Over the years much has been documented about selective and opportunistic behavior.  Most fly fishers consider this discussion to be focused upon trout as they feed.  Selective and opportunistic behavior also pertains to where trout hold in a lake.  It can be said that when it comes to stillwaters trout are selective on depth and typically opportunistic when it comes to feeding.  Find the depth trout are holding at on a given day or part there of and success should follow.  

Even when trout appear to be focused upon one food item such as chironomids finding the depth is more the key than finding that perfect pattern.  With the popularity of strike indicators fly fishers can now surgically search the water column for feeding fish.  There are times when a depth change as little as six inches can make all the difference in the world.