Are heads of porgies poisonous? I found on the Wikipedia article for porgies, there is a quote
    from a reference saying that:


  
  Johnson, G.D. &amp; Gill, A.C. (1998). Paxton, J.R. &amp; Eschmeyer, W.N.. ed. Encyclopedia of Fishes. San Diego: Academic Press. p. 184. ISBN
  0-12-547665-5.  Eating the head is known to cause hallucinations,
  lasting many days.
  


I was wondering if their heads contains some poison or other bad
    things, and thereforeare not eatable and  must be removed before
    cooking?

All fishes are caught in Maryland.
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.