The Science Behind Tarpon

The proverb “know your quarry” applies to fly-fishing for tarpon as much as it does to hunting. The more you know, the better your odds are of finding them and coercing them to bite a hook bound with feathers, fibers and animal hair. Additionally, increasing your knowledge will inevitably build your respect and admiration for this remarkable fish.

Relative to other economically significant fish, tarpon have received little attention from the scientific community. However, there have been a handful of research projects dedicated to tarpon, and these have greatly increased our understanding of the silver king.

Fossil Record and Life History
While dinosaurs like T. rex and triceratops roamed terra firma during the Cretaceous Period (about 120 million years ago), ancestors of modern-day tarpon shared the ocean with other prehistoric fish. Mother Nature must have known a good thing when she saw it, because as far as evolution goes, the transformation of these early fish into Megalops atlanticus (the tarpon we pursue today) was complete about 18 million years ago. To put this in perspective, humans did not branch away from chimpanzee ancestors until 6 million years ago.

Despite spending the majority of their lives in coastal waters, tarpon actually begin life far from it. Spawning takes place offshore in the Gulf Stream waters of the Atlantic Ocean (about 20 miles east of Florida) and in the Gulf of Mexico (about 150 miles west of Florida). This has been verified through both satellite tagging and the capture of tarpon larvae. Tarpon have a unique larvae phase, called leptocephalus, that they share with bonefish, ladyfish and eels. In the first development stage, leptocephali have a limited ability to adjust for the ocean’s salty environment. Therefore, they must be born in waters with stable salinity. This explains why tarpon spawn offshore. If these leptocephali were in Florida Bay, the tide change would alter the salinity of the water and the larvae would die from either shrinking (in higher salinity) or exploding (in lower salinity). Ironically, scientists conducting research projects in the 1970s and 1980s tried to discover tarpon eggs and larvae by chasing nearshore tarpon schools. It was not until the 1990s that large numbers of larvae were discovered 150 miles west of St. Petersburg, Florida, in the Gulf of Mexico. Another interesting aspect of leptocephalus larvae is their unique behavior of being swimming machines that do not eat, as opposed to the larvae of, say, snapper or billfish, which feed immediately after birth. Instead, leptocephalus larvae have long, slender bodies and very low energy requirements, and they avoid predation by being transparent. During this phase, their teeth point outward and work similarly to a weed guard on a fly to prevent items from entering their mouth. There has been some discussion in scientific literature about whether the teeth are used to pierce jellyfish and suck out nutrients — but this is still a topic of debate.

When leptocephali reach their final development stage, they swim inshore and settle in stagnant lagoons that act as nurseries for the young fish. Because these areas have poor water exchange, oxygen levels are low, which keeps out predatory species that require more oxygen, like sharks. Here juvenile tarpon thrive. Predators are limited, and tarpon can breathe by taking gulps of air from the surface and feed on other low-oxygen-requiring species, such as guppies, killifish and finger mullet. Anglers can use these nursery areas to their advantage. If you are fishing in the tropical waters of the west Atlantic, keep an eye out for stagnant bodies of water; you could very well find a good number of juvenile tarpon.

At the age of about 3, tarpon move out of the lagoons to find both more and larger prey. They spread out to a variety of nearshore habitats and have been documented to actually follow schools of mullet and other prey.

When tarpon reach sexual maturity around 8 years old, their growth and behavior change. Instead of primarily increasing in length, they increase in both length and girth. And they begin to join other mature tarpon in spawning aggregations. The growth rates are different for each sex. Males are smaller and rarely exceed 100 pounds. This is because males don’t need to have a large body to spawn as many times and with as many females as possible. Females, on the other hand, grow to larger sizes to produce and release as many eggs as possible. Think of eggs as marbles, and consider the number of marbles you can hold in a coffee mug versus fit in a five-gallon bucket. A larger container equals more eggs.

Morphology
Tarpon’s body structure helps them both capture and avoid becoming prey. Their forked tail permits fast, sustained swimming speeds and can propel them acrobatically out of the water, a virtue loved by fly-anglers. The upturned mouth of these fish has a massive gape that allows them to flare their gills to suck in water and inhale prey into their “bucket mouth.” Watching this process in action is precisely why many anglers pursue the silver king.

Tarpon have an interesting way of reducing mortality from predation. Most marine fish have spines to poke a predator when one attempts to consume them. Tarpon do not have spines but instead wear armor in the form of large, thick scales. A predator not only needs to catch these fast fish but also has to pierce the scale wall.