I feel this is the basic question that every trout angler asks themselves, "How many fish are in that stream?" How many is enough? How many should there be? What is a "good" stream? There is no good answer to any of those questions. It depends what are you looking for. Some prefer higher density streams because - like me - you enjoy catching fish on dry flies. More fish = more dry fly eaters, in my experience. Others want to chase big fish which are more likely to occur in lower density streams. Oh, you are looking for lots of big fish...good luck with that. It is all pretty complicated but more is not always "better".
Wisconsin is blessed with thousands of miles of trout streams. Some, I suppose, see this as a curse too. Where to get started? Well, I suppose it depends upon what you are looking for. Unquestionably, not all stream are created equal.
There are over 13,000 miles of trout streams in Wisconsin that are categorized into three categories depending upon the stream reach's ability for natural reproduction. Class 1 streams support wild, self-sustaining trout populations, these are approximately 40% of Wisconsin's trout streams. Class 2 streams are about 46% of the state's trout stream miles and have natural reproduction but not enough to "maintain a desired fishery", whatever that means. Class 3 streams make up about 14% of the state's trout stream and they are streams that have little to no natural reproduction and according to the DNR, require stocking to maintain trout fisheries.
Sometimes these categories tell you a lot about the stream and its potential; other times they seem a little outdated and not terribly useful. It certainly depends upon where in the state you are to determine what the categories really mean. In much of northern Wisconsin - say Florence County - most class 1 streams are likely to be Brook Trout streams and maybe nearly unfishable for fly anglers. In the Driftless, say Vernon County, I don't always see a lot of difference between class 1 and 2 streams. Even some of the class 3 streams have not been stocked in many years. Some streams listed as class 2 and 3 have 10 fish bag limits so certainly they are able to "maintain a desired fishery". Certainly there are some overclassified streams as well - streams that used to be class 1 have certainly moved to class 2. And lastly, stream classes may vary by region pretty significantly. I would not expect Waukesha County class 1 or 2 streams to have sort of trout densities that Vernon County streams do.
By now, I know what you're starting to say to yourself, "When the Hell is he going to talk about how many trout are in 'that' stream?"
Here's your answer to the how many trout question - it depends. The stream class, the location, the regulations, stocking history, and other factors may all inform us about that answer. Fact is, there is not a simple answer. Class 1, 2, and 3 streams don't have specific numbers or densities associated with them and even if they did, those categories are rarely updated to reflect improvements or degradation that occurs over time. Fortunately there are some data online. We'll start with a pretty coarse look at trout densities. Trout are often measure in number per mile - not exactly the best measurement of density because streams vary so much but it is useful for setting expectations, after all, at the end of the day, I usually think about how much time I've been on the water and the linear distance I've covered.
Brook Trout density is certainly quite patchily distributed and looking at the figures, 12 inch and larger Brook Trout are pretty rare just about everywhere. Looking at the legends, median densities range from 0 (obviously) up to about 4,000 Brook Trout. For larger Brook Trout (>12"), watershed median densities top out a 78 per mile.
With Brown Trout, median densities range up to nearly 3,000 fish per mile and 10 to 15 inch browns range up to a bit over 1,000 fish per mile. Brown Trout over 15 inches are pretty rare everywhere - high densities are around 50 per mile. Rather simply, trout densities have a huge range of variability and big fish are always pretty rare.
What about Individual stream Reaches?
At finer resolution, stream by stream, there is a ton of variability among trout streams. From streams with thousands of fish per mile to less than 100 per mile. And I'll argue trout numbers per mile tells a quite incomplete picture. Size distribution tells us an awful lot about what to expect when we fish a stream. I pick the two areas - the Central Sands and the Driftless mostly because there is more data that is readily available for them.
The first two figures are central Wisconsin - Central Sands - streams that are both quite solid fisheries. The figures represent the number of fish captured by electrofishing and measure - not fish per mile. The first stream, had Brown and Brook Trout densities in the 55% percentile - so a little bit better than the statewide average. The second stream has Brown Trout densities in the 90th percentile - one of the state's better streams in terms of numbers - but Brook Trout densities were lower (15th percentile) which is often the case when Brown Trout densities are high.
Length-frequency distributions like these are great for thinking about what your expectations should be. Obviously anglers are catching trout of different sizes than electrofishing gear which is much more indiscriminate. But it does tell us that in stream 1, you should expect 5-8 inch Brook Trout to plentiful but they captured just 13 Brook Trout that were 9 inches or larger. In the second stream, Brown Trout outnumber Brook Trout and the size distribution for Brown Trout is quite excellent. In the second stream, while there looks to be but one trophy, there are a number of memorable fishes (see What is a Trophy Fish? for sizes that fisheries biologists have labeled as "trophies").
The Driftless Area has some of the highest densities of trout, Brown Trout in particular, of anywhere you will fish. Our sidewalk to street wide streams have densities that rival trout densities anywhere (for better or wosre...). Below, I present Brown Trout populations in 3 stream reaches, 1) a high density stream reach with lower than average >12" trout, 2) an average density stream with greater than average >12" trout, and 3) a moderately-high density population with higher than average populations of larger fish.
Stream 1 is one of the most densely populated streams in the Driftless Area. The number of fish over 12 inches is around the statewide average of 33 / mile despite the fact that the overall Brown Trout density is generally over 3,000 fish per mile. About 1% of Brown Trout in this stream reach are 12 inches or larger (a real, measured 12 inches, NOT an anglers 12 inch guesstimate).
Stream 2 is a lower density population with total density going above and below the Driftless median over the years. However, the number of fish 12 inches or larger is consistently better than the Driftless median. I'll certainly write more about this down the road but there is a lot of thought that carrying capacity for a stream may be best measured in biomass and number population. Streams can have a lot of small trout or they can have fewer larger trout. Much of this is probably dictated by food availability. As trout grow larger, it is more difficult to make a living eating only insects.
Stream 3 is really the best of all worlds - the trout numbers are pretty high (though the decline over the last 4 years is concerning) and they're larger than average. This is probably one of the state's better trout streams.
What else is interesting in these figures aside from the amount of variability is that much but not all of the variability is looks to be regional - winter mortality, drought, or some other factor. 2011 and 2012 were peak density years but 2013 and 2014 were low years across the region.
How many Trout are "Enough"?
How many is enough - well, that depends upon the experience you are looking for. If you're looking for numbers of trout that are willing to eat your dry flies - as I often am - higher density streams will (usually) provide more action. Lower density stream will be more likely to provide a chance at larger fish, maybe even a truly memorable trout. Not that higher density streams can't do this, they're just much less likely to have larger trout. Of course, you might find one of those unicorn streams where both numbers and densities are above average. If you do, expect to share it with others.