Understand Fish Metabolism and Become a Better Angler


“What goes around comes around.”

In other words, as the French author Jean-Baptiste Alphonse Karr wrote in the middle of the 19and century, “The more things change, the more things stay the same.” Whether you realize it or not, hunters and anglers apply this mantra more than most recognize – with changing but predictable annual seasons, different approaches to hunting and fishing. Understanding and adapting to your career’s unique interactions with the environment and other organisms goes a long way toward success on the court and on the water.

Animals are usually innate creatures. To be successful, hunters and anglers often aim to mold these habits, which are driven by a multitude of factors such as day length, food availability, hormones, weather, and more. All of these factors change throughout the year, but the changes are most apparent during periods of seemingly abrupt and drastic seasonal change.

Even as I look across the snowfields that still linger in the north, spring hangs in the air. Ice fishing sleds will soon be replaced by boats. Shed woods are placed there for the taking. Turkey seasons are about to begin. In the south, the azaleas will soon bloom and the pre-spawn bass models will go into full spawn mode.

Many hunting opportunities occur during specific seasons, however, barring certain regional closures or state regulations, fishing is easily a year-round activity for many fish across the country. So, as we “move” our clocks forward and look to welcome spring fishing conditions, let’s dig deeper into how changes in water temperature affect fish, their ecosystems, and your ability to catch them throughout the season. year.

Cold-blooded basics
All freshwater fish are cold blooded. As a result, their habits and habits are closely related to water temperature. In other words, the ability of fish to perform many bodily functions, their metabolism is a key underlying factor for the “who, what, when, where, why and how” of daily fish life. Metabolism, as in humans, refers to how fish convert food into energy and growth, which is directly related to eating habits. Each species of fish has evolved to live at an optimal thermal temperature and generally exists within a temperature range surrounding that perfect heat level.

Broadly speaking, there are three thermal guilds for freshwater fish: coldwater species, including trout, salmon, and sculpin species; fresh waters including walleye, northern pike, muskellunge and yellow perch; and warm waters, including species of bass, catfish, and sunfish. Nuances exist where individuals of species adapt to local temperatures that often do not reach their species’ preferred optimum temperature, and furthermore, the optimum temperatures of some species change with their life stages (juvenile vs adult ) and metabolic needs change. The temperature range for each species has been derived from various laboratory experiments under “ideal” conditions (e.g. unlimited food availability), which obviously may differ from factors in their natural environment, including food availability, habitat and oxygen.

As cold-blooded organisms, fish can “survive” very low temperatures, but there is a heat threshold at which their metabolism becomes extremely inefficient. Ice fishermen can attest to this often “slow” fishing. Alternatively, at the upper end of a species’ temperature range, especially for cold water fish, there is a threshold at which their bodily functions cease to function and they cannot survive –trout anglers are familiar with the consequences of approaching these thermal maxima, including summer stress-related mortality.

For example, cutthroat trout’s optimal metabolic temperature is 57°F, but their appropriate temperatures range from 41 to 61°F. Deadly stress occurs around 75°F. However, the added stress of fishing at the line can be fatal in temperatures below 75°F, depending on many other environmental and physiological factors. In contrast, the temperature range for smallmouth bass extends from 40°F to 97°F with an optimum temperature of around 72°F. With increasing warm weather patterns including drought , water diversions and alterations, the overlap of these species could have profound effects on freshwater ecosystems where they have not traditionally shared space, evident in the recent discovery of smallmouth bass in a watershed of the Upper Yellowstone River in Montana.

Death, taxes and bioenergetics of fish
As the days lengthen and lake ice begins to melt in northern latitudes, more sunlight enters the water column, stimulating the annual production of microorganisms and algae. The same acceleration occurs, albeit to a lesser extent, in ice-free waters to the south.

