There was a paper released a while back about this very subject. Here is the Opening paragraph.....
Are Circle Hooks Effective?
Among studies the results have been quite disparate, overall the mortality rates were consistently lower for circle hooks than J-style hooks. In addition, circle hooks were more frequently hooked in the jaw, and less frequently hooked in the gut than conventional hook types. There is no doubt that in some marine fisheries, such as tuna, billfish, and striped bass, capture efficiency remains high and injury and mortality rates are drastically reduced.
You can download the entire thing at
http://fryingpantower.com/downloadvi...Effective.html
You will have to register to download it.....It keeps the file harvesters and Hot-linkers out.
Hope that helps.