Updated: Apr 6, 2021
Fish bioenergetics is a pretty simple idea - for fish, growth is limited by how much a fish eats and how much energy is used to "do stuff". It is basically an energy balance equation where what can be used to grow is what is left over after growing gonads, cellular respiration, and what was not able to be digested or lost as urine. What fish eat it is termed "consumption" and is the ultimate limit to growth - though there are other factors. Being cold-blood (ectothermic), fishes are not able to control their own temperature, thus their metabolic activities vary as a function of temperature. Anglers know this - many fish get lethargic when the temperature is too cold or too warm. As trout anglers, we know not to fish when the water temperature gets too warm because the stress of catching them is more likely to cause mortality.
Bioenergetic is a way that fisheries scientists quantity how what an individual fish or a age cohort of fish consumes is turned into body growth. Growth (delta B which means a change in body size (weight)) is typically the response variable of interest, that is we are interested in how food is turned into more fish. And often, we are interested in seeing how changes to a parameter is likely to affect growth. For example, we might be interested in how climate change is expected to affect fish growth. Or we might be interested in how an invasive species may change consumption and thus change fish growth - like how Great Lakes Smallmouth Bass are growing faster and larger due to Round Gobies. It also informs us about the presence or absence of fish species by predicting if a fish species is going to be able to consume enough food and oxygen to survive.
The above figure comes from field data - this is the model was created from the presence and absence of Brook Trout in streams with different maximum summer water temperatures. Certainly the presence or absence of Brook Trout in not just due to temperature and the ability to consume enough food and not expend too much energy (that's what the biogenergetic model estimates). However, competition and predation are also temperature dependent, in part because Brown Trout and Northern Pike are negatively affected by the cold water temperatures in which Brook Trout thrive (or at least survive). And for trout, because they have a high oxygen demand, warmer water is less able to hold sufficient oxygen to sustain trout.
The basic bioenergetic model:
C = (R + A + S) + (F + U) + (delta(B) + G)
C = Consumption (calories consumed - what they eat)
R = Respiration (basal metabolism, aka energy necessary to survive)
A = Active Metabolism (energy required to move, get food, etc.)
S = Specific Dynamic Action (energy required to digest food)
F = Egestion (feces - what is not digested of what was consumed)
U = Excretion (urine)
delta (B) = changes in body weight (growth)
G = Gonads (reproduction; energy used to grow sperm and eggs)
As mentioned above, the equation is often reconfigured so that changes in body weight (delta(B)) is the response variable which mean all the parameters are subtracted from consumption (C) and what is left over is growth.
While temperature is not explicit in the model, it affects every parameter in the model. Again, because they are cold blooded and their body temperature matches the environment. Enzyme activity is temperature dependent so how quickly and thoroughly they digest their food, their basal metabolism, the cost of movement (A), and the other parameters are all temperature dependent. Quite simply, fish metabolism is a function of temperature and different fish species have different metabolic curves. And, again, we as anglers know this. Most fish are more "sluggish" in coldwater and again above a certain water temperature. Those that have trout fished in the winter may have experienced a rather significant decline in fishing quality as the sun melts the snow and cold water enters the stream. Suddenly trout do not need to eat as much because they can not digest food quickly enough to need more of it (consumption is temperature dependent - thus so is growth).
The figure above demonstrates how a metabolic curve is calculated. We see that the aerobic scope is the difference between the maximum metabolic rate and the standard (resting) metabolic rate. The greatest scope for growth, a species thermal optimal, is the temperature where this difference is maximized. It is an optimality model as I described in the Optimal Foraging Theory post. Critical temperatures (the red arrows) are temperatures that are lethal to that fish species and the temperature labeled "pejus" is the temperature below and above which performance is significantly negatively impacted. The "sweet spot" is the area between the two pejus temperatures.
The figure above, a series of Fry curves, illustrates the range at which a species can sustain capacity (survive). The peak is the optimal temperature at which that fish species is best able to utilize oxygen and thus should be most able to be active and able to grow (there is a scientific debate about this I am not going to get into). The ends of the lines are the critical thermal temperatures, the range of temperatures for each line informs us about the range of temperatures in which they survive. The length of the lines tell us about their thermal range. To be sure, it does not tell us that Brook Trout instantly die at 23*F, rather they can not sustain themselves for long at that temperature. It also does not mean that trout are able to grow faster or "more" than Goldfish or bullhead, just that they have a larger difference between their maximum and resting oxygen consumption.
