SCIENCE OF TARPON

by Michael Larkin, PhD. Photos by Neal Rogers, Brian O’keefe, Greg Poland

A tarpon’s forked tail permits fast, sustained swimming speeds and allows it to propel itself out of the water.

MORPHOLOGY AND COLORATION

The first tarpon ancestors arrived on this planet 160 million years ago. The tarpon we fish for today (Megalops atlanticus) arrived 23 million years ago. That’s 137 million years of evolution to make the perfect gamefish! The form and function of the fish was tinkered with and modified until evolution settled on today’s Silver King. Here are evolution’s results: The body structure of the tarpon helps it to both capture prey and to avoid becoming prey. Its forked tail permits fast, sustained swimming speeds and allows it to propel itself out of the water. Tarpon scales are thick and act as a protective suit of armor. However, the scales also protect them by making it difficult for visual predators to focus on them. The scales along their sides are overlapping and have mirror-like reflective properties. These scales flash sunlight, making it difficult for predators to focus on a specific part of the tarpon. This is especially helpful when tarpon school, because light flashing from the scales makes it difficult for predators to single out a specific tarpon.

Tarpon scales also have coloration to camouflage them. Tarpon are dark along their backs and light on their undersides. This is called countershading. The dark coloration on the top of the fish helps them blend in with the dark waters below them. This allows tarpon to hide from either predators attacking from above or prey looking downward. The light coloration on the underside of the fish helps them blend in with the light streaming down from above. So when a predator or prey look up at tarpon swimming above, the tarpon will blend in with the light shining at the water’s surface.

Tarpon are dark along their backs to help them blend in with the bottom or deeper waters below when a predator or prey views them from above. Tarpon have light coloration on their undersides to help them blend in with light from above when viewed from below.

Stiff Tongue Plates on the roof of the mouth.

HOW TARPON FEED

Tarpon have an upturned mouth with a massive gape. They flare out their gills to suck in water and inhale prey. The large “bucket mouth” has plates on the roof, which work in tandem with a stiff tongue to crush prey. This results in the tarpon’s mouth acting as if it were lined with concrete and makes it difficult to set a hook on them.

CATCH AND RELEASE

Tarpon are long-lived fish (greater than 50 years), and their longevity makes them sensitive to population decline from fishing mortality. To ensure we have abundant tarpon populations for years to come, please follow the advice below, which was determined from research projects that explored tarpon release mortality.

AVOID GUT-HOOKING THE FISH

As expected, gut-hooking a tarpon will increase its chances of mortality. I understand this is a fly fishing magazine and this action primarily relates to using live bait. Just keep in mind that fishing with live bait increases the chance of gut-hooking a tarpon. Limit the amount of time you let the tarpon eat the bait before setting the hook to reduce the probability of gut-hooking the fish. Also, circle hooks have been shown to reduce gut-hooking, so use them when you can. (Using flies tied on circle hooks is also a good idea, not to mention a very effective way to hook tarpon.)

AVOID THE GILLS

The tarpon’s gills are vital to its survival. Even though tarpon can breathe air, their gills are still the primary mechanism they use to obtain oxygen. The gills are sensitive to physical damage and should be left alone at all times. If a hook gets lodged in the gills, then carefully remove it. Keep in mind that the best option may be to cut the line

Tarpon gills have exntensive surface areas. Never touch!

and gently pull the hook out though the gill cover. Never reach your hand through the gill cover to subdue a tarpon, as your hand likely will damage the gills.

Never touch the gills of a tarpon. The gills are sensitive and this can kill the fish. If you need to control the fish, focus on holding the mouth with one hand and supporting the fish’s body with the other hand.

REVIVE THE FISH

Don’t just pitch the tarpon off the side of the boat to release it. Instead, keep the fish in the water and allow water to pass over the gills. If you’re in an area with no current, then motor the boat slowly to force water to pass over the gills. This will give the fish a chance to recover from the fight. Ensure the fish is swimming on its own before you let it go, and keep in mind this may take at least 20 minutes.

Always take the time to revive the fish before releasing it. Keep the tarpon’s head in the water and allow water to flow over the its gills. Don’t let go until the fish is responding and swimming on its own.

HOW MUCH DOES YOUR TARPON WEIGH?

