Are heads of porgies poisonous?
There are better references in Wikipedia, to be sure. 

Apparently, the Romans ate the Sarpa Salpa, a type of bream, for it's hallucinatory effects. These hallucinations are caused by a toxin called ichthyoallyeinotoxism, present in the plankton and algae the fish eats.

It's best to avoid these toxins as the hallucinations are said not to be pleasant.