Science of the fight

This post was published in the June/July edition of GAFF Magazine. The article can be found here.

I remember watching fishing icons like Bill Dance on Saturday mornings on TNN as a kid. Not much has changed. I still watch Saturday morning fishing shows with my bowl of Cinnamon Toast Crunch and chocolate milk, but these days I have a better understanding of what the fisherman is putting a fish through. It is quite interesting how a typical teleost (teleost is the group of fishes that have a true bony skeleton) responds to the stress of being caught.

Fish stress 2

Any stressor initiates a cascade of events within a fish, but the intensity of the stressor determines the level of the response. These levels can be classified as primary, secondary or tertiary responses. Tertiary responses will degrade a fish’s physical performance, growth, reproductive capacity, etc., and does not entirely pertain to the discussion here. When a fish is initially hooked it perceives the event as a stressor. (There is some debate on whether or not fish feel pain, like when being hooked. Fish lack the cortex of the brain typically used in processing pain, but new data suggests they have other ways of processing noxious stimuli. I think the jury is still out on a definitive answer.) In a primary or secondary response there is an increase in production of specific hormones called corticosteroids and catecholamines. These hormones prepare the fish for a fight or flight response by: increasing the permeability of water and dissolved ions across the gills and into the blood; increasing the oxygen consumption rate; increasing heart rate; and increasing blood flow to the gills and muscles. Essentially the physiological changes are “fine tuning” the fish for a peak physical performance to overcome the stress of being caught. Heat shock proteins are activated from the front portion of the fish’s kidney. (The kidney is the dark red line right below the spine. When cleaning a fish you probably take your thumb and run it along this line to remove it. It isn’t blood. It’s the kidney.) These proteins are incredibly important because they act as molecular chaperones that aide in moving and folding other vital proteins. As the stressor is prolonged, the accumulation of corticosteroids and catecholamines will trigger an increase of glucose. This is important because the white muscles found in fish use the glycolysis pathway to transform glucose into a hydrogen ion and compound called pyruvate. This transformation releases energy and produces a high energy molecule called adenosine triphosphate (ATP), which is the molecule that all life on Earth uses as energy. Once the glucose levels are used up, the fish tires. Furthermore, a byproduct of glycolysis is lactic acid. If the angler fights a fish too long, lactic acid will accumulate in the musculature of the fish and create a condition called acidosis. We have all seen a fish played until exhaustion, and it is the crystallization of the lactic acid in the muscles that prevents them from working properly and the fish swimming off. In my opinion, the heaviest tackle (within reason) must be used to fish for a particular species. We as fisherman tend to use extremely light tackle to increase the fight in the fish, but what this really does is allow for glucose levels to be quickly depleted and lactic acid to build up. Heavier tackle will reduce the fight time and lactic acid accumulation. Is your head spinning yet?

Once an angler lands a fish, they do their best to revive and release it so future anglers can enjoy it too. What most anglers do not realize is they intend well, but many are counterproductive when releasing fish. Let me explain. We all understand a fish pulls dissolved oxygen out of the water with its gills. It’s how they breathe. Water enters a fish’s mouth and passes over the gills. What is less known is how this actually works. The blood in the gill lamellae (lamellae are the bright red structures we see in gills) flows in the opposite direction of the water. This is known as counter current gas exchange. If the blood and the water flowed in the same direction (i.e. the blood and water flowed from the head to the gills), the oxygen exchange between the water and the blood will be very poor. Very inefficient. An oxygen concentration equilibrium will be achieved if the oxygen concentration of the blood closely matches that of the water, and therefore no oxygen will be dissolved into the blood stream. This makes for a bad day in the life of a fish. However, if the flow of the blood and water are in opposite directions then this scenario ensures the highest oxygen concentration in water is in contact with the lowest oxygen concentration in the blood. The maximum amount of oxygen can be delivered to the blood in this situation. Make sense? Let me explain it in other words. A study of water flow across the gills was conducted in 1963 to record the gill’s efficiency of removing oxygen from the water. What was discovered was fascinating. When water flows in the natural direction – from the head to the gills – the gills are 80% efficient in pulling oxygen from the water. Now reverse that flow – from the gills to the head – and the gill’s efficiency to pull oxygen from the water drops to less than 10%.


Once a fish is landed it is a natural reaction for an angler to “pump” the fish back and forth to the increase water flow over the gills, but what happens is the angler is being less efficient in reviving the fish than allowing the fish to recover at its own pace. We are all guilty of this at some point or another, and maybe it is science’s fault for not properly educating the public on this manner. If an angler must aide in the fish’s revival, then the appropriate action is to pull the fish in a circle or a figure eight type pattern. This forces water through the mouth and over the gills – the natural direction of flow. If the angler is in a river, point the fish’s head up river to allow the current to passively flow over the fish’s gills. Remember, 80% is much better than 10% any day of the week and twice on Sunday.

Hopefully, this “brief” explanation has scratched your curiosity’s itch. I don’t know. Maybe it didn’t. I also hope it didn’t leave your head spinning like it did in your high school freshman biology class. I find it quite fascinating to know exactly what a fish is doing right down to the molecular level. The more knowledge we have, the better equipped we are to preserve and conserve our beloved sport. Keep this in mind the next time you have a fish on the line, and it will allow the best possible scenario to ensure its survival. Minimize the time it spends out of water, take a quick photograph and release it to fight again another day.


– Chris


For further reading:

Barton, B. A., 2002. Stress in fishes: A diversity of responses with particular references to changes in circulating corticosteroids. Integrative and Comparative Biology 42:517-525.

Hughes, G.M. 1963. Comparative physiology of vertebrate respiration. Cambridge, Mass: Harvard University Press. 146 pp.

Ferguson, R.A. and Tufts, B.L. 1991. Physiological effects of brief air exposure in exhaustively exercised rainbow trout (Oncorhynchus mykiss): Implications for “Catch and Release fisheries. Canadian Journal of Fish and Aquatic Sciences 49:1157-1162.

Meka, J.M. and McCormick, S.D. 2004. Physiological response of wild rainbow trout to angling: Impact of angling duration, fish size, body condition, and temperature. Fisheries Research 72:311-322.

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