In my story, in the midst of a human-merfolk war, merfolk are captured by humans, and are periodically de-scaled (they grow back) to sell their colorful scales as jewelry.

In these farms, the merfolk are held in pools with measures to guard them and such.

My question is, would they suffer from anoxic water (https://en.m.wikipedia.org/wiki/Anoxic_waters) and what would that look like?

I've hypothesized that they could be kept alive by means of sluice gates (this is an industrial era world) that exchange water between the pool and the ocean.

More wealthy farms, that can afford to build their farms away from land, employ pipes and steam pumps to circulate water.

I do not need a precise answer. Answers with very hard science focusing on fish are acceptable, answers focusing somewhat on humans and response to low oxygen are also helpful. My merfolk are basic, being just humans with fish tails. They are warm blooded, and can breathe water as well as air - and yes I know that's probably unrealistic, I simply want a bit of guidance on believability. They can't get their oxygen from surfacing in the pools, as other than times they are descaled, the scale farmers do not let them.

  • $\begingroup$ Humans with fish tails in the place of their legs would be air breathers. They would drown if they were to remain underwater for any great length of time. So, anoxic water would have no effect upon them. If these merfolk are water breathers, you'd need to make that clarification. $\endgroup$
    – Monty Wild
    Nov 13, 2021 at 5:24
  • $\begingroup$ @MontyWild they are water breathers. Please try to be respectful to those not as knowledgeable in biology, I am trying my best. $\endgroup$ Nov 13, 2021 at 6:11
  • $\begingroup$ Anoxic water would be exactly as much and as little of a problem to merfolk farming, as it is for Koi farming. The internet's full of how-to guides for the latter. You will also find detail about water purity, thermal control, circulation, feeding, cleaning, medical needs, breeding and inbreeding hazards, etc, etc, etc. $\endgroup$
    – PcMan
    Nov 13, 2021 at 6:13
  • $\begingroup$ The second clarification we would need is if merfolk are 'cold blooded' as are fish, or warm-blooded as are mammals. This makes a very great difference in oxygen consumption. $\endgroup$
    – Monty Wild
    Nov 13, 2021 at 6:29
  • $\begingroup$ "They can't get their oxygen from surfacing in the pools, as other than times they are descaled, the scale farmers do not let them." That seems dumb. It's easier to put a metal grate on top of an otherwise-open tank than to build a fully enclosed tank. Are they chained to the bottom or something? $\endgroup$
    – Matthew
    Nov 16, 2021 at 19:02

3 Answers 3


Since you will need to keep their pools somewhat clean, you will need to ensure a flow of water, so that the flow can transport with it all the wastes.

That flow will also ensure fresh and more oxygenated water getting in, and water can pass through gates which instead block merfolks. Additionally, you can also place underwater plants which would oxygenate the water and improve a tad the morale of the prisoners. If you want to use them for supplying jewelry, you want them to live long, I assume.

  • $\begingroup$ An important question will be: What sells better, the scale of a young mermaid or the scale of an old mermaid? This has to be decided by the writer and it will have a strong impact on whether or not the slavers want their prisoners to live long or not. $\endgroup$
    – Stef
    Apr 21, 2023 at 7:02

Not necessarily. First of all, the merfolk might be air breathers. Even among fish, air breathing isn’t unheard of; air holds more oxygen than water, and isn’t vulnerable to algal blooms. Even some ocean species, like Tarpon, can breathe air.

If the merfolk aren’t air breathers, they could be maintained by a sufficiently large tank with aquatic plants and a water pump. In the land dwellers of this region want to capture merfolk, they will be incentivized to develop the technology for water circulation and tank maintenance.

Improved ability to circulate water means you could keep merfolk in a smaller tank. This is inhumane, of course, but if you were concerned with ethics, you wouldn’t be kidnapping merfolk in the first place.


There are a variety of options

The wastewater treatment industry has developed many ways of aerating waters low on oxygen so that the oxygen levels can remain high for life to be able to happen. Between surface aerators, bubble diffusers, and blowers of different types it would be relatively simple to maintain a suitable oxygen level in your farms.


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