The larger fish also contain high levels of mercury.
Yikes!
That statement might be a bit over the top but I know what you're trying to say...I wouldn't want people to think theyre going to be walking thermometers after eating a good sized fish. Not every large fish contains high levels of mercury and some large fish have very low levels of mercury. It would be more accurate to say larger fish are more likley to contain higher levels of mercury.
Ciguatera can be considered regional but in areas not considered to be "regionally affected" it may be present on one section of reef and not on others. It's a very perplexing critter...Here's a very good (but fairly technical) article written by two of the preeminent authorities on the subject...
http://www.ncf-net.org/library/CiguateraToxicon.pdfand a study performed in Belize
http://www.sil.si.edu/digitalcollections/atollresearchbulletin/issues/00569.pdfWith respect to mercury...
http://www.greenfacts.org/en/mercury/l-2/mercury-2.htmNote sections 2.3 & 2.4 specificaly
specific study on mercury in southern Belizean coastal waters
http://www.int-res.com/articles/esr2008/theme/Goliath/goliathpp7.pdf