These studies have been around for decades. I believe it was Lee Wulff that was one of the first advocates of "fly angling only" and "catch and release" waters, this back in the late 40s early 50s. I imagine that any state that has these regulations conducted their own studies as well. So, the study you mention doesn't surprise me at all.
That fly angling has less "impact" on fisheries is so widely accepted it's a "given". That fly angling is difficult, expensive, and elitest is pure non sense (though there are those who do feel superior because the are fly fishers but, that's true in a lot of sports). I'll also state that my angling efforts aren't limited to fly fishing; to spin is
not to sin.
Before I moved out here (Oregon) from Michigan I thought their DNR was one of the best in the country. I am most impressed with ODFW's fishing regulations and their logic behind those regs don't confuse me in the least, even designating Rainbow Trout over 12" as "Steelhead" on certain waters (too many idiots out there who don't know the difference). Whilst I miss night fishing for trout, I do understand their reasoning behind that general prohibition as well (there are a few exceptions).
Any who, I'm not at all familiar with the California fishing regs and was surprised to see there are so few fly fishing only waters. I did (do) find this discussion interesting as well as entertaining, if ya'll catch my drift...
Dano