There are countless variables as to why these different situations occur but energy metabolism is a primary factor that we, as anglers, tend not to consider. The larger the fish, the more mass it has to move in any direction. Trout are fine tuned to their domain and they are extremely efficient feeders that waste little or no movement in doing so. This is one reason why we rarely see the smarter, larger fish moving vertically (unless conditions are optimal); it is more efficient to move laterally. Moving vertically often takes more energy due to varying currents within the water column.
As a general rule, trout will not pursue a food source if more energy is going to be exerted than gained by consuming it. Of course, this depends on such conditions as water temperature, time of the year, food sources available, water levels, etc.. If the conditions are ideal and the trout's metabolism is at its peak, it will move more to find food. We can see this variation in behavior throughout the year. All we have to do is observe. For instance, during the cold months of winter or the hot months of summer we know that the trout's metabolism is at its lowest so they tend to move less. Whereas, during the months when water temps are ideal (between 55 and 65 degrees) we see fish moving more because their metabolic rate is at its highest.
Water temperature also controls the trout’s metabolism. While very cold water can hold the maximum of dO2 it also slows the trout’s metabolism to the point of suspended animation ( a cryogenic effect ). This is the way it works: from 32 f to 44 f the trout is slowed to the point of needing very little food and he has a over abundance of dO2, up to 30ppm. At 50 f to 55 f the trout’s activity increases and they actively feed for long periods of time and they still have an over abundance of dO2. When the water temperature reaches the 55 f to 65 f range you have the ideal fishing conditions. The trout’s metabolism is in high gear and they feed constantly, dO2 is in the 18 to 12ppm range and there is plenty of food. The food; aquatic insects and their larvae, minnows of all types and crustaceans are prolific and abundant. The fisherman only has to give a proper presentation and he will hook a trout. The great decline starts when the water temperature climbs to 68 f. Brown, Brook and Cutthroat trout start to feel what I call the frying pan effect. Unless there is a lot of turbulence to oxygenate the water, the dO2 falls rapidly to perilously low levels. The trout’s metabolism is racing furiously along and he is burning oxygen as fast as he can adsorb it from the water. As the sun heats the water, he uses the dO2 faster and faster. With out some type of escape valve he will suffocate.
The trout reacts to this danger in several ways. The first reaction is to decrease activity as in “the dog days of summer”. Fish sulk on the bottom and feeding seems to be nonexistent. When and if they feed it will be in the wee hours of the morning when the water is at its coolest. Water takes a long time to release heat and pre-dawn is when it will be at its coolest. The trout’s second reaction is to move to a place where there is more dO2 available. This could be as close as the head of his pool where a riffle provides the turbulence necessary for oxygenation of the water or a considerable distance. If there is a spring feeding the stream, you will find trout stacked up down stream of the plume of colder water. Ground water can be 10 to 15 degrees colder than the stream. A high shady bank can attract many trout also. FISHING FOR TROUT by Bryant J. Cochrane, Jr.
There is a positive correlation between metabolic expenditure and food intake in both dominant and submissive fish. The dominant fish usually makes better choices than its subordinates and will obtain a greater energy intake because it is often moving less.
Just as any animal in the animal world, most studies have concluded that those that practice a high return/high cost foraging strategy will actually expend more energy than they acquire, whereas those that minimize energy expenditure obtain a higher energy gain. This is one reason why we always see the smaller, subordinate fish moving more and being more opportunistic whereas the dominant fish pick their feeding times and foods wisely.
So, which is most important to a trout, shelter? food? oxygen? As a biologist I have to go with shelter followed by oxygen, water temperature and food. A trout can survive for a few weeks with out food if necessary, but with no place to hide from danger he won’t be there, even if there is plenty of dO2. As a fisherman I am most interested in the food supply, specifically the ease of a trout’s obtaining it. Let’s face it. If a fish isn’t eating you are not going to catch it. If he is too scared or too stressed by lack of dO2, he is not going to eat, and if Mr. trout isn’t opening his mouth you are just practicing casting. FISHING FOR TROUT by Bryant J. Cochran, Jr.
Try to think like a trout. Read the water, pick up rocks, look for the shadows, shapes, and flashes of fish, etc.. Bottom line, observation is key as a fly angler. Observe more than you cast. Position yourself in areas where your glare window is less and you can see more. Take 1/3rd of the time that you spend fishing and spend it looking. Believe me, you will be rewarded. If you fish with a buddy, take turns spot fishing for one another. This is productivity at its best. The result will be more fish hooked and less time aimlessly casting.