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Guest Blogger: Michael Vorhis, author of ARCHANGEL suspense thriller, OPEN DISTANCE adventure thriller & more to come

Ever hear those ubiquitous analogies about trout fatigue, such as “a hooked fish fighting your rod exerts more energy than a human doing three sets of heavy squats followed by five consecutive 100-yard dashes”? Every wonder from whence those exaggerations come…how much empirical calculation goes into them…how accurate they really are?

Spoiler: They’re not calculated. They’re more what you’d call a colorful explanation. They’re trying to make an impression. But…what’s the real science behind such claims?

Gills vs. Lungs

For extracting oxygen from the medium around them, gills are actually far more efficient than lungs. Their problem is that they need to be. The best of water holds a tiny fraction, volume for volume, of dissolved oxygen compared to what comprises air. At sea level one litre of air will include over 200 milliliters of oxygen (21% by volume…it’s the same percentage at altitude, although the oxygen molecule count reduces as you go up the mountain due to the air’s expansion). By contrast, a litre of 25-degree-C water has at most 5 milliliters of O2 dissolved in it (half a percent or less). So assuming muscles all need a comparable amount of oxygen to do a given amount of work, gills need to encounter 40 times more water, by volume, than do lungs. And warmer water can show a far greater disparity, since water’s ability to dissolve O2 depends on temperature and other life within it.

That’s just looking at it volume-for-volume. If we assess the energy required to do the oxygen harvesting, the picture looks even more bleak. Moving 40 times the water across gills uses far more than 40 times the energy, compared to moving air in and out of lungs. Two grams of oxygen can be gleaned from about 7 grams of air moved into and out of lungs…which is relatively light work. By contrast, getting two grams of oxygen by flowing water across a brace of gills requires pumping more than 441 pounds of water past them (or else swimming them past that much water). The water “moved” is nearly 29,000 times as heavy as the air moved by a lung.

So for an organism to extract sufficient oxygen to serve muscle metabolism, its oxygen extraction equipment must be far more efficient if the O2 is coming from dissolved O2 in water. Gills are far more efficient, but not enough to fully offset their challenges. The net assessment is that a gill/water system falls short of a scheme using lungs and air.

As another way of illustrating the challenge of moving that much water, our human lungs could actually get oxygen from liquid water, but the muscles that operate them are far too weak to move that much water in and out. Trying to breathe in water would simply boost our need for yet more oxygen to drive those lung-driving muscles; we’d pass out in very short order due to asphyxiation long before we get enough…and drown. (Science fiction and Navy research have attempted to address this reality using special liquids that include phenomenal amounts of dissolved oxygen, such that small volumes could be pumped in and out by our breathing muscles and still get enough oxygen to keep the human being alive. To my knowledge this remains in the realm of theory, experimentation, and action thriller literature.)

Size of the Animal
Ever wonder why the biggest fish, who don’t need to support their full body weight, aren’t monstrously bigger than the largest mammals? Oxygen is the reason. They cannot get enough of it to support more than a certain body size. Some are big, but there’s a limit; and the bigger they are, the more they must swim, very efficiently and all the time, dragging those gills through huge quantities of water. When whales (courtesy of their pre-existing mammal legacies) ‘chose’ lungs and air over gills and water-dissolved O2, they kept the doors open to significantly larger body sizes for themselves, and to higher metabolic rates courtesy of the plentiful oxygen they could tap into. It enabled them to support much larger body sizes and to hunt and evade despite the water’s temperature and oxygen content. They became the advantaged ones of the ocean. (And back in the dinosaur days, air held up to six times the oxygen molecule count it does today; every air-breather could grow to enormous size and have enormous amounts of energy.)

So to survive, fish must extract oxygen very efficiently, and they must also do less work per minute–they must limit their energy expenditure. They must contain their body mass to a manageable size–to stay within an acceptable ratio of body mass to oxygen-harvesting-system capacity. As they grow, their gill surface area must increase exponentially…else they’ll starve their muscles of O2 with the slightest exertion. (So we can blame the lacklustre average size of the fish we catch on that stupid water and its stupid inability to hold more stupid O2….)

Aerobic vs. Anaerobic
Any muscular system has two metabolic modes: Aerobic and anaerobic. The first one is a mode that performs a degree of work sustainable for long periods of time (nutrients and hydration permitting). The organism’s ability to harvest oxygen from its surroundings and supply it to the muscles at the rate those muscles are consuming it permits this–the oxygen-harvesting system is sized to handle that workload.

