How to Identify a Productive Brook Trout Stream (and Where To Find Brook Trout In It)

Introduction

With its colourful spots and patterning, the brook trout (Salvenius fontinalis) is without a doubt one of the most beautiful fish out there. In addition, its willingness to take a dry fly makes it widely popular and inhabits (or perhaps haunts) the dreams of countless fly fishermen around the world. 

Interestingly, in comparison to the brown trout, it is technically not a trout but a char. It is therefore more closely related to the lake trout than it is to the brown trout. The easiest way to tell the two genera apart is: White spots on a dark background, and dark spots on a light background for brook trout and brown trout, respectively.

Brook trout are native to the northeastern parts of United States and eastern Canada but have also been introduced to Europe and Asia. 

It can be found in spring-fed creeks, streams, small to medium rivers, and lakes but what type of habitats does it prefer? That is, how to identify a productive brook trout stream, and where do I find the brook trout in it?

What to look for in a brook trout stream

Temperature

A study conducted on 33 streams in northeastern Vermont by Vermont Fish and Wildlife concluded that temperature was one of the most important factors in predicting brook trout abundance, the other being wood density, see wood density section below (1). 

Brook trout prefers water temperatures in the range of 50-68 F (10-20 C) and in the study above the largest predicted population of brook trout (measured as lbs/acre) could be found in streams where the temperature did not go above 68 F (20 C) for a single hour during the period of May through September. With each 100 hours of temperatures above 68 F there was a decline in the predicted overall population and in one stream that reached 80 F (26.7 C), no brook trout were identified at all. 

These results clearly highlight the importance of temperature on how productive a section of a stream could potentially be for someone fly fishing for brook trout. Therefore, always make sure to bring a thermometer when out trying to discover some new potential waters. 

Figure 1. A small resident brook trout caught by the author in a small spring-fed creek in Ontario, Canada.

Other important habitat factors that boost brook trout populations

Wood Density

As mentioned above, the study conducted by Vermont Fish and Wildlife identified stream wood density as one other critical factor positively correlating with brook trout population size (1). They found that this was especially true in colder streams where the temperature did not go above 68 F (20 C) for a single hour during the period of May through September. 

For warmer streams the wood density had less impact on brook trout populations, again highlighting the importance of temperature. 

The study did not discuss why the wood density is correlated with larger brook trout populations but fallen wood provides plenty of cover (shade, protection from above predation), and a potential source of food (insects living on the decaying wood) all of which would be beneficial to the brook trout.

Although sometimes hard to fish around, try to identify stream sections with lots of wood in addition to a low water temperature. 

Stream bed substrate 

Always look for a stream or creek bed consisting of a mixture of sand and gravel. This substrate mix not only provides good spawning grounds but also a prolific insect life making it an ideal habitat for brook trout. 

Clear water 

Brook trout in streams largely rely on drifting aquatic insects as a food source. Turbid water makes it harder for the brook trout to see its prey drifting by in the current which can lower feeding rates and hence survival. 

Search for streams with water with little sediment and organic matter which remains clear throughout most of the year.  

Figure 2. The author fly fishing for brook trout in a clear coldwater stream with a gravel/sand substrate.

High percentage spots (or micro habitats) to target on a stream section

In the 33 streams investigated, the brook trout population ranged from 0-1725/mile (1.6 km). The average was 440 brook trout per mile of stream. 

Most of us are all familiar with the old 90/10 rule meaning 90 percent of the fish is located in 10 percent of the water. The question is: What is that 10 percent of water?

Below are four high percentage spots to find brook trout within a stream. 

High oxygen spots 

High oxygen spots like those generated below both large and small waterfalls are especially productive in stream sections that are prone to higher water temperatures. 

Behind Larger Rocks

Rocks break up the current and brook trout can be found holding close to the current seam (the border of slow and faster moving water) formed by the rock. The trout generally holds in the slower current of the seam to conserve energy and darts into the current seam to eat drifting insects.   

Deadfalls in the Water

Provides shade and protection from predation from above and breaks up the current. Always great spots to target and can be fished efficiently by swinging a fly close to the deadfall. 

Undercut banks

Provides shade, protection from predation from above, and relief from the current. 

Undercut banks are prime lies and generally holds the largest trout and are well worth targeting if you can find one. 

Final Note

Sadly, due to the sensitivity of brook trout to environmental factors, and the introduction of non-native species such as brown trout, a decline in population size has been observed over the years. 

It is up to all of us as avid fly fishermen to locally lobby for the preservation of our beautiful creeks, streams, rivers, and lakes so that generations to come can continue to enjoy the beautiful brook trout. 

References

1) Jud Kratzer – Biologist report: What makes a good brook trout stream?, 2012

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