The Gulf of Mexico Fishery Management Council recently modified its Reef 
Fish Fishery Management Plan to require that non-stainless steel circle 
hooks be used when fishing for reef fish using natural baits.  Although the 
Council was primarily concerned with reducing red snapper discard mortality, 
the rule, which took effect June 1, 2008, was applied to all reef fish 
fishing.  For purposes on analyzing the proposed action, the amendment 
assumed that circle hooks could reduce discard mortality by up to ten 
percent.  Based on public comments, that may be a conservative estimate. 
The following rationale and references are out of Reef Fish Amendment 27 
(the amendment is available on the library section of the Council website, 
www.gulfcouncil.org):

- For many species, circle hooks reduce hooking mortality rates more than 
J-style hooks (Cooke and Suski 2004).
- Reduction in release mortality associated with the use of circle hooks 
results primarily from the tendency of circle hooks to jaw-hook fish (Cooke 
and Suski 2004).
- Some studies indicate catch rates and mean length at capture for red 
snapper are greater for circle hooks compared to J-hooks (Sullivan et al. 
1999; Henwood et al. 2006); however, Powers and Shipp (personal 
communication.), found similar mean length at capture, but lower catch rates 
for red snapper caught with circle hooks compared to Jhooks.
- More red snapper caught with rod-and-reel gear die from hook mortality 
caused by Jhooks
than all other causes combined, including depth, stress, and handling (Burns 
et al. 2002). Additionally, Burns (personal communication.) has shown tag 
return rates for red snapper caught on circle hooks to be greater than 
J-hooks.
- Preliminary data suggest that venting increases survival in red snapper 
caught in deep water (Burns and Porch, personal communication).
- Ease of hook removal is a major contributor to release survival (Cooke and 
Suski 2004).
- Venting, when properly executed, increases survival of released fish.
- Large hooks in general result in some size selectivity towards larger 
fish; however, they do hook smaller fish as well (Cooke and Suski 2004).


You might also check the following references that are available online (do 
a google search on the title to find them):

World Wildlife Organization. 2005. Year-long Study Shows Circle Hooks Help 
Save Sea Turtles
Ng, A. and A. Ng.  2003. Effects of hook treatment and water depth in a 
commercial hook-and-linr grouper fishery, Onslow Bay, NC
Bacheler, N.M. and J.A. Buckel. 2004. Does hook type influence the catch 
rate, size, and injury of grouper in a North Carolina commercial fishery? 
(only abstract is online for free - full article can be purchased from 
publisher)

There's lots more references on the subject, but these are some that I know 
of that are available online.

Steven Atran
Population Dynamics Statistician
Gulf of Mexico Fishery Management Council
Tampa, Florida

(Sorry for the delay in replying, but the Fish-Sci listserv for some reason 
will not accept e-mail sent from my web based e-mail client.)