By Ken Moore
One thing that I have consistently observed with my fishing buddies is that no one is paying attention to the temperature of the water. Friends, water temperature and oxygen levels (which we will discuss at a later date) drive everything from trout behavior and location to the very insects that they eat. If you want to take your fly fishing to the next level, then this article is for you.
The following four parameters drive a trout’s behavior, predation, metabolic rate, food source(s),and once a year, spawning. Predation begins once a fish has hatched as it always has the pressure of being dinner for something bigger than itself. Imagine how you would behave if every-time you moved you might be eaten. Food sources for fish are very important and as fly fishermen we are consumed thinking about what fly or streamer to tie on. Water temperature will narrow your focus, and it will tell you two vital facts that you need to know. 1) What is the probability of the fish consuming the fly that I am presenting? 2) How likely is the trout willing to exert energy to consume that fly? The first thing I always do when I get to the water is pull out my thermometer and I continuously monitor the water temperature throughout the day. It is not uncommon to see the temperature of the river rise and fall 3-12 degrees Fahrenheit (F) throughout the day. If I had to narrow it down and pick one item in my bag that will instantaneously tell me when, where, and how to fish my flies it would be a $12 thermometer.
Because fish are cold-blooded (ectothermic) their metabolism is ruled by water temperature. As the water temperature rises and falls so does their metabolic rate. To put it another way, as the water temperature increases so does the quantity of how much a trout will eat and how far they will move to eat. Water temperature will even indicate the type of water they will be lying in, a riffle, a run, or a pool. The ideal temperature for increased metabolic rate, when they begin to eat and move, is 48-50 F. Most trout depending on the species and strain have a maximum metabolic range of 55-65 F meaning they are hungrier and more aggressive at these temperatures. At 55- 65 F a trout will eat three to four times more food than they do at temperatures less than 45 F, this is why I measure water temperature early and often when I am on the water. At 67-70 F trout enter stress mode, and their willingness to eat and move for food decreases dramatically at these water temperatures. The correlation between trout feeding and insect hatches is not happenstance; water temperatures decide when the invertebrates also hatch.
Let us examine trout behavior and the importance of temperature. Tom Rosenbauer of the Orvis Company once wrote: “water temperatures below 32 degrees are lethal for trout because fish don’t do well encased in ice.” I fully agree, but at temperatures above 35 F, trout and their food sources (macro-invertebrates) become predictable. If the water temperature is cold, say 33-44 F, I don’t expect much activity. Although I have had nice days fishing at these temperatures, you won’t find me fishing riffles or fast-moving water. The fish just won’t move, far from their comfy spots at the bottom of the river. I like deep dark pools and slots behind structure where they feel safe from aerial predation, here they do not have to exert much energy. I take my time working likely fish holding spots and I move and fish very patiently and deliberately. I get my flies to the bottom and I put the fly right in front of their nose; meaning I am more comfortable at casting multiple drifts and at varying angles of attack before I move. I like to fish stoneflies, mops, jigs, and even egg patterns. My goal is for the fish to see my offering as it rolls and bounces along the stream bottom
At 42-48 F things are beginning to look up and trout begin to spread out a bit throughout the river. I focus on eddies along bank sides and larger pockets of water that allows the fish to hold tight to structure. This is the time to begin considering riffle run transition zones and the head of pools. If you are a dry fly aficionado, this is a good time to break out your dry fly rod and start presenting midges and Baetis flies. A confidence fly for me is a soft hackle that I present up and across the river on the swing. I often find trout holding in shallower water and small riffles especially if there are large mayflies present when the water temperature is 46-50 F.
From 52-58 F a trout’s metabolic rate is coming into full swing and you will find trout throughout the river even in the faster water. I like to imagine in my mind, trout putting on the feed bag. At these temperature ranges trout begin to move away from one another into prime feeding lanes but they continue to remain near the bottom of the river. At 55-60 F, I have seen trout more willing to move from the bottom of the river to mid-level in the water column. I have had some of my best days with tight line nymphing and streamers at these water temperatures. I focus on a structure that allows the trout to conserve energy, i.e.shallow water and fish the inside foam lines close to the bank. I rarely put my focus on the slowest parts of runs or the back half of pools unless I’m targeting apex predators.
When water temperature is 59-64 F you are in the highest probability for fish dining at the end of your tippet. It is within these temperature ranges that fish are eating three to four times more aquatic food than when the temperatures are in their 30’s and lower 40’s. I put my focus on faster water that has a holding spot for trout, deeper heavy pools that exhibit a standing wave, and high flowing riffles. I learned this lesson well on the Snake River with the water temperature between 58-62 F. I found rainbow trout nosed into the fastest parts of riffles gorging themselves on everything that the river would produce. It didn’t matter what I threw, as long as I could put the fly in the right spot, it was going to be eaten by a big fat rainbow. A day like that only happens because you put together the correlation of how water temperature affects trout feeding behavior, insect hatches, and understanding current breaks, and how trout seek to conserve energy expenditure.
At higher water temperatures above 68 F the oxygen concentration declines significantly and just like you, fish need oxygen to breathe. It is a good time to switch from trout to other species like smallmouth bass. On an equipment note, I have used regular analog thermometers for years but have recently taken to digital thermometers. I find that they are significantly faster at capturing the water temperature and the numbers are easier to read because they are larger. I have not found a fishing company that makes a waterproof digital thermometer, so I have substituted to a small meat thermometer that you use in cooking steak or chicken. I prefer a 4 1/2” folding meat thermometer with a 3” probe this way it fits in all my fishing bags with no problem. Many of these are now “waterproof” but I don’t suggest giving them a dunk in the river or lake. I also prefer the type that turns itself off. You can buy these folding meat thermometers for $12-$20 dollars at any store that sells kitchen supplies or on Amazon.
In summary I know that water temperature is one of the most critical variables that affect a trout’s feeding behavior. If the water temperature is trending to match the prime metabolic rate of trout (55-65 F), I can expect a fine day of catching. If the temperature is trending away from prime, I expect to have a slower day and need to adjust my presentation style. I also recognize understanding a trout’s environment is a very dynamic world. Beginning to understand the mechanics of their behavior and their food source availability due to water temperature increase your chances of catching trout. Remember, during the day the temperature of the water fluctuates, and this knowledge helps you decide when and where you want to target your favorite fishing holes and what to expect.
(please note, contrary to what was published in the Barbless, Rick Newman did not write this article, only helped with editing.)