This essay originally appeared in the Lakeland Times in Minocqua, Wisconsin. Carol is an Aquatic Invasive Species expert and outreach specialist with the CFL and Wisconsin DNR.
by Carol Warden
Throughout my life I’ve been surrounded by water. Whether it was lakes, rivers or the sea, the water and I have had an intimate relationship. I remember thinking as a child, “These fish are so lucky; they never have to get out of the lake. They get to swim all day. How do they breathe underwater and what’s stopping me from doing it?”
A fish depends on oxygen just like we do except fish breathe oxygen that is dissolved in water. Instead of having lungs to breathe, a fish has gills. Lungs and gills have the same purpose; they both supply the body with oxygen. Unlike human lungs that sit inside our bodies, gills of a fish are on the outside of their bodies. They sit right behind the mouth on both sides of the fish like where an ear is on our head. On some fish, the gills curve from the eye to the mouth. On other fish like sharks, the gills look like slits from a knife on the side of their bodies.
Most fish can open their mouths and let water in then when they shut their mouths the water is forced out past their gills and the gills absorb the dissolved oxygen in the water. Other fish, like tuna, don’t have the best system to force this water out and so they have to swim continuously so water is constantly passing over their gills.
Gills are very delicate, almost paper-like structures that depend on the buoyancy of water to remain open. The fragile gills will collapse if the fish is taken out of the water because air doesn’t have the same density and buoyancy water has to keep the gills open and functioning. This is why a fish cannot live outside the water. Gills also have much more surface area proportionate to their bodies than our lungs do proportionate to our bodies. This added surface area allows the fish to be very efficient at removing oxygen from water. Since water only holds a fraction of oxygen air does, it is important for fish to be really efficient breathers. Some fish can remove up to 85 percent of dissolved oxygen from the water that passes over their gills.
Alas, I will never be able to join the fish and swim all day. Gills are great for air and water exchange while our human lungs are only adapted to exchanging air. If our lungs were on the outside of our bodies like fish, we would lose too much water from our bodies because we don’t have the constant replenishment of moisture like fish do living in water. The same is true for a fish out of water. Even if their gills did not collapse, they would eventually lose too much water from their bodies to survive. They require a constant flow of water over their gills to absorb dissolved oxygen and to keep their gills moist. Also, our lungs do not have enough surface area to pull out the amount of oxygen from water that we humans require to live. I’m condemned to a life on land.