Understand Fish Metabolism and Become a Better Angler

“Plus ça change, plus c'est la même chose.”

That is, as French author Jean-Baptiste Alphonse Karr penned in the mid-19^th 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 yet predictable annual seasons come different approaches to hunting and fishing. Understanding and adapting to your quarry’s unique interactions with the environment and others organisms goes a long way towards success in the field and on the water.

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

Even as I gaze across the snowfields still lingering in the north, spring is wafting in the air. Ice fishing sleds will soon be replaced by boats. Shed antlers are laying there for the taking. Turkey seasons are about to begin. Across the South, azaleas will soon bloom and pre-spawn bass patterns will shift to full-on spawn mode.

Many hunting opportunities occur during specific seasons, however, save from certain regional closures or state regulations, fishing is easily a year-round pursuit for many gamefishes across the country. So, as we “spring” our clocks forward and look to welcome spring fishing conditions, let’s take a deeper dive into how changing water temperatures affect fish, their ecosystems, and your ability to catch them throughout the year.

Cold-Blooded Basics All freshwater gamefishes are cold-blooded. Accordingly, their habits and patterns are tightly coupled with water temperature. That is, the ability for fishes to carry out many bodily functions, their metabolism is a key underlying factor for the “who, what, when, where, why, and how” of fishes’ daily lives. Metabolism, like in humans, refers to how fish convert food to energy and growth, which directly relates to feeding habits. Each fish species has evolved to live at an optimal thermal temperature and typically exist within a range of temperatures surrounding this perfect level of warmth.

Generally speaking, there are three thermal guilds for freshwater fishes: coldwater species including trout, salmon, and sculpin species; cool water including walleye, northern pike, muskellunge, and yellow perch; and warmwater including bass, catfish, and sunfish species. Nuances exist where individuals of species adapt to local temperatures that don’t often reach their species’ preferred optimal temperature, and further, some species' optimal temperatures shift 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 can obviously differ from factors in their natural environment including food, habitat, and oxygen availability.

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

For example, cutthroat trout’s optimal metabolic temperature is 57°F, but their suitable temperatures range from 41 to 61°F. Lethal stress occurs around 75°F. However, additional stress from angling can be lethal at temperatures below 75°F, depending on numerous other environmental and physiological factors. In contrast, the temperature range for smallmouth bass extends from 40°F all the way to 97°F within an optimal 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 throughout 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 Fish Bioenergetics As the days lengthen and lake ice begins to melt in northern latitudes, more and more sunlight is able to penetrate into the water column, stimulating the year’s annual production of microorganisms and algae. The same ramp-up is occurring, albeit to a lesser extent, in ice-free waters to the south.

A similar ramp-up pattern is present in fish metabolism related to their preferred water temperatures. In some ways you could visualize this like the slow rise of a roller coaster. Say, when you hop on a roller coaster, you’re at the lower end of a fish’s temperature range. As conditions warm slowly and the ride creaks up the initial hill, there’s more potential energy from both a physics standpoint and excitement building within the riders. Those potentials are akin to the growth potential of fish. The higher the roller coaster gets on that initial rise, the more a fish can eat and grow. At the top of this rise, a fish’s optimal temperature, their growth potential is the greatest; however, if temperatures keep increasing, the growth potential of fish quickly falls off. However, unlike a roller coaster, the stress felt by fish falling off that drop is not fun or exhilarating because they may not recover.

Luckily for fishes, they aren’t strapped into a ride—when more favorable temperatures and conditions are present elsewhere they can move to these habitats to maintain within 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 from a budgetary perspective. That is, as cold-blooded organisms, fish growth follows the Second Law of Thermodynamics, which essentially means that the amount of energy coming in must equal the amount of energy coming out.

More simply, food consumed by fishes are their “wages.” In converting this food to growth, some energy is used or lost by metabolic processes including daily movements and digestion—their “taxes,” if you will. In the end, the amount of caloric energy left is what fish can allocate to individual growth as well as reproduction (growing eggs and milt)—their biological “savings.” Obviously, fish that can maintain and increase their wages while moving to reduce their taxes will be the healthiest, “richest” individuals, and most robust populations.

Fishing for Thermal Perspective From an angling perspective, the patterns of fish metabolism and growth are relevant across all seasons and waterbodies. Under the ice, when temperatures are lowest and metabolism is slowest, small bait and lure presentations, passive and deadstick techniques can be most advantageous. This carries over into spring where small and relatively slow presentations still make sense from a metabolic and ecological perspective. Fish are willing to exert some effort to eat, but not too much. Food availability starts to increase, but smaller lures translate to a “match the hatch” perspective as young-of-the-year from spring spawners hatch and are abundant in the environment. Likewise, it takes less energy and handling time for a fish to digest many smaller meals rather than fewer large prey when their metabolism is still relatively low. Bass anglers, for example, are familiar with targeting rip-rap rocks during this period—the thermal mass of rip-rap material combined with heat gained 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 more efficiently digest, allowing for greater growth potential as well as better egg development, which is driven also by temperature, often measured in total degree-days.

By mid-summer, food availability is increasing along with fish metabolism. In lakes and reservoirs, once the water column stratifies, fish species segregate into thermal habitat that is most efficient for their metabolic needs. Thus, having an idea of water temperatures and the depth of the thermocline, the preferred temperatures of your target species as well as their prey, and where these might overlap, can help anglers locate fish, especially when venturing onto new waters. In some cases, by the dog days of summer, 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 instances they’re still trying to put reserves away “in the bank.”

Coming out of often tough late-summer fishing patterns, fall fishing can often be fruitful. For many species, this is a time when more energy can be put towards individual growth in preparation for the long winter and before a great deal of energy is used for reproductive development and related activities. For example, in autumn muskie anglers often turn to bigger lure and live bait presentations. Fall patterns present a sweet spot for their metabolism—under these thermal conditions, it’s most efficient for the fish to balance energy expenditures needed to obtain fewer, bigger prey while digestion is also efficient given surrounding water temperatures.

All told, regardless of season, when it comes to learning and adapting to fish patterns driven, there is no substitute for on-the-water experience. So, while I hope this background on water temperature and fish metabolism provides some context to adapt your angling practices throughout the year, once again, pardon my French, but—

Get your ass out there and go fishing!

Feature image via Sam Lungren.