (earth radius)^2 x (0.70)

about 3.5×1018 m2, which is again the surface area of the oceans on earth. Dividing the above volume of fish by this area gives us the change in sea level, and is about 2.8 microns. Curiously, Wolfram informs us that this is close to the radius of an average bacteria.


So again, the short answer is that fish are few and far between and are tiny compared to the ocean, but Wolfram’s tangent makes me wonder: why is this answer so close to the radius of an average microorganism? What if this number was really telling us something about how biomass is distributed in the pyramid of producers and consumers? In other words- the food chain.

I’m speculating wildly here, but for the oceans to support this much animal mass, there must be something providing food to support it. If you consider that 1-2 billion tons of fish, but instead imagine that was the mass of bacteria, then this observation that the “sea level drops by 1 bacteria’s length” makes sense, at least if you’re a theoretical physicist who answers questions he knows nothing about. This number would mean that if you drew a straight line down in the ocean, on average you would hit about 1 microorganism, which is probably some kind of plankton. Of course, the real number is probably much higher as microorganisms and phytoplankton are certainly going to be more abundant by mass than bigger organisms like fish, but I still think this is an interesting observation.

This makes sense to me because real estate is a valuable resource to these organisms because it guarantees them sunlight. That sunlight is used to photosynthesize, and if there is anything that I know about life, it’s that it exploits every niche it can; if the conditions are right for photosynthesis, someone will be there to do it. If a biologist wants to weigh in on this and correct me I’d appreciate it, because I may very well be drawing connections where there are none. But I think the hypothesis is interesting – if the conditions are right then there will be life, because life, uh, finds a way.




asked by /u/kwmcmillan with special thanks to /u/atomfullerene

cover image credit: Wikimedia Commons




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