Understanding Water Temperature for Better Fishing
It doesn't seem like a big deal, but water temperature can often be the most important determining factor to make or break a successful fishing trip
by Bill Bernhardt
Ambient water temperature is often the primary controlling factor in determining a fish’s willingness to feed because it not only determines the level of oxygen in the water, it also raises or lowers their metabolic rate.
However, there are numerous different factors that affect the ambient water temperature and thus, each factor should be carefully considered when planning when and where to fish because different fish species often have very different preferences for ambient water temperature.
Temperature and Fish Biology
How Fish Breathe
It is also important to realize that contrary to popular belief, fish do not use their gills to break down water molecules in order to extract the oxygen that they need to survive but, instead, they use them to extract something called "dissolved oxygen" from the surrounding water.
Dissolved oxygen is a free roaming oxygen atom contained in the water that is not attached to a pair of hydrogen atoms. Dissolved oxygen enters the water when aquatic plants engage in the process of photosynthesis as well as when air becomes trapped in the water via the action of rapids, breaking waves and rainfall.
Metabolism vs Breathing
The next thing to understand about ambient water temperature and how it affects a fish’s metabolism and feeding behavior, is that fish are cold blooded creatures and thus, their body temperature is dependent on the temperature of the surrounding water. When the water is cold, their metabolism is at its lowest and, when the water is warm, their metabolism is at its highest.
Paradoxically, the colder the water is, the more oxygen it can hold and the warmer it is, the less oxygen it can hold. Therefore, when a fish’s metabolism is at its lowest the availability of oxygen is at its highest and, when their metabolism is at its highest, the availability of oxygen is at its lowest. Thus, fish are least able to breathe when they are most inclined to feed and, they are least inclined to feed when they are best able to breathe!
Consequently, ambient water temperature plays a very significant role in fish feeding behavior and the wise angler learns to use this knowledge to his advantage.
Temperature At Work
While direct sunlight inevitably warms the water and cloudy skies or a low ambient air temperature cause it to cool somewhat, it is important to note that rain can rapidly raise or lower the ambient water temperature depending on the size of the body of water. Also, it can rapidly raise the water level while causing a significant amount of turbidity. At the same time, it can wash a significant amount of food into the water in the form of terrestrial insects, causing fish to congregate in schools near inflowing creeks and streams.
Of course, we all know that the sun's radiation heats both small and large bodies of water when it shines upon them. When it's cloudy and the Sun's rays are blocked, then the water cools slightly by radiating the heat it has absorbed from the Sun to the surrounding air. However, when the ambient air temperature is exceptionally cool, then the water temperature often remains cool despite the heat absorbed from the Sun. Furthermore, water is a poor conductor of heat and thus, it both heats and cools relatively slowly. Consequently, minor fluctuations in either sunlight or air temperature have little effect on water temperature in small bodies of water and almost none in large bodies of water; especially where currents exist to distribute the absorbed heat.
But rain can rapidly raise or lower water temperatures in small bodies of water and, while it does affect larger bodies of water in the same way, is does so far more slowly. On the other hand, rain can also significantly raise both the water level and the level of turbidity in any given body of water.
Flow, Surface Area and Depth
Also, because most creeks and streams start life as ground water that springs forth from the Earth and then flows downhill as gravity dictates, the temperature of the water contained in creeks, streams, and rivers is often significantly lower than that contained in ponds, lakes, or the world’s oceans. Of course, this is due to its recent emergence from the ground as well as the fact that gravity causes water to flow downhill and thus, the current in bodies of water located at higher elevations rapidly distributes any heat the water absorbs from the Sun.
Therefore, bodies of water that are fed by springs, creeks, streams, and even some rivers are often significantly cooler than those that are fed by rainwater alone. However, there again, the level to which inflowing creeks, streams, or rivers can lower the ambient temperature of any given body of water is dependent on both the surface area and depth of said body of water. Consequently, the larger a given body of water is, the more surface area it has and thus, the greater the amount of sunlight that shines upon it and therefore, the more quickly it will heat up on sunny days. Also, the smaller the body of water is, the more quickly it will heat up due to the fact that there is less volume of water to be heated.
In addition, the shallower a given body of water, the more rapidly it will heat up due to the fact that there is less water to be heated and, the deeper it is, the less rapidly it will heat up due to the fact that there is more water to be heated.
Until now, we’ve been discussing micro-changes in water temperature, but it’s also important to be aware of how the changing of the seasons affects ambient water temperature. As mentioned previously, because water is a poor conductor of heat, it tends to act like a heat reservoir that absorbs heat relatively slowly and also radiates its heat relatively slowly as well.
While it is true that during the warmer months of the year, the water near the surface is often significantly warmer than the water at deeper depths and, during the cooler months of the year, the water near the surface is often significantly cooler than the layers of water at deeper depths, this is not always true. Therefore, you should also be aware that sudden changes in ambient air temperature and can cause the layers of water near the surface to either rise or fall more rapidly than the layers deeper in the water column and thus, fish will sometimes seek out deeper water during these periods of rapid change in ambient air temperature because the ambient temperature of the water is more stable at deeper depths and more reflective of the season.
Seeking ‘Just Right’
Because the ambient water temperature determines the amount of dissolved oxygen any given body of water can hold, fish move horizontally in the water column in order to find locations where the water temperature is to their liking, such as congregating near inflowing springs, creeks, streams, and rivers as well as near hydroelectric power plant outlets. But they also move vertically in the water column for the same reason. The further from the water’s surface a given layer of water is, the further from the Sun it is and thus, water at deeper depths is inevitably cooler than the water nearer the surface and thus, is also inevitably holds more dissolved oxygen while, the water near the surface inevitably holds less dissolved oxygen.
However, the changing ambient water temperatures also causes their metabolism to either slow down or speed up. Therefore, fish often seek out the optimum water depth to find the perfect balance between water temperature and dissolved oxygen levels.
Temperature, Lure Size, and Lure Presentation
It is important to note that, because ambient water temperature affects a fish’s metabolic rate in addition to the amount of dissolved oxygen it can hold, it also affects their willingness to chase a lure. Therefore, when the ambient water temperature is low and a fish’s metabolic rate is low, so are their energy requirements as well as their energy levels. Not only are fish less inclined to feed when the water temperature is low, they are also less inclined to chase a lure because they have less energy to do so.
Consequently, experienced anglers are well aware that they need to use smaller lures and slow down their presentation during the colder months of the year and use larger lures with a faster presentation during the warmer months of the year.
Different Species, Different Temperatures
I is very important to note that different fish species have different oxygen requirements and thus, different fish species are often more active during different times of the year.
In addition, it is important to note that each fish species has a minimum water temperature below which it will not actively feed due to a low metabolic rate and a maximum water temperature above which it will not actively feed due to the lack of the availability of dissolved oxygen.
For instance, Trout, Muskie, and Pike are all cold water fish species and thus, they require a higher degree of dissolved oxygen in the water than warm water fish species such as Largemouth Bass and Catfish. Consequently, the dissolved oxygen requirements of any given fish species determines when that species will be most active and thus, most inclined to feed.
Wise fishermen will endeavor to learn what at range of water temperatures their preferred fish species is most active and thus, most inclined to feed in order to experience the best fishing.