Shoot poachers. ETA: and pool together subspecies (like some rhino subspecies) which in isolation will certainly go extinct, since there's not really any point in them suffering without company and probably a hope of cross-breeding. There was a story about European/American bison crosses that were later all killed to prevent 'genotype pollution', but from the poi t of view of minimazing suffering it always striked me as stupid.

Also, bees and some related species experience a decline nowadays; it is not technologically impossible to plant most suitable plants to support their existence (instead of just importing the most fashionable species.)

But as long as about 20 people out of 1000 would swerve to make a snake completely flat, I would continue to think that the highest returns here can be obtained only through strict prohibition. Think of Madagascar. Heck, a party of 500 salamanders dried out within hours of confiscation in the Boryspil aeroport once, because there is not even a room to keep confiscated animals! Many animals are too stressed by capture to survive transportation, which is why 'to raise a reptile' means for experienced batrachologists 'to keep it alive until you obtain F2 of it', for example (here I mean my own biology teacher.)

Or, you can make 3D images of animals to replace all those zoo sufferers...

(I also agree with that downvote. I should have thought past my knee-jerk reaction.)