Catching a large tarpon and only knowing its length is like ordering chicken at a steakhouse: It will get the job done but you will have missed out. This is because length only captures one dimension of the threedimensional fish, whereas weight captures the complete size of the fish. However, lifting a large tarpon out of the water to get the weight is a risky exercise for both you and the fish. The tarpon can flop around and hurt itself, and you could get smacked or knocked overboard if the fish is not under control. A safer way to determine a tarpon’s weight is to keep the fish in the water and measure its length and girth. In the 1920s William W. Wood developed an equation for predicting a fish’s weight from length and girth measurements. Wood’s formula is simple enough that it can be easily calculated on piece of scrap paper. The equation is:

WEIGHT = (GIRTH SQUARED * FORK LENGTH)/800

Weight is in pounds and both girth and fork length are in inches. Wood’s formula is an easy way to provide a ballpark estimate of your fish’s weight. Wood’s equation was developed by assuming a fish’s body shape is similar to two cones placed together. The closer the fish’s body resembles two cones placed together the more accurate the equation is at predicting the weight. The shape of the body of some fish, like bonefish, are similar to two cones placed together, which makes the equation very accurate for bonefish.

A problem with Wood’s equation is that the bodies of some fish deviate from the shape of two cones. If you apply the Wood’s equation to tarpon, it results in a weight that can be underestimated by as much as 15 percent. So a tarpon that weighs 180 pounds could be predicted by Wood’s equation to be only 153 pounds. That’s a big difference! To fix this

problem, scientists at the University of Miami developed a new equation specifically for predicting tarpon weight called the Ault-Luo tarpon equation. Instead of using Wood’s assumption of the body shape of two cones, the Ault-Luo tarpon equation assumes a tarpon’s body represents an ellipsoid. The downside of the Ault-

Luo equation is you probably won’t be calculating it on a piece of scrap paper: A calculator is needed.

Here is the Ault-Luo tarpon equation for predicting a tarpon’s weight: Weight = 2.828 + 0.0000296*(Girth squared)*(Fork Length) + 0.006123*(Girth*Fork Length) + -0.008284(Girth squared) + 0.1845(Girth) + -0.1943(Fork Length) Weight is in kilograms, and both girth and fork length are in centimeters.

WHY DO TARPON ROLL?

Tarpon have extensive gill surfaces to obtain large amounts of oxygen to maintain their large bodies and active lifestyle. In addition to these gills, their gut is connected to their swim bladder, allowing them to swallow air from the water’s surface. Once the air reaches the swim bladder, oxygen is removed by “lung-like” tissue. This unique swim bladder gives tarpon two methods to obtain oxygen.They have the capability to switch between the two methods, but it depends on the conditions. For example, the primary organ for obtaining oxygen in water with normal oxygen levels is the gills; however, in water with low oxygen levels the primary breathing organ is the swim bladder.

There are advantages to having such a swim bladder. It allows tarpon to live in water with low oxygen levels where there is food (such as other fish tolerant of low oxygen levels: guppies, killifish, and juvenile mullet) and very few predators (e.g., sharks). Tarpon can enter and survive in oxygen-poor stagnant pools and mangrove-lined

estuaries. This advantage is especially important for juvenile tarpon, because it allows them to thrive in a protected habitat during this critical life stage. Anglers can use the behavior to their advantage. If you are fishing in tropical waters of the West Atlantic, keep an eye out for stagnant waters because you might find tarpon there.

Another advantage to the the tarpon’s air-breathing behavior is rapid recovery of oxygen debt, because the air contains considerably more oxygen than water. Air contains about 21 percent oxygen, whereas water has less than 1 percent oxygen. This is why tarpon will roll at the surface during an extended fight; also, this is why tarpon are able to fight for such long periods of time. This also explains why experienced tarpon anglers will attempt to prevent the tarpon from gulping air by using the “down and dirty” technique. By doing this, the angler is essentially keeping the tarpon from catching its breath. Experienced tarpon anglers will target tarpon at sunrise. At sunrise there is enough light to see around you, and tarpon often show themselves by frequently rolling at this time. The tarpon are rolling because dissolved oxygen levels are lowest at sunrise because photosynthesis (the chemical process that requires light to produce carbohydrates and oxygen) has ceased during the night. The low oxygen levels force tarpon to roll frequently to meet their oxygen demand. Thus, the early-bird angler has the advantage of knowing where the tarpon are located.

