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I know this is going to sound really weird! But I was confronted with this claim and the person strongly had faith in it! I said faith since he couldn't provide any evidence.

  • Is there any kind of fish that is forced to hold his urine when in deep water ?
  • If yes, is there any relationship between the case with fish's scale ?
  • Does that make the fish's meat not proper (poisonous) to eat ?

I don't know what depth would be proper to define here but I guess the matter to be rare and not so important to define.

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    $\begingroup$ Is there any rational for which a fish would be forced to hold its urine in deep water? $\endgroup$
    – Remi.b
    Apr 3, 2016 at 0:06

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In a salty environment there is the risk of loosing water by osmosis. There are different strategies to prevent this. One of them is to hold the salts of the urine (not the urine itself) by reuptaking them from the kidney. The resulting high internal salt concentration prevents the water loss.

This strategy is used by Sharks and rays/ skates, they take up urea and TMAO from the kidneys. Those metabolites are highly concentrated in their blood, which makes them unhealthy to eat, when fresh. However, after fermenting they are edible: Hakarl is an icelandic dish of fermented shark. The linked Wikipedia-Article gives more details about the topic.

I guess your friend was referring to this phenomenon, but his memory was a bit imprecise.

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Fishes have urinary bladder too. It would be hard to hide from predators with an urine trail pointing towards you. I don't think this is size dependent.

enter image description here - ref

Urine is usually not that dangerous and we remove inner organs, like urinary bladder by the preparation of the meat, so this is not a problem.

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