In Cuba, if you catch a picúa (barracuda), folk wisdom says to give a piece to the cat first. If the cat refuses it or gets sick, the fish is “ciguato” and you don’t eat it.
It’s a folk belief, not a test. It doesn’t work.
The danger is real, though: ciguatera, a food poisoning caused by toxins from microalgae living on coral reefs. Herbivorous fish eat the algae, bigger fish eat them, and the toxin piles up the food chain, so the big predators (barracuda, grouper, moray) carry the most. The signature symptom is bizarre: cold things feel hot, or burn.
And there’s no trick. The toxin has no smell, colour or taste, cooking and freezing don’t destroy it, and the poisoned fish looks perfectly healthy. The other popular checks (scales falling off, bulging eyes, the colour of the blood) are just as useless. Only a lab can tell.
The custom is old: in the 19th century, in naturalist Felipe Poey’s time, it was already usual to try suspicious fish on an animal first. The actual advice is simpler: don’t eat the risky species. The US CDC says never eat barracuda or moray.
Poor cat. The same ciguatera shows up in The exiles of Bikini Atoll.
https://www.juventudrebelde.cu/opinion/2013-08-24/picua-por-pargo