Trout are an interesting case. Having evolved in cold, highly oxygenated waters, trout are not terribly efficient in their use of oxygen but they don't need to be. They require more oxygen than do most fish species but they live in cold environments that are relatively high in oxygen. They peak at temperatures well below where most other species find tolerable which is why we find them in cold places were other fishes are not able to survive. As you may recall from somewhere, enzymes catalyze reactions (make it easier for reactions to happen) and enzymes function best within a small range of conditions. Our stomach enzymes function best at a low pH and almost all of our enzymes function most efficiently at about 98.6*F (about 37*F). Fish are much the same but being cold-blooded their enzymes function over a larger temperature range. However, enzyme function is still a function of temperature in cold-blooded organisms. All fishes have curves that describe an enzyme's function as it relates to temperature meaning that for all fishes, there are temperature "sweet spots".
To explore the curves of a few of these species, let us begin with the Gates Sockeye (a strain specifically evolved to a particular stream). Gates Sockeye are well adapted to a particular environment where stream temperatures remain within a relatively small range. Thus, this strain of Sockeye have very low thermal tolerance (which the Farrell paper explains in more detail). To look at Brook and Brown Trout, we see both have wide thermal tolerances but the shapes of the curves are much different. Brown Trout metabolism increased nearly linearly whereas Brook Trout show a strong peak at approximately 16*C (~61*F) but as temperatures warm, their drop in performance is steep. This very likely explains a lot of why Brook Trout "drop out" in warmer streams, particularly if there are not coldwater refuge areas available to them.
Why You Should Care About ANY of This
Ok, you have read this far and probably want something more out of this than my fish geek talk about bioenergetics, Fry curves, and thermal optimums.
For the angler, this may mean that in July and August as stream temperatures increase, access to slightly colder water will make all the difference in your success. Finding water that is 58-62*F may mean that the Brook Trout are active whereas in water only two or three degrees warmer will have them sluggish and uninterested in your offering. In the winter, it means finding the warmest water you can. Look for springs which watercress often gives away, small spring tributaries, and other inputs of warmer spring water. Fish before the snow starts to melt. Days that are about 33 to 36 degrees are perfect as your guides do not ice up and snow melt is slow. If there is one thing that will turn fish off in the winter, it is the stream temperature dropping.
Consumption (C) is a function of temperature. To move out of the fish world for a bit, you are probably familiar with the Marine Iguanas of the Galapagos Islands which have been in any number of nature shows. Marine iguanas feed on algae in the cold waters surrounding the islands but digest their food on dark volcanic rocks. This allows them to digest their food much more quickly which increases consumption because they can go feed again more quickly. Increased consumption generally means there is an increase in growth potential - so long as other factors like activity do not increase at a much faster rate. Fishes generally do not have this option although there are some lake and oceanic fishes that will forage and digest at different depths - and thus temperatures - to increase consumption. For the stream angler, it may mean that fish move throughout the river network, seeking optimal temperatures. The trade-off being that the increased activity to move needs to be compensated by increased consumption. As discussed in optimal foraging theory, there are costs to moving but if it means a greater density of food or higher quality food, the move may well be worth it.
Lastly, not all consumption is the same. I do not want to get too far into the weeds here - read the optimal foraging theory post for more details - but not all food is created equal. Just like our food, what fish eat varies in calories and nutritional content. Some prey items are high in fats and thus calories (fats have about 2 times as many calories per gram than do carbohydrates and proteins). Some prey items are easier to catch so A (active metabolism) in our bioenergetic model is lower which may make that prey item more profitable despite being lower in energy density (calories). And some items are less digestible than others meaning that F (egestion or feces) is higher which would limit the potential for growth. And for the angler, consumption - getting that fish to eat - is what it is all about.
For the conservationist, it means that connectivity is important as cutting off headwaters may extirpate (local extinction) Brook Trout as they will not have access to thermal refuge when lower reaches exceed their thermal maximum. It is part of why road crossings has received so much attention lately (that and access to spawning areas). It means that the effects of droughts may be long-lived if there is not a nearby source of colonizers so trout may not recover. Unfortunately, it means that some species or locally adapted strains with small thermal ranges may be victims of climate change.