Why are large reef fish sometimes poisonous to eat?

Ciguatera. A microscopic alga growing on damaged reef produces a toxin; small fish eat the algae, larger fish eat them, and the toxin accumulates and concentrates at every step. The biggest, oldest predators - barracuda, grouper, snapper, moray - end up carrying enough to poison a person, while the same species caught small and young is fine.

There is no way to tell by looking, and nothing you can do to the fish helps. The toxin is odourless, tasteless, and unaffected by cooking, freezing, smoking or salting. Symptoms include a strange and characteristic reversal of temperature sensation, in which cold things feel burning hot, and they can persist for months or recur years later after alcohol.

It is the commonest form of fish poisoning worldwide, with tens of thousands of cases a year, and it is spreading as reefs are damaged and waters warm - the alga colonises dead coral. Island communities have long had local rules about which fish are safe from which reef and at what size, which is exactly the kind of accumulated practical knowledge that looks like superstition until you find out what it is tracking.