The Forgotten End of the Temperature Spectrum: Winter Fishing
Words by Sascha Clark Danylchuk. Photos by Joe Klementovich.
This post was created in collaboration with Fulling Mill and a similar version appears in their blog here.
Winter Fishing
We often think about what happens to fish as water temperatures heat up in the summer, but neglect to consider the other end of the temperature spectrum. This is true in fisheries science as well — perhaps you could chalk it up to scientists favoring fair weather field work — there are many more studies that examine the impacts of warm water on fish than cold. The lack of studies, however, does not mean that the subject area is unimportant. Especially in temperate and more northern (or southern) latitudes, fish spend a considerable proportion of their lives contending with cold water temps.
Below is some biologically relevant information on how fish respond to cold water temperatures, as well as some insights from studies on ice fishing. Combined, these scientific facts provide some considerations and precautions that anglers should take when fishing during the winter months.
What happens to fish as temperatures decrease
Fish survive between a thermal maximum and minimum, above and below which is fatal. Even within their thermal tolerance range, fish have another smaller range. This is known as their thermal optimum, or the water temperatures at which they thrive and prefer to live in. The thermal optimum, maximum, and minimum varies by species, but can also vary depending on life stage, size, as well as what temperatures fish are acclimated to.
Fish are cold-blooded animals so as water temperatures decrease, so do fishes metabolic processes. In fact, water temp is often referred to as the master factor for fish. A fish’s ability to swim, feed, digest food, avoid predators, and defend its location all decrease as water temperatures decrease. This leads to more sluggish, less hungry fish in the winter.
What to do when fishing in winter
Fish in streams and rivers often move into deeper areas (sometimes forming aggregations) in the winter because there is less optimal habitat as temperatures drop and ice begins to form. Deeper pockets can also be slightly warmer if there are groundwater seeps. One consideration to make when fishing during the winter is that removing fish from their deep pockets and thermal refuges could be detrimental. Their decreased swimming abilities at low temperatures could make it more challenging for them to return to the preferred spot from which they were caught.
Recommendation: Consider releasing your fish into the same pool from which you caught it during the winter.
Prior to winter, fish in temperate and northern latitudes will pack on the fat that will be later used as energy, especially since feeding tends to decrease during winter months. For some species, prey items are also not as readily available in winter. As winter progresses, energy reserves can become depleted, and if they run out of energy to maintain basic bodily functions, fish can die — essentially from starvation. This is different from winterkill, which mostly occurs in lakes that become completely frozen over when there is not enough dissolved oxygen in the water and fish die of hypoxia (lack of oxygen).
Anything that causes the accelerated depletion of energy stores can make the situation worse. Fighting at the end of a fishing line increases the metabolism and muscular activity that is fueled by energy. During the winter when fish haven’t been feeding and are already depending on limited energy stores, long fight times as well as anything else that could require energy, such as healing a hook wound or replacing a slime coat, could impact fish overwinter survival.
Recommendation: Reduce fight time and use barbless hooks.
Tailwaters
While tailwaters (water below a dam) can be popular spots for winter fishing because they are often free of ice, they can present worse situations for fish. Tailwaters are usually warmer and have an increased flow rate. The increased water temperature raises fish metabolism and fish are able to be more active, but they have to contend with the increased flow rate of the water. Both of these lead to fish needing more food/energy, however, prey availability is usually low — fish are hungrier, but cannot find enough food. This can result in mortality, especially for smaller fish that have lower energy stores.
Recommendation: When fishing tailwater in the winter pay attention to the health of the fish. If they look especially skinny consider fishing further downstream.
Lessons from science on ice fishing
There have been a handful of studies examining the impacts of ice fishing on fish. Despite the differences between ice fishing and, for instance, fly fishing in open water, there are some parallels we can draw, especially in regard to how fish react to angling at very cold-water temperatures. Two trends that stand out and one aspect that needs to be examined further are:
1) During winter, fish have a muted physiological stress response and mortality rates are generally lower. The stress response measured by examining blood concentrations of glucose, lactate, and cortisol (read here for more information) often decreases at lower water temperatures. By holding walleye in a pen, this study was able to show that all fish were still alive 24 hours after angling. This is good news for anglers — fish are less physiologically impacted by angling during the winter.
2) Although stress responses are often diminished at lower water temps, they can also be prolonged and/or delayed. A study on northern pike, found that it took 45 mins to 4 hours to see changes in blood chemistry following the angling event. As a comparison, in warmer water temperatures we often see these types of changes within minutes. During the winter, this means that fish may not incur the physiological impacts of angling until hours after they are released, and these impacts may last hours longer. We often say that just because you saw your fish swim away does not mean that it’s ok, and this is even more relevant at colder water temperatures.
3) While not specifically addressed, several of the studies also point out some of the potential impacts of air exposure during winter fishing. One study noted that fish showed signs of freezing damage to eyes and gills. Very cold air temperatures and windchills could cause damage even during brief air exposures.
Recommendation: If the guides on your rod are freezing up, consider how delicate gill tissue might respond to air exposure. Just one more reason to Keep Fish Wet.
Takeaways
Until we have some more conclusive science on the impacts of winter fishing at cold temperatures, it behooves us to employ the precautionary principle and extra careful when fishing during cold temperatures. Returning fish to the same lie where you hooked them, limiting fight time, using barbless hooks, and minimizing air exposure are all important actions that anglers can take to help create better outcomes for fish after release.