Obviously this amount of sustainable work is modest in any given moment, else the oxygen supply rate would be exceeded. In fish, aerobic motion includes cruising at speeds intended for distance, swimming at speeds necessary to hold against modest currents, and the moves required to maintain position within a flow (such as to hold posture and bearing while in feeding stations). Steady-state swimming is aerobic work; the gills have the ability to harvest and deliver oxygen at a sustainable rate during these activities.

ANaerobic mode is, obviously, “not aerobic.” In solid state memory access vernacular, it’s “burst mode.” Anaerobic motion makes an animal capable of extreme speed, extreme maneuvers, to catch prey or to avoid being preyed upon. It’s a mode crucial to survival; maximum muscle power occurs as a result of anaerobic energy production. But activating it comes at a price.

The price is that the oxygen needed to fuel such a high-output instantaneous deed must be made up after the fact. It’s a little like when you tell your boss on a Friday evening that the big system validation report is 99% done…then over the weekend you’ve gotta make it so, or you’re a goner. When anaerobic work is performed, an “oxygen debt” is built up, and very quickly. Failure to pay it back very soon after the work is done results in failure of the muscles themselves, then the brain…and asphyxiation follows all too soon.

Fish use anaerobic motion to chase evading prey short distances, to avoid otters and bears, to wriggle out of a hawk’s talons, to leap waterfalls, to sweep redds clean of gravel, to deposit eggs or milt, to pull and dart against a fly rod. Depending on the species, in evasion scenarios they’ll take themselves to the point of system collapse on the physiological ‘belief’ that they’ve nothing to lose by doing so–they risk death either way. So they’ll go until their muscles fail, which isn’t very far from where the brain fails too.

Immediately after building an oxygen debt, a fish has to cut its energy expenditure and harvest oxygen…to pay its debt. It needs to drop activity well below the steady-state aerobic threshold and park somewhere in current that brings a lot of water over its gills…or else fan or cruise water across them very efficiently. Those gills are good at the job, but again the amount of oxygen in the surrounding water isn’t much. The exhausted fish is seriously depleted and may no longer know much of what’s going on or what just happened. It’s got a few moments to set things right or lose it all. It may even still be in your hands but not be aware of it.

Warmer water holds less oxygen than cold. In it, cold-water fish are already near the oxygen-debt threshold when doing the slightest amount of swimming work. They might have a short burst or two left in them, which they can attempt out of pure courage, but their capacity to survive a sustained fight against an angler’s rod, or to recover after even a couple of brief sprints, is severely compromised. They just can’t harvest the oxygen quickly enough to come back from such an ordeal.

Hypoxia in Fish
Like mammals at high altitudes, fish are susceptible to hypoxia–to oxygen depletion so serious it threatens survival of cells and systems. Fish species differ in their degree of hypoxia tolerance. A fish’s tolerance can be represented in different ways, including critical oxygen tension (the lowest water oxygen tension at which a fish can maintain a stable oxygen consumption rate), or the amount of time the fish can spend at a particular hypoxic level before it loses dorsal-ventral (i.e., vertical) equilibrium. It depends on the species. We’ve all seen fish that kept going belly-up while we tried to revive them.

Like ourselves, fish respond to hypoxic states with a combination of physiological, behavioral, and cellular responses in an effort to recover basic function. Since 95% of the oxygen consumed by fish is used for ATP production (often called “molecular currency,” ATP facilitates energy transfer cell to cell), the greatest challenge hypoxic fish face is maintaining metabolic energy balance while they try to recover. Therefore, for a fish to survive hypoxia requires a coordinated response–physiological and behavioral–to secure more oxygen without delay. This is where our actions can assist–we can be part of the behavioral side of things–don’t just put the fish under the surface, keep the water around it moving.


The common simplistic colloquial “sprinter analogies” are entertaining but don’t really explain the reasons or realities of the gill-wearing life. Hopefully the above information gives a little more detail. Bottom line, while “X number of 100-yard dashes” is not a characterized value, we can still relate experientially. Fish are a lot like us in their reliance on oxygen to enable the tasks that survival requires, and we both have physical limitations we go up against from time to time…and a few seconds can make the difference. Handling those occasions even a tiny percent more efficiently can mean the difference between acquiring energy and being ‘acquired as energy’ by something else.

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