I would like to add some of my own observations, comments and speculations on the matter of competition between brook trout and brown trout. I have measured and documented each trout caught since 1996. All were from small to medium sized freestone streams. Some hold sympatric (two or more species – brook and brown trout) populations, and some allopatric (single – brook trout) populations. My angling experiences and a modest collection of technical literature suggest clues as to how brown trout displace brook trout, even in our mountain streams where brookies would seem to have a definite advantage over brown trout. For years I have contemplated the question: How do brown trout displace brook trout in freestone streams? These are the streams in North America where brook trout evolved and would seem to dominate. However, in some of these waters brook trout appear to be losing the battle for dominance.
It is well known that brown trout are harder to catch than brookies. Ask any dedicated angler. Studies by fish biologists have shown that this is indeed the case.  I fish a few streams with sympatric populations of brook and brown trout. One has recently shown a marked increase in the extent of the brown trout takeover and the other is slowly being taken over. In both streams brook trout seem to be steadily declining in the lower sections. I suspect that there are a lot of reasons for this, much of which has been attributed to the difference in temperature tolerance of the two species. I personally think that this is a rather small factor. The difference in temperature tolerance is less than 20 F. And trout of both species will move upstream into cooler water in the early summer as water temperatures rise. Brook trout are especially prone to make these movements. Early angling literature describes these movements in some detail.  Brook trout seem to begin making these movements when stream temperature reaches about 650 F. Several times during early summer fishing trips, I have personally observed large schools of brookies gathered in pools in numbers way beyond what those the pool could possibly support on a long term basis. In one case I fished on upstream for several hours and upon returning to the pool, they were gone. Apparently they had moved upstream into cooler tributary waters. I have seen brown trout gathered in the mouths of cooler tributary waters but never in the numbers brook trout do. And I have never seen evidence of their having moved on upstream, although I’m sure they do, eventually, as the larger waters continue to warm in the summer sun.
My conclusion is: brown trout do not replace brook trout. They displace them.
The mechanisms seem to me to be as follows:
- Fisheries biologists, by and large, agree that brook trout are far more vulnerable to angling pressure than are brown trout.
- Brown trout spawn, on average, a few weeks later than brook trout. Typically starting in early November, after brook trout spawning is mostly done
- Brown trout are known to use brook trout redds to excavate their own redds, thereby displacing or eating the previously deposited brook trout eggs.
- Brown trout lay larger and therefore more viable eggs and consequently more viable young in the early stages of life.
- One trend that I have noticed thru the years is that smaller (YOY to fingerling size) brown trout density in streams is typically much lower than that of brook trout. Typically the few browns caught tend to be of legal size to ten inches. But ultimately these few larger browns do take over the really prime lies and therefore continue to grow and outlive the brookies. In time the browns can and do get much larger than the brookies. In olden days brook trout filled all the niches in PA streams and achieved much larger ultimate size than today. According to Cooper both species grow at about the same rate, given that both are living in low to moderately fertile waters. But the brook trout tend to peak out at slightly less than a foot. I think what happens is that brown trout, although initially smaller in number tend to live longer and grow a bit larger than brook trout, on average. It is well established that once trout get to be about ten inches in length they must switch over to larger prey like minnows and/or crayfish if growth is to continue.
- So as brown trout grow they eventually come to depend on smaller brook trout as their primary food source.
- As long as smaller brookies are abundant, the larger browns can continue to grow sometimes reaching sizes well in excess of 12 inches. I have caught wild browns in small freestone streams as big as 17 inches. I have lost or seen a a few that were probably 20 inches. This is the same ultimate size brook trout once achieved when PA was a brook trout angling Mecca.
Conclusion: In streams where brook and brown trout are living in sympatry, brown trout are typically present in lesser numbers than brookies. Their larger size allows them to utilize the extensive brook trout population as an expanded food base, extend life span and achieve larger size than sympatric brook trout. Consequently, the brook trout are confined to inferior stream positions, but because they can spawn at an earlier age and smaller size than brown trout, they continue to maintain a population. However, life span and ultimate size is greatly reduced. This needs to be recognized and addressed by the PFBC. Otherwise, we are slowly but surely going to lose our state fish as a viable angling resource. “We haven’t lost them as a species, but we have lost a lot.” 
 Cooper, E.L. Effects of Exploitation of Eastern Brook Trout and Wild Brown Trout Populations in the Pigeon River, Otsego County Michigan), Transactions of AFS, Vol. 1,1952
 Nesmuk, Trout: Meeting Them on the June Rise, From: Fishing with the Fly, H.R. Nims & Co., 1885
 Charles Gowan, Personal communication.
Editor’s note: Ken Undercoffer who wrote this article is a lifelong devotee of the conservation, protection and restoration of our native brook trout populations. He has served on 2 different occasions as President of PA Council, and currently is PA Council’s delegate to the Coldwater Heritage Partnership (CHP) and representative for the Eastern Brook Trout Joint Venture (EBTJV). Ken can be reached at [email protected] or 814-765-1035.