Tarpon also roll frequently at night. Tags with depth sensors deployed on tarpon found that rolling activity peaked at night. Again, this is because of the low oxygen levels due to the disruption of photosynthesis. Keep this activity in mind when tarpon fishing at night because on calm nights you can hear rolling tarpon. Water temperature also plays a part in the frequency that tarpon roll. If the water temperature exceeds 79 degrees F, a tarpon will increase its rolling frequency to adjust for its elevated metabolism and the lower oxygen concentration of the water (the higher the water temperature, the lower the water’s oxygen level). This increased rolling behavior can be used as an indicator of the presence or absence of tarpon in an area. If you’re fishing in waters above 79 degrees F and you don’t see any tarpon rolling, then it’s a safe assumption they’re not in the area. Move on.

There’s more to a tarpon’s rolling than simply breathing. Tarpon also roll to adjust their buoyancy. When a tarpon approaches the surface to roll, it will first exhale air to empty its swim bladder. Then it gulps air from the surface to refill the swim

bladder and adjust its buoyancy. If it gulps too much air, the fish will expel bubbles on the way down. This leaves behind a type of “bread crumb” trail revealing the tarpon’s direction of movement. By relating the tarpon’s roll location to the expelled bubbles, it’s possible to determine the fish’s direction of movement. Cast ahead of the bubble trail!

COPY CAT

As already discussed, a tarpon’s rolling allows for breathing and to make buoyancy adjustments. There is also a behavior component to rolling. Tarpon will roll to imitate one another. In a school of tarpon, if one tarpon rolls then most likely another tarpon will also roll within one second of the first. In fact, scientists explored this and determined that if one tarpon rolls there is a 70 percent chance that a nearby tarpon will roll also. The scientists placed glass dividers in the tank to see if the behavior was determined from visual observations, or if the tarpon detect others rolling using their lateral lines. The tarpon separated by the glass dividers still imitated each other, revealing that their behavior depends on sight. What’s really interesting is that the scientists took this experiment even further by placing a painted wooden tarpon in a tank with a school of tarpon and engaged the wooden tarpon in rolling behavior. Sure enough, they were able to get the other tarpon in the tank to imitate the rolling of the wooden tarpon. Then the scientists attached some string to three separate items: a white spatula, a piece of red rubber tubing, and a solid glass rod. They moved the string to get each item to imitate rolling tarpon, and again they were able to get the tarpon to roll with these objects. The scientists found that the key to getting a tarpon to imitate these crude objects was the way the object approached the surface of the tank. If the object was raised too slowly, or raised with the object parallel (instead of at an angle) to the surface, the tarpon ignored it.

WHY DO TARPON JUMP?

When a fish is confronted by a predator it has limited choices. It can aggressively defend itself, engage in evasive behavior, or flee. A tarpon jumping out of water is a combination of the latter two choices. Jumping out of the water allows the tarpon to elude the predator, and then the tarpon may separate itself from

the predator enough to allow it to flee. There is a physical advantage to jumping out of the water. Water is 800 times more dense then air. Therefore, jumping out of the water affords the tarpon significantly more maneuverability.

There have been many observations of tarpon jumping out of the water when a large shark, such as a hammerhead, is chasing them. Why tarpon jump when hooked is still not clear. There must be some stress response caused by the pulling of the line that triggers the jumping behavior. The tarpon may be jumping to take advantage of the additional maneuverability from being airborne that assists with dislodging the hook.

WHY DO TARPON FORM A DAISY CHAIN?

Tarpon are certainly not the only fish to daisy chain. Jacks and koi also daisy chain, and other species daisy chain as well. It’s still unclear why tarpon engage in this behavior. One hypothesis was that the behavior was related to spawning, such as courtship or pre-spawning behavior. However, this hypothesis was disproved because tarpon spawning occurs offshore, and the majority of observed tarpon daisy chains occur in nearshore waters. Also, in the late 1930s, scientist deployed plankton nets around daisy-chaining tarpon with the hope of capturing fertilized tarpon eggs. They were unsuccessful. To further debunk this hypothesis, juvenile tarpon have been observed forming daisy chains. Another possibility is that the daisy chain is a defensive behavior. It allows the tarpon to form a wall of flashing silvery scales, which makes it difficult for predators to single out a specific tarpon. Additionally, the circle allows tarpon to use their excellent eyesight along with their lateral line to continuously scan for predators in every direction. Predators will be quickly detected, and the predator’s element of surprise will be eliminated. There is anecdotal evidence to support this hypothesis. Captain Will Benson of Key West, Florida, has observed a string of migrating tarpon stop about 100 feet from his boat and start daisy chaining. The tarpon act as if they sense danger ahead, and they form a daisy chain for protection. In these cases, the tarpon frequently turn off the fly and refuse to eat. Will has also seen spooked tarpon swim over to join a daisy chain.