Maybe that empty pool you see isn’t empty at all.
When making your way to cover the water, be it a small stream, river, or lake one will usually come across sentinel fish. The definition of a sentinel is a “guard”. This is not completely accurate when it comes to my definition for fishing but the outcome is true. When foraging for food, smaller fish often take greater chances.
In streams, you will see the small guys in the tail outs, and near the banks. Often they will hang closer to the walking path, or boat launch. The larger wiser fish lay back out of sight only to appear when a small window opens. This window provides safety and easy feeding. Take large brown trout, they typically feed in the low light conditions and come out of their sleepy holds when things set-up nicely.
One of the reasons we like epic dry fly days is the volume of food (bugs) help us catch fish. Why, because the fish let down their guard and feed recklessly on the surface. But notice how rising trout react on sunny days versus cloudy. Often the sunny day risers will swim to the surface then immediately back to the bottom opposed to cloudy days where they will simply hang just below the surface. How many times have you fished on a blue bird day only to get skunked or have marginal results. Trout do not like the sun because it is difficult for them to see nearby predators. Of course these are generalities but accurate in many cases.
I’ve written about stealthy fishing before and here again the sentinel fish are another reason to become the hunter. It may be easily written-off when you spook smaller fish but the repercussions are huge. Those little guys (sentinels) that scurry away will often put the others on high alert. On many occasions when I fish tough, spooky water, regardless how hard I try to go unnoticed, it takes a few tries before I get an opportunity. If the water is super difficult; low, clear, quiet conditions, I may find my only success is in low light or water with surface disturbance to shield my presence.
My point? Recognize the sentinels. The first fish to send an alert. Achieve success by stealthy approaches, timing, and further immersion into this game we call fishing.