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Someone told me that one can kill a fish by putting it in iced water. He stated that it'll kill the animal extremely fast and without any pain.

I have my doubts... Can anyone explain why this should work? Or is it nonsense?

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  • related - it's not painless
    – Raystafarian
    Commented Dec 2, 2015 at 16:20
  • @Raystafarian your related answer just lightly touches on freezing in the comments. See related What happens when a gold fish pond freezes?
    – James Jenkins
    Commented Dec 2, 2015 at 19:27
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    @OddDeer Sorry, I wasn't paying close enough attention to attribution in the comment "it's just in reference as to what is deemed "humane" for (pet) fish" - you probably want to edit the question to reflect the focus on a wilderness scenario. I know I generally don't bring an icemaker to the back country, and I suspect fish I find in already-icy waters have adapted to handle that. Your explanatory comment here doesn't seem to match the question as written.
    – WBT
    Commented Dec 4, 2015 at 14:37
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    In what survival situation would you have (a) tropical fish, (b) ice, (c) no other source of food, and (d) no way to get back to a civilized settlement that might have other food before starving? Also, the fact that Pets already has multiple questions about humanely killing fish seems to justify pointing to at least one of them.
    – WBT
    Commented Dec 4, 2015 at 15:17
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    I'm not a mod, but I don't think your question makes sense as you're trying to explain it. It needs editing. The question makes sense in the context of Pets (it's even been asked there) and it might be a reasonable guess that you didn't know Pets.SE existed, but I cannot imagine any wilderness survival situation where one would have both ice and fish that are not adapted to icy water, unless one is a very poor planner (carried ice but not food) and still close enough to where one got the ice that it hasn't melted. Can you imagine and describe such a situation?
    – WBT
    Commented Dec 4, 2015 at 15:42

3 Answers 3

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You can kill tropical fish by freezing them, or even just letting them cool down. From the outdoors perspective, most game and edible fish are not tropical. There is a post on a sister site Fish “coming back to life” after being frozen and there are many Google finds about frozen fish coming back to life. From personal experience I recall my grandfather putting a fresh salmon in the freezer and when days later we put it in the sink to defrost, it started flopping around still alive.

In short, putting most fish in cold water or freezing them is not going to reliably kill them. Some die, some don't. Whether it is humane remains questionable: I'm not sure there is enough evidence to support a conclusion on the fish's experience of being frozen. Many game fish (and pond goldfish) are frozen during winter as part of the natural life cycle.

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    Good answer, answers the question and doesn't get into the debate of what is humane.
    – kaay
    Commented Dec 3, 2015 at 12:10
  • Can we be certain that thawing fish nerves aren't being activated making it seem as if it's alive?
    – Howdy_McGee
    Commented Dec 3, 2015 at 18:50
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    @Howdy_McGee that is a slippery slope into the scope of our sister site Philosophy
    – James Jenkins
    Commented Dec 3, 2015 at 21:21
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Ice water will not kill any fish quickly. Some fish even live in ice or icy water. A tropical fish may go into shock, but it won't die right for a long time. I think people may believe the fish is dead because the body has stiffened, but that's just a function of the cold and doesn't indicate a dead fish.

The fastest way to kill a fish would really be to behead it, but that assumes you are cleaning it immediately, and it can also make filleting a little more difficult. An alternative is to pour distilled grain alcohol over the fish's gills. That will also kill it quickly as well. The best way in my opinion is to cut the gills out at the bottom of the gill covers. This bleeds the fish out, helping it to die and also has the benefit of improving the fish for eating.

Because they are cold-blooded, fish will live a long time, even if they can't be resuscitated. So all you can do is the best you can. Place a folded wet towel over the fish's head so it's in the darkness and it will calm down significantly. Anecdotally, it may help the fish as much as it helps you.

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  • While out fishing, when the fish is small enough I've always just broke its spine by hand. That seems quick, humane, and efficient to me.
    – Erik
    Commented Dec 2, 2015 at 21:20
  • Decapitation doesn't cause instantaneous death (in fish or in humans). The chef Dave Arnold looked into the subject and recommends a spike to the brain, following by bleeding out. If you're trying to improve the fish for eating, there's also benefit to following up with a wire down the spinal chord.
    – Peter Taylor
    Commented Dec 2, 2015 at 21:47
  • @PeterTaylor I didn't say instantaneous death anywhere in my answer.
    – Escoce
    Commented Dec 2, 2015 at 21:58
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It is absolutely not humane - in fact it is by FAR one of the most CRUEL methods to euthanize a fish. Regardless of them being tropical or not, before they are sufficiently cold enough to die, their blood crystallizes and they basically end up with ice shards shooting through them. Ouch. This is a very common question on aquarium forums and message boards though.

A better method to euthanize a pet fish is to overdose it with clove oil in a bowl of its tank water (clove oil is ordinarily an anesthetic is smaller quantities).

A better way to kill a fish you want to consume (or pet fish if you have the knife and fortitude to do it) is to simply chop its head off in one blow. Or to sever the spinal column (I think that would do it).

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    I would be interested in a reference that supports, freezing is cruel, and that fish "blood crystallizes". Many fish have natural antifreeze in their blood sciencedaily.com/releases/2010/08/100825103832.htm
    – James Jenkins
    Commented Dec 2, 2015 at 19:06
  • What do you think causes the "ouch" in crystalizing blood? Personally, but I am a mammal, don't have feelings inside my veins. Do you mean the heart attack or other things that follow the crystalization?
    – phresnel
    Commented Dec 3, 2015 at 11:17
  • @phresnel Remember that every cell contains water. The ice is going to form near the sensory neurons as well, and ice formed in blood can activate the sensory neurons, so you can feel the ice forming as pain. The only question is: Is the fish put asleep from coldness before the pain starts? Maybe yes, maybe no.
    – yo'
    Commented Dec 3, 2015 at 13:46
  • For humans for instance, most people who freeze to death first can't fall asleep from cold, only to get into a koma from the coldness and never wake up. But for the fish that survives it, the waking up is very likely quite painful. (Anyway, I don't say anything for sure nor provide any evidence.)
    – yo'
    Commented Dec 3, 2015 at 13:46
  • Sounds like chopping the head off is most "humane", how you native English speakers call it :)
    – phresnel
    Commented Dec 8, 2015 at 10:23

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