So, I was traveling for a while and lost track of this thread. I was surprised to see it was still alive. Let me toss in a few other things here.
I've researched the river extensively and dug up some interesting data from a variety of reports. The water just isn't cold enough there for these fish in the summer. I think that they use it to migrate around, but its not where they would live most of the time. No, they live in the best spots in these smaller streams. What's different here is how many big ones are in this small area. Or maybe its not different--more on that in a bit.
As Mike has pointed out, a good forage base is key for these fish. That means a good circumneutral to basic water--think limestoners or similar. Another option is for large numbers of forage fish to be present--so the lower reaches of the larger streams on the Susky are good spots, too. So are spots where cold limestoners enter larger streams. There are a lot more spots like this than most people realize, and it often means fishing in areas that you wouldn't traditionally think of. I know of another stream in Lancaster County where you would never imagine trout, yet it also has produced some massive browns like this. Its also not stocked, but shares many of these characteristics.
Now, on to the preservation of fish like this. First, its wrong to assume that non fly-fishers are killing these fish. The majority of avid spin or bait fishermen I know about are catch and release too. Its also wrong to assume that locals don't know about fish like this or just because you don't see pictures posted online that they aren't being caught. There are just too many people in this area for any place to go unnoticed. I personally heard of this spot from three different people over the course of a year. All of them felt that it was relatively unknown, and yet clearly they all knew about it. I also randomly heard about it from a guy who saw me fishing in a different part of the county--a linesman repairing a power line.
I've fished there twice and both times ended up speaking with several locals. All of them know about the fish and all of them say they've been there for years. One older gentleman told me they've been there for decades. The locals all wanted to know that I was doing C&R and once they saw I was were actually happy to share all kinds of stories about the huge fish there. Its not just brown trout at this spot, either--there are VERY large bass. The forage base is there for these fish to feed and grow.
I don't want to get sucked into a spot-burning debate; its fruitless. I will say that there are more fish out there like this than people realize, although typically they are in deeper bigger waters and not easy to spot. Actually, later in the year when the water is higher and not as clear even these fish would be much harder to spot. I have seen fish this large now in at least a dozen streams in Lancaster and York Counties. The common factor is always how much forage base is available and how much holding habitat there is. Remember, most of the big guys are only out and feeding at night, so we don't ever even see them. Many of these streams are in very built up areas with lots of people around, so they manage to elude lots of fishing pressure (or more people practice C&R than is thought). I've even seen one right along a major road in an urban area that was over 24".
I shared the photo because we all love to fish here and we all love to see big beautiful fish (OK, most of us do). I don't typically hunt larger fish as I prefer to fish tiny streams with a little 6' 6" rod, far from any civilization. For me this was a rare catch and a memorable moment and I thought others would enjoy my sharing it. I'm glad its provoked some interesting discussion. I will say that its got me thinking about spending some more time this year trying to catch fish like this on a few other streams where I know they live.
Jeff