Getting a little more in the weeds here. I've always noticed a difference in brook trout at Bob's. I know it used to be stocked with brook trout. In 2021, there were no brook trout stocked, at least none were recorded.
Big disclaimer, I'm not entirely sold on identifying genetic purity via outward appearance. However, I think in some cases it becomes obvious when there's hatchery mixing going on. For example, these "patriot trout" (selectively bred brook trout to enhance the squiggly magenta "dots") mixing with wild fish produce obvious physical traits that imply the fish is the product of a wild native brook trout X a laurel hill trout farm brook trout.
In addition to the brook trout I posted above, this fish is what I would consider a clearly wild brook trout with seemingly "pure" genes. I usually look at the black line on the anal fin and how clean that line is as a good indication of how "native" the fish is. Note this was June 15 2019, roughly 3 years ago to the day from when I fished it Sunday.
In contrast, this is almost certainly a stocked brook trout (note trailing edge of caudal fin and rough black line on anal fin).
This fish is a little harder to tell, but when compared with the first photo and the other fish I posted earlier, you can clearly see the difference in fish. I suspect this fish might be a stream-born fish, but it could be stocked as well. Its fins are clean, but the black line on the anal fin isn't clean. It's fairly small for a stocked fish, but it's legal sized. The thing that makes me think it's stream born, and possibly a cross between a wild native fish and a hatchery fish is that it's very different than the other obvious stocked fish AND the clearly "native" fish.
I think another "tell" on these fish is the spots. Look at the difference between the fish below up close and the fish above. Again, I don't claim that outward appearance can identify genetic "purity", but there's clearly 2 different "looks" going on here.
So why does all this matter? We know hatchery introgression is bad for the wild fish. Hatchery fish aren't suited for survival in the wild and the traits they've been bred for (ferocious appetite, fast growth, no fear, etc.) don't translate to survival in the wild. So, in theory, there could have been a lot of hatchery introgression going on here because of the relative fitness of the brook trout being stocked. We know introgression is relatively rare, however, those studies were based on pretty terrible source stock from hatcheries. Not all stocked trout are created equal, and some may be more fecund or successful at survival than others.
I'm just trying to wrap my head around all the factors that might be at play here. I don't think there's any two ways about it that the brook trout population here has collapsed over the past 20-30 years. I hope PFBC surveys this again to compare against historic records. I'm willing to accept that I'm wrong, but I have a hard time believing my angling experiences would be that far off from reality.
So hatchery introgression may have played a role, but the other thing that's changed a lot during that time is the amount of "habitat improvement" projects that have gone on here. Prior to about 2000, from my recollection, the stretch from about the ballfields in Pavia up into the SGL were relatively natural. I know from researching the habitat work that a lot of it started in around 2002 which is about 2 years into the formation of the "Stream Guardians" branch of the Pavia Sportsmens Club. Between 2002 and today, there have been a LOT of in-stream structures built in the stretch from the ballfields into the SGL.
Some of these structures are obsolete jack dams that create movement barriers during low flow. If we get a low baseflow year I'm going to measure the vertical drop on some of these dams. I think anyone who's fished here knows what I'm talking about. Some down by the old CCC camp have to be 2 feet or so of vertical drop off the plank platform on the upstream side of the dam. I think the height combined with low flow would make them difficult for brook trout to pass.
So in theory, brook trout could've been trapped down in the lower end of the stream as temperatures increased. These pools created by the dams are home to a lot of holdover stocked trout and larger wild brown trout all year round. So for a brook trout to get through there, they have to avoid predation AND make a vertical jump w/ little to no water assistance.
Thankfully, we have some data on Bob's back to 2010 anyway. Here's the temperature profile from 2010 to today. Obviously, there's nothing that stands out here. The high during that period was on August 9th 2021 at 76.6 F at Pavia. Prior to that it was July 22nd 2011 at 75.4 F. The high average is right around 20C/68F over that timespan. I don't think it's unrealistic to project back prior to 2010 and assume the trend was likely very similar. The 2010 to 2022 timespan certainly doesn't show a noticeable average increase. It's worth noting that up in the SGL/Class A section there are several feeders that likely stay much cooler, and the temperature would go down from the recorded temp at Pavia, so even though it saw high 70's in Pavia, several miles up in the watershed was likely several degrees cooler.
So I don't buy an explanation that water temperatures favored brown trout as an explanation. I personally believe, as is probably the case in a lot of places, that there have been multiple factors at play. Habitat structures that favor brown trout and stocked trout, the continual shift in C&R/Harvest likely resulted in far more nonnative (larger stocked and wild nonnative fish) staying in the stream for longer, heavy stocking, possibly genetic introgression, and the typical invasion/invader benefit favoring nonnative fish.
I'm not sure there's anything that could be done about all of this at this point. Unfortunately, I think brook trout are never coming back here. It's worth mentioning that further south, and at a much lower elevation (it's worth pointing out that Blue Knob is the 2nd highest point in Pennsylvania) there is a population of allopatric brook trout in a relatively untouched network of smaller streams where the main stream is close to the size of Bob's creek that weathered the same environmental conditions but remains intact and relatively robust for a brook trout population in this area. To me, that reinforces that the habitat work and the nonnative fish introductions, possibly angling pressure, and hatchery introgression all likely contributed to the decline in brook trout here.
