Fish in general, except some tuna species, are ectothermic (cold blooded) creatures. In large part, their metabolic processes are regulated by the temperature of their environment. For example, below is a graph from some of my recent bioenergetics modeling of the energy consumed on a daily basis by a year class of Arctic char in my research lake and the corresponding environmental temperature.
As you can see the consumptive demand of the char rises with temperature and drops dramatically again as the temperature falls. This happens on both sides of a fish' optimum temperature as you can see in the next graph from a study on char growth at different temperatures.
As a fish reaches its own optimum temperature, the catalytic enzymes which determine the speed of its metabolism are at their most efficient speed. As it passes this temperature the enzymes begin to change conformational shape resulting in a rapid decrease in metabolism and stress. Luckily, this stress is reversible if temperatures cool. However, given that we're coming into winter here in the northern hemisphere I'll focus on the cold side of the spectrum.
Most trout are at their best between 55-65 degrees. Given that the Poudre in the canyon started at 38 degrees last Saturday I would say that things are definitely beginning to drop into winter mode. This means a giant reduction in energy demand and feeding activity. I always laugh when I walk into a fly shop in the winter and someone says, "get out there and fish, they still gotta eat." In many places during the winter, that's simply not the case. As water temperature falls below 35 degrees fahrenheit most trout are so far into the left side of the energy demand curve above that they can go most of the winter without eating and still be around when the baetis start hatching in the spring. All is not lost though for those who have rivers near them that stay open in the winter. Indeed some of my favorite days on the river are when the banks are white with a fresh snow. If nothing else, you usually have a lot less people to share them with. Below are 6 ways to cope with the challenge of winter temperatures.
1. Keep a thermometer with you and use it. When there is skim ice about and the water is 33 degrees or below you're probably wasting your time for the most part. You may be able to come back later if the water warms. If possible, fish closer to the dam if you're on a tailwater or seek stretches of river with springwater input or warm effluents like those from wastewater treatment plants.
2. Shift your choice of water. As trout take in less energy when it's cold they also need to conserve it. This means backing out of the riffles and pocketwater that are so amazing to fish earlier in the year and focusing on where current slows in runs and pools. This isn't a hard and fast rule though so I usually fish a few of the best looking banks and pockets in the winter to spot check if fish are still there. Last year I was surprised by quite a few fish holding in skinny water on the Provo River while on my visit home for Christmas.
3. Fish the choicest pools and deep runs more thoroughly. Fish will not be willing to move as much and may require several changes in rig, depth, angle, or fly to get them to eat. Patience is always rewarded in the winter. Sometimes it just takes a lot of drifts to get your fly precisely in front of a trout's nose.
4. Once you've found fish, stay on them. Fish that used to be spread out in faster water will be congregated together in the best winter holding water. If you catch one there will likely be more. Change flies every 20 or so drifts without a fish and you might be surprised how many you get out of a small area. On that same trip to the Provo last year, I found a tiny slack bank behind a bush that produced 3 doubles and 6 more fish before I finally decided to have mercy and move on. The action hadn't really slowed when I left. The spot couldn't have been more than 3 x 3 feet but the fish were packed in regardless.
5. Pull out the suspension rig in slow water. Especially in the typically low base flows of winter, pools will often be flat and the fish unapproachable from a Euro-nymphing distance without spooking them or getting poor drifts. Don't be afraid to pull out your favorite dry fly or indicator to fish these spots.
6. Fish flashier flies than you might at other times. I'll revisit this one in a future post.
Happy winter fishing!