What was the meaning of the fish?

This interview with the film's director indicates that the soldiers cut up the fish to intimidate the tail-enders before battle was joined:

BJH: These moments are what make filmmaking fun and interesting. It wasn’t actually in the script, but when I was making the storyboards, I thought of this idea I mentioned before—a primitive aspect, like tribes in Africa, a ritual before battle to intimidate their enemies. Putting blood on their faces and whatnot. So I came up with this idea of the fish, but I also wanted to bring it back somehow, because it was such a cool concept. We decided to have Chris slip on it. It’s a bit of a strange moment but very natural for me, and Chris took to it immediately. He sort of laughed and said: “We’re doing all this cool action—you want me to slip on a fish? Why not?” We shot it very quickly.

and here

Q. Is there any cultural significance to put the axe blade in the belly of a fish before the massacre?

BJH: So it’s a sci-fi movie but I thought it would be appropriate to have a primitive type fight, like an ancient tribe ritually putting blood on their face. It wasn’t in the script, but as I was storyboarding I included it. But it was actually hard to shoot on set: the fish smelled awful, getting the blood on the axe was hard, and the timing was difficult (it was fake blood in a condom, and it was hard to cut it with the axe).

Note that none of these interviews indicate that the fish was poisoned.

I think fish symbolizes people, particlarly ones in the tail section. Mason said that they eat sushi twice a year in trying to control the population of the aquarium. Likewise, the population of the train can be controlled by a massacre.

Found this interesting interpretation of the fish, it attempts to tie the events of the movie to astrological symbols/cycles. http://webcache.googleusercontent.com/search?q=cache:0w577xTVOowJ:www.reddit.com/r/YMS/comments/2a2hny/id_love_to_hear_adams_thoughts_on_snowpiercer/+&cd=10&hl=en&ct=clnk&gl=us&client=firefox-a

Per fellow user's request, my quick summary of the post linked above:

The post explains that the cycle of the train follows the zodiac cycle, Pisces, fish cut, Aquarius, aquarium then Taurus for the steak. If the front of the train is the top of the cycle this syncs up quite well actually, the front being the first symbol, the ram (ramming through the snow). Then there is mention of a Greek myth involving Eros and Aphrodite turning into fish in order to escape Eros' father, Typhon. If you believe this tie into astrology it could indicate that cutting the fish is meant to show that there is no escape.

I think there are multiple meanings and people above touched on some of them. It definitely has a lot to do with the idea of "balance" that Tilda Swinton talks about with the Sushi. Interesting that the aquarium becomes a symbol of balance after Curtis slipped and lost his balance on the gutted fish.

While I can find no direct quotes from anyone involved with the film to confirm my suspicion, within the movie itself I took it to be a symbolic gesture of intimidation. Much like when someone draws their finger across their throat to imply an opponent's death or defeat.

In a grander sense, I think it was a symbolism to mark the end of Christian charity. As you might or might not know, the fish has been associated with Christian symbolism. I think that the act of gutting the fish with their ax was symbolic of the renunciation of the Christian idea of brotherly love, which was an omen to show that they were prepared to murder their "brothers."

Joon-ho Bong was the director and if youre familiar with asian film making then you will recognize some cultural references in the screenplay....the fish (which looks like it might be koi---japanese, chinese, korean symbolism around koi is pretty ancient & all of them look at the koi as a symbol of strength and perseverance) has multiple cultural layers of symbolism so much so that it is a virtual archetype and will have many interpretations from many different cultures...but given the major influences for the film (asian director and french author) i think the following is likely being referenced: "The fish signifies freedom from all restraint. As in the water a fish moves easily in any direction, so in the Buddha-state the fully-emancipated knows no restraints or obstruction."

the train is like a fish (in shape and movement through the snow), yet the social environment of the interior of the train is the total opposite of the above analysis....the people are fish whose natural affinity is freedom and movement. the perfected state of free will which means neither being an agent of oppression nor submitting to or tolerating oppression.

it seems to be an asian-european fusion & utilizes tropes from these two paradigms. i think, really, this film is commentary on the rationalization of oppression hierarchies we see at work in the world today (ie the rhetoric around "sustainable balance", everyone "knowing their place", etc so called "social darwinism" and randian style "objectivism" which states that the "strong" are justified in their exploitation of the "weak"). the juxtaposition of the utter impoverishment that is separated from utter luxury by only a few cars is reflective of the surrealness of the real world (esp in the usa) where wealthy neighborhoods are separated from deeply impoverished ones by only a few blocks. these blocks become real symbols of this chasm.....the main focus is about class struggle. i wonder if the author is a marxist? def marxist-like influences.

They slice into the guts to rupture the fish's gallbladder to coat the edges of their axes with it's fluid. I KNOW this is the most likely answer being that I've fished for 22 years and have been filleting fish just as long. It's the first rule of cleaning fish is to remove it's innards and if you so much as poke the gallbladder sack then the fish is ruined.

It's definitely a crude intimidation gesture. The raw fish is filled with bacteria that would cause a slow, painful death.

The fish was no "symbol" it was cut so that their weapons would have an edge because the mixing of fish blood and human blood would most likely cause a very bad illness to death

The fish is poisonous, they pass it around to spread its blood on their weapons as intimidation.

I think the fish is symbolic to a warrior getting ready for battle, like painting his face before battle.