Winter Fishing—The Forgotten End of the Temperature Spectrum
This blog was created in collaboration with Keep Fish Wet. A similar version can be found here on their blog.
We often think about what happens to fish as water temperatures heat up in the summer. However, we neglect to consider the other end of the temperature spectrum. This is true in fisheries science as well. There are many more studies that examine the impacts of warm water on fish than cold. Perhaps you could chalk it up to scientists favoring fair weather field work. The lack of studies, however, does not mean that the subject area is unimportant. Many people, especially in recent years, are trying winter fishing. Additionally, 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 smaller range, the thermal optimum, is the water temperatures that fish thrive and prefer to live in. The thermal optimum, maximum, and minimum varies by species. The thermal optimum also varies depending on the life stage, size, and temperatures that fish are already acclimated to.
The metabolic processes of fish decline with water temperatures because they’re cold blooded. 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 Winter Fishing
Fish in streams and rivers often move into deeper areas (sometimes forming aggregations) in the winter. This is 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. So, consider that removing fish from their deep pockets and thermal refuges could be detrimental in the winter. This is because fish have a harder time returning to their preferred spot after being caught in the winter due to decreased swimming ability at lower temperatures.
Recommendation: Consider releasing your fish into the same pool from which you caught it during the winter.
In temperate and northern latitudes, fish will will pack on fat that will be used as energy during winter. This is 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. So, if they run out of energy to maintain basic bodily functions, fish can die—essentially from starvation. This is different from winterkill, however, which mostly occurs in lakes that become completely frozen over and thus don’t have enough dissolved oxygen. Essentially, when there is not enough dissolved oxygen in the water 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, thus burning energy. During the winter when fish are depending on limited energy stores, long fight times could impact fish overwinter survival. Additionally, anything else that could require energy, such as healing a hook wound or replacing a slime coat, impacts survival.
Recommendation: Reduce fight time and use barbless hooks.
Winter Fishing in Tailwaters
While tailwaters (water below a dam) can be popular spots for winter fishing because they are often free of ice. However, they can present worse situations for fish. Tailwaters are usually warmer and have an increased flow rate, which raises fish metabolisms. So, 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. These are especially in regard to how fish react to angling at very cold-water temperatures. There are two trends that stand out, and one aspect that needs to be examined further:
Stress Response Effect
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 (Logan et al. 2019) was able to show that all fish were still alive 24 hours after angling. This is good news for anglers. It shows that angling has a lesser physiologically impact during the winter.
Stress Response Time
At lower water temperatures, stress responses are often diminished. However, they may also be prolonged and/or delayed. A study (Louison et al. 2016) 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. This means that in the winter, the physiological impacts of angling on fish may not take effect until hours after the fish 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.
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.
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 to be 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. For more information about best practices, visit Keep Fish Wet.