All Hail the Silver King: what makes Tarpon so formidable? By Noah Bressman

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Early in June, I was fishing with a guide and a friend near Charleston, SC. My buddy, Ben, was fighting a good 4-5 ft blacktip shark when suddenly, my drag started peeling out and I saw a giant silvery fish jump clean out of the water, vigorously shaking its head. “Cut the line Ben, we’re going to chase Noah’s fish,” the guide said, to Ben’s disappointment. We chased the fish a quite a distance, with many jumps, runs, and gulps for air, not to mention plenty of stubbornness. After an hour, I finally landed the fish, my first Atlantic tarpon (Megalops atlanticus). My arms were completely dead from fighting that 100+ lb. fish, and it was a struggle to hold the tarpon’s head out of the water for a picture before releasing the beautiful animal. For once in my life, I needed to take a (quick) break from fishing to recover. How could a fish wear me down so much? To figure this out, I interviewed Dr. Jiangang Luo, a scientist at the Tarpon and Bonefish Center at the University of Miami, to learn about their fascinating biology.

Many of my friends were surprised to hear that I caught a Tarpon as far north as Charleston, SC, but according to Dr. Luo, this is quite common, depending on the time of year. Tarpon prefer a water temperature of about 26˚C (79˚F), and will migrate great distances to find water with their optimum temperature. Individuals tagged in the Gulf of Mexico and south Florida in the winter have been found in the Chesapeake Bay in the summer. While the Chesapeake Bay is typically as far north as tarpon will migrate, some individuals have been carried by the Gulf Stream all the way to New England and Canada. Tarpon will also migrate into brackish and freshwater regularly. Since many species of herring and shad also migrate to estuaries, and tarpon look like giant threadfin shad, tarpon were originally classified into the group containing herring and shad. However, tarpon were reclassified in the mid-20th century after it was discovered that tarpon share an unusual larval stage with eels, known as a leptocephalus. While tarpon begin to resemble shad at about a month old, says Dr. Luo, this is just a case of convergent evolution, and they look much more similar to eels as larvae. 

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The eel-like leptocephalus larva of the tarpon. (Image from the Bonefish and Tarpon Trust, https://www.bonefishtarpontrust.org/conservation/research/tarpon-life-cycle)

The large, silvery scales of tarpon may be one of the reasons they were thought to be large shad, but the scales have multiple functions. In addition to providing some defense to toothy predators, like hammerhead sharks, which are frequently known to steal tarpon from fishing lines, the scales may provide a few forms of camouflage. When schooling, the shiny scales make it difficult for predators to single out an individual fish in the school. Additionally, when in open water, the scales can reflect the open ocean environment, potentially making them appear somewhat invisible to predators. Dr. Luo hypothesizes another function of their large, overlapping scales: osmoregulation. Osmoregulation describes how fish balance the concentration of water and salts in their bodies. Many fish are unable to balance these concentrations as they quickly move between fresh and saltwater, because they may lose too much water or too many salts through diffusion to the environment. However, by essentially having an impenetrable shell of bone around their body, Dr. Luo thinks tarpon are able to quickly move between fresh and saltwater because diffusion through their skin is restricted by their scales. Aside from their scales, tarpon have many other fascinating anatomical adaptations. They have a giant, expandable, bony mouth that can inhale large prey whole using suction feeding (see previous post  on suction feeding). Because of their hard, bony mouth, it’s important to make sure you have a good hookset when you hook a tarpon, to prevent them from shaking the hook loose on a jump. Their giant mouths make a deadly combination with their giant eyes. Their large, reflective eyes not only allow them to see fine details and distinguish poorly-tied flies from prey, they improve night vision. While on a night dive at Lighthouse Reef Atoll in Belize, I saw about a dozen me-sized tarpon swimming around, sucking in everything in sight (including a foot and a half long goatfish, which was quite the sight!). They could do this because their eyes have a reflective layer called a tapetum lucidum, which reflects additional light from the moon and the stars into their retinas, allowing them to see better than other fish at night. 

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Musculature of a tarpon that washed ashore during a cold snap in 2010. Note the many blood vessels and red color of the oxidative muscle fibers, which give tarpon their endurance. (Photo courtesy of Dr. Jiangang Luo)

As for their fighting ability, they are a slab of muscle. Most of their body consists of muscle, giving them immense power. These muscles are highly vascularized and they have a relatively high concentration of red oxidative muscle fibers, which means their muscles have great endurance (essentially, the redder the muscle in a fish, the greater the endurance). As tarpon begin to tire, they start to gulp air at the surface. Since they can actually breathe air to some extent using a modified swim bladder, they get a burst of oxygen and energy each time they gulp, allowing them to recover a bit. Therefore, it is important to that you continuously put pressure on a hooked tarpon and do not give it a chance to relax and recover during the fight, or else the fight will last longer. Once you have caught the fish, though, it is very important to let it recover. Keep the fish in the water for taking pictures, and just raise the head out of the water quickly, to avoid further stressing the fish out. Dr. Luo highly recommends against not boating the fish, as a thrashing tarpon can cause damage to you, your boat, and itself. If you insist on weighing the fish, there are less-stressful methods to get weight than a scale at the dock, such as by taking multiple measurements with a tape measure (Ault and Luo, 2013). Additionally, once you are done taking pictures, please revive it with “fish CPR” by holding its mouth open and moving the fish through the water. If the fish is too large to move by hand, hold its mouth open and gently drive your boat so that water moves past the tarpon’s gills. You will know when the fish has revived enough because it will try to swim away on its own. Both CPR and not boating tarpon greatly increase their chances for survival after being caught, so they can continue to reproduce and allow other sportfishermen to enjoy these amazingly powerful and exciting fish for generations to come. 

Noah Bressman is a PhD candidate studying fish biology, functional morphology, and biomechanics at Wake Forest University. Below is Noah with an Atlantic Tarpon (Megalops atlanticus), caught on a live menhaden shad by a sandbar in Charleston, SC.

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References

Ault, J. S., & Luo, J. (2013). A reliable game fish weight estimation model for Atlantic tarpon (Megalops atlanticus). Fisheries research139, 110-117.

For additional information on tarpon, check out the Tarpon and Bonefish Research Center’s website: http://www.bonefishresearch.com/index.html

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