A similar acceleration pattern is present in the metabolism of fish depending on their preferred water temperatures. In some ways, you might visualize this as the slow rise of a roller coaster. Suppose when you jump on a roller coaster you are at the lower end of a fish’s temperature range. As the conditions slowly warm up and the race creaks over the initial hill, there is more potential energy both from a physical standpoint and excitement in the runners. These potentials are similar to the growth potential of fish. The higher the roller coaster goes on this initial climb, the more a fish can eat and grow. At the top of this elevation, a fish’s optimum temperature, its growth potential is greatest; however, if temperatures continue to rise, the growth potential of fish declines rapidly. However, unlike a roller coaster, the stress felt by fish falling from this chute is neither fun nor exhilarating as they may not recover.

Luckily for the fish, they aren’t attached to a ride – when more favorable temperatures and conditions are present elsewhere, they can move to those habitats to maintain their preferred temperatures. As such, they can continue to grow, reproduce and survive, hopefully without a catastrophic roller coaster crash.

Another way to think about fish growth is to a budget perspective. In other words, as cold-blooded organisms, the growth of fish follows the Second law of thermodynamicswhich basically means that the amount of energy going in must be equal to the amount of energy going out.

Put simply, the food eaten by fish is their “pay”. By converting this food into growth, some of the energy is used or lost by metabolic processes, including daily movement and digestion – their “taxes”, if you will. Ultimately, the amount of caloric energy remaining is what the fish can allocate to individual growth as well as reproduction (egg and milt rearing) – their biological “savings”. Clearly, the fish that can maintain and increase their wages while lowering their taxes will be the healthiest, “wealthiest” individuals, and hardiest populations.

Thermal perspective fishing
From an angling perspective, fish metabolism and growth patterns are relevant for all seasons and all water bodies. Under the ice, when temperatures are lowest and metabolism is slowest, small bait and lure presentations, passive and deadstick techniques can be of most benefit. This continues into the spring where small and relatively slow presentations still make sense from a metabolic and ecological point of view. The fish are ready to make an effort to eat, but not too much. Food availability begins to increase, but smaller lures result in a “matching hatch” outlook, as young-of-the-year from spring spawners hatch and are abundant in the environment. Similarly, it takes less energy and less handling time for a fish to digest many small meals rather than fewer large prey when its metabolism is still relatively low. Bass fishermen, for example, are used to targeting riprap during this time – the thermal mass of the riprap material combined with heat from the sun warms the surrounding water, attracting baitfish and their predators. Here, bass can both be more active in pursuit of prey and able to digest more efficiently, allowing for greater growth potential as well as better egg development, which is also dictated by temperature, often measured in total degree-days.

In mid-summer, food availability increases with fish metabolism. In lakes and reservoirs, once the water column is stratified, fish species segregate into the most efficient thermal habitat for their metabolic needs. So, having an idea of ​​the water temperatures and the depth of the thermocline, the preferred temperatures of your target species and their prey, and where they may overlap, can help anglers locate fish, especially when venturing into new waters. In some cases, during the scorching days of summer, the fish are active in efforts to keep up with their metabolism, or in other cases relatively sedentary in efforts to reduce their metabolic losses. In both cases, they always try to put reserves “in the bank”.

Coming out of often difficult late summer fishing patterns, fall fishing can often be fruitful. For many species this is a time when more energy can be devoted to individual growth in preparation for the long winter and before a large amount of energy is used for reproductive development and related activities. For example, in the fall muskellunge anglers often turn to larger lure and live bait presentations. Autumn patterns present a sweet spot for their metabolism – under these thermal conditions it is more efficient for fish to balance the energy expenditure needed to obtain fewer and larger prey, while digestion is also efficient. given the surrounding water temperatures.

All in all, whatever the season, when it comes to learning and adapting to fishing patterns, there is no substitute for experience on the water. So while I hope this information on water temperature and fish metabolism provides context for adapting your fishing practices throughout the year, again, pardon my French, but… .

Get your ass out and go fishing!

Feature image via Sam Lungren.

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