FWIW: A long time ago (mid 80s) when Ann and I were studyin' up about going cruising, we did some ciguaterra research
. At that time, folk wisdom suggested the "ask the locals, they know..." method. Sounded good to us. However, some further reading revealed that at that time if one excluded infant mortality and old age, ciguatoxic poisoning was the next most common cause of death in the Tuamotus. Hmmm...
Then we got there (Tuamotus). One day in Raroia I caught a couple of smallish spotted Groupers. Was a bit leary of them, so we walked towards the village carrying one of them for ID. First villager we asked said "oh yes, very good to eat". Ahaa, good news!
Walked onward and met the village gendarme whom we knew somewhat. When he saw the fish
he said "you're not gonna eat that, are you? Full of "le gratte*", they are!"
Much later, a couple of years ago in New Caledonia's northern lagoon
in the Belep group we caught a small Spanish mackerel (locally called Tazar). We'd been told that these smaller fish were quite young and were safe to eat. None the less, we cut it into steaks and carried some of it into the village, where we encountered several Kanak women
. Showed them the fish, asked if they would like to have it, and they happily accepted the gift and we felt justified in having some ourselves. That was our third case of Ciguaterra... hopefully our last!
So there you are... ask a local, they know!
*local French term for Ciguaterra, means 'the itch', one of the symptoms of Ciguaterra.