That study was from fish tagged last year on Palestine. They tagged 200+ flatheads and only 4 (1-2% of tagged fish) were turned in during the whole season by hand-fishing. That information came from a game warden I spoke to on the lake around towards the end of the season (late July) and I verified that by reading the result of the study for 2014 on their website somewhere. He said a few tags were also turned in by jugliners and trotliners (I think it was like 7 or 8 total). You can find the actual results of the tagging study from TPWD online.
I think the mentality of noodlers is evolving to where most of the bigger fish are let go nowadays. Of course there are exceptions, just like there is for any other fishing method. I usually keep a few of the 15-25 lbers at the beginning of the season for eating and that lasts me all year. The rest, and anything over 25 lbs gets let go right after we catch them. From what I've seen, most of the really good noodlers do the exact same thing. Most of us do it for the thrill of finding a hole and seeing what size fish is in there and then getting it to bite, not for the meat.
To be honest, its a very effective way to catch fish but if you do it legally (by Tx rules) it is a lot of hard work and takes ALOT of time and patience. Contrary to popular belief, it is not as simple as just sticking your hand in a hole and the fish bites, you grab on, and pull him out. It may work like that about 5-10% of the time on the good lakes, but the majority of the time you have to position the fish (if you can even reach it) and then kind of coax it into biting or get your hand in their gill plate. You also have to do this with perfect timing all while blocking the hole with your body. If you do it towards the end of when your breath runs out you have to let go of the fish and come up for air and repeat the process. We'll spend hours working a single fish if we think its bigger than 40 lbs. I remember working a very big fish (probably 60+)through a lightning and windstorm, in a pretty big chop at Palestine one time for almost 7 hours and we never even caught it.
The only time we keep larger flatheads is for noodling tournaments. Most of those tournaments limit a 3-4 man team to 3-5 fish or a single fish for two days, because they have to be kept alive. If we noodle a lake in close proximity to the weigh-in, we usually release them alive in the body of water they came from. If not, they are cleaned and eaten. During tournaments though you are looking for quality and not quantity... so we pass on a lot of fish to find the bigger ones.
Yellows and Blues start cleaning holes in April in Texas, depending on where you live. A better indicator is water temp, and average lake depth (the shallower and more turbid the water, the faster it warms, the sooner the fish will move to spawn). Texoma is as far north as you can get in Texas, so May is probably prime for that lake. Where I live it usually happens a little sooner than the end of May, peaks in June, and is pretty much done by the end of July.
North Texas starts in May sometime, peaks in June, and can last into August.