Idyllopus asks a good question: how do you humanely euthanize a fish? As a fish biologist, I get this question fairly often.

Another question I get is, "Fish can't feel pain, right?" It's usually phrased exactly that way, too—they aren't looking for an accurate answer, they're looking for a reassurance that casual brutality towards cold and slimy creatures is acceptable. The actual answer, though, is "Of course they can feel pain, you clueless boob! Mind if I put this barbed hook through your lip?"^* The fish cutaneous sensory network is intricate and exquisite, and they react vigorously to noxious stimuli. We often don't recognize their responses because fish faces are rather expressionless, but if you're in the know you learn to notice the signs. Zebrafish, for instance, blanch noticeably when they're stressed or fearful or in pain.

So how should one kill a fish? People recommend some incredibly brutal methods. Throw them in a blender, they say, it's quick—yeah, and I imagine that throwing cats in a woodchipper would be quick, too, but no one suggests that humane societies should adopt it. There's also the 'club them over the head' method, or 'pick them up by the tail and whack them hard against a table edge'. Those work, if the executioner is swift and sure, which most people aren't. In most cases you end up with a fish frantically flopping on the table, or a bleeding mess of an animal that's feebly twitching, so you have to whack it a few times. (This is how my father and I used to kill salmon, though: we had a heavily weighted club, and we were also very quick and confident.) I think plucking an aquatic animal out of its environment and swinging it through a hostile atmosphere also counts as inhumane.

Less nasty techniques are the freezer and alcohol strategies. I don't think putting a fish in a freezer is humane: they don't seem to react strongly to slowly freezing to death, but then they can't—their metabolism is shutting down. Fish tend to be very sensitive to cold, though, and seek out optimal temperatures and avoid the cold, and can respond to changes of a few degrees with shock, so I have my doubts that this is a good way for them to go. Putting them in water with a few percent alcohol might be OK; they do get drunk, pass out, and die, just like people can.

Here's the way I euthanize fish, though, and since I've killed many thousands, I can say it's the cleanest, least painful way to do it, for both me and the fish. It's an anesthetic used for frogs and fish that goes by various names: ms222, MESAB, 3-aminobenzoic acid ethyl ester, tricaine methanesulfonate, or, as most of the pet and aquaculture supply houses call it, Finquel. For routine anesthesia, I use a 0.2% solution of the stuff—let a fish swim in it for a few minutes, they lose consciousness, you can do various surgeries on them, and then put them in clean fresh water, and a few minutes later, they're awake and swimming around again. If I need to euthanize them, I use a 0.4% solution (or more crudely, I use my 0.2% stock and sprinkle a few extra crystals of the ms222 powder in the beaker), put the fish in it, they fall asleep…and after 3-5 minutes, their heart stops. It will kill them at lower doses, but simply takes longer.

I get my stuff from Sigma, catalog number a-5040, for those of you who can purchase through academic suppliers. Otherwise, here are a few commercial places that will sell it to you: Doctors Foster & Smith, PondRX, and Argent Labs. It's about $15-20 for a 5 gram bottle, which sounds expensive, but a little goes a very long ways. I bought a 25 gram bottle 8 years ago, and I've still got lots left…and I euthanize fish far more often than your usual pet fish owner.

It's good to be prepared, too. Several years ago, my colony was suddenly struck with hemorrhagic septicemia, a bacterial infection that causes blood vessels to rupture and fish to die slowly and unpleasantly and messily, and after spending days trying to treat it with antibiotics and water changes and new tanks and hoses and so forth, I had to spend a sad afternoon putting about 400 fish out of their misery. Using an anesthetic in bulk was the only reasonable way to do it.

^*While I am fully aware that fish can feel pain, I still enjoy fishing and eating fish. I just don't delude myself into thinking the fish are enjoying themselves in a friendly tussle out there on the end of the line.