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The sky-trout is a magic fish which can levitate and swim in the air. While this protects its gills from collapse, it doesn't help with dehydration

The fish is a type of trout around 2-3 feet long, which is found in eastern Europe. It has only been around 20 generations since the first sky-trout gained levitation, hence they aren't much adapted for keeping hydrated. They live in freshwater when not levitating

How long could these fish float in the air before their skin/gills start to dry out?

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    $\begingroup$ In real life it's the dehydration that causes gill collapse, so... they can fly for as long as they can stay hydrated. $\endgroup$ Mar 5 at 1:36
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    $\begingroup$ This sounds like a roundabout "how long can a fish survive out of water". $\endgroup$
    – Daron
    Mar 5 at 2:44
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    $\begingroup$ Slugs do just fine for years by secreting slime. Not seeing the worldbuilding problem, more of a feed waiting for a punchline. Tell us how long you want it to be OK outside water and ask us how to do it perhaps. $\endgroup$ Mar 5 at 3:14
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    $\begingroup$ Indeed, but fish produce slime also. It's up to you. Just tell us what your requirements are, what you need it to be able to do and we can help you get there. $\endgroup$ Mar 5 at 3:29
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    $\begingroup$ I do think this is more of a biology question than worldbuilding. Really the answer is "almost as long as you'd like". If we assume that a levitating fish has also evolved to be good at it then it may actually be an amphibious fish (e.g., lungfish, mangrove killfish). So the answer could be "3 minutes" (it's basically a goldfish) or "a couple of months" (if amphibious). $\endgroup$
    – JamieB
    Mar 5 at 6:51

3 Answers 3

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Because this is a specific type of fish, you could just look up stats for the European trout. It will depend on the relative humidity of the area, the wind, and what kind of activity the fish has, but you could probably figure it out.

More generally, you should check out other fish that spend time outside the water. Gaining traits from mudskippers, lungfish, and flying fish doesn't seem that much more crazy than gaining levitation. They would probably want to stay close to the water. Air is not unbreakable for fish (just like water isn't unbreathable for vertebrates!) But it does have complications that make would make trying extremely unpleasant for a dedicated fresh water fish like a trout.

You could also check out amphibians. Many frogs and salamanders have skin similar to fish. They don't have scales of course, but it's something to check out. These animals actually try to avoid open air, because they can maintain their moisture better in on and in the ground. Also makes them less likely to get eaten by birds

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30 seconds to 10 minutes

Anglers try to keep time out of water under 30 seconds to avoid stressing out the fish. Fish already struggle to survive and stress doesn't help. But going over isn't certain death, until it is.

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Days to Months

Frame challenge! You may wish to consider some other inspirational fish that would give you a lot more freedom in species design for a magical levitating fish. Any common fish isn't going to get a lot out of levitation (and may easily self-harm) due to rapid onset of stress.

For some alternative thinking, I present to you the mangrove killfish. "The mangrove rivulus can spend up to 66 consecutive days out of water, which it typically spends inside fallen logs, breathing air through its skin."

Or, alternatively, the lungfish and perhaps the Australian lungfish. Most lungfish require air to breath, as their gills are no longer efficient enough on their own, but the Australian lungfish can do both. "The Queensland lungfish can live for several days out of the water, if it is kept moist."

So you can of have a lot of leeway here depending on how you envision your fish.

More like the Australian lungfish: it's a true fish, prefers the water, with efficient gills, but can hang out over the water pretty well as long as it likes, breathing with its singular lung, dipping in and out as needed to stay moist. I imagine levitating fish would use this to hunt insects over the water or around the shores.

More like the African lungfish: still a fish, but more of an air breather. Can secrete protective mucus that allows it to levitate around for months without worrying about drying out. This may be a better option if you envision levitating fish that have some migratory ability. (Just a school of migratory levitating lungfish going by. Nothing to see here, folks.)

I suspect the mangrove killfish option is better for smaller fish, where the "oxygen through the skin" option is efficient enough. (I bet it would work well in something like a flounder though -- any fish with a high ratio of skin).

If you want to stick with more common fish, i.e. a levitating trout, then candied_orange's answer is best.

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    $\begingroup$ The question says the fish is a trout. Not a killfish or lungfish. $\endgroup$
    – Daron
    Mar 7 at 16:48
  • $\begingroup$ @daron Frame challenge but you're right that I should probably have clarified that's my intent. I'll add that to the answer. $\endgroup$
    – JamieB
    Mar 7 at 16:57

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