The Science of Tarpon Vision

This is the first article in a series that will reveal research results on tarpon. There are so many fascinating results that it’s difficult to decide where to begin. I’ve decided to start with tarpon vision because I’m confident this topic has sparked many conversations on the bows of flats skiffs.

Let’s begin with the first time tarpon were discussed in the scientific literature. The zoologist Achille Valenciennes first described tarpon in 1847. Achille noticed their large eyes, which inspired him to choose the scientific name Megalops atlanticus for this amazing gamefish. Megalops means large eye and atlanticus refers to the Atlantic Ocean they inhabit. These large eyes make tarpon effective visual predators both during the day as well as at night. During the daylight hours, the tarpon’s large eyes help it to see fine details. At night, the large eyes increase visual sensitivity. These large eyes also have a layer of cells, called a tapetum, that act like reflective tape at the back of the eye. This is why the tarpon’s eyes glow at night when you shine a light on them. This “reflective tape” contains high densities of light-detecting cells and maximizes the capture of light. This gives tarpon a visual advantage when feeding at night or in other low-light conditions such as turbid water. This also explains why anglers often find tarpon in shadows under mangroves, docks, and bridges. Essentially, in low-light conditions tarpon can see their prey better than their prey can see them.

If I had a nickel for every time a guide yelled at me, “Put the fly in front of the fish!” I would be a rich man. Research has shown that this is not simply a form of ridicule; in fact, the

guides are correct. Research on the location of vision cells around a tarpon’s eye reveals that its eyesight is keenest in upward- and forward-looking regions. This makes sense, considering it has an upward-turned mouth. Keep the location of their vision cells in mind when you are casting to them, as the best placement for an offering is above and in front of a tarpon’s face. This also correlates with research on how tarpon feed. Some scientists refer to a tarpon’s feeding method as a suction strike. The tarpon will approach its prey from below with a closed mouth and closed gill covers. Then, when the tarpon is ready to strike, it lunges forward as the mouth opens and the gill covers flare out. The flaring of the gill covers creates suction, allowing the tarpon to slurp down its prey. Thus, the tarpon’s upwardand forward-looking vision complements the suction-strike behavior.

The vision cells that determine what colors tarpon see are another matter. First you need some information on the biology of fish eyes, because they are very different from human eyes. Human color vision is fixed for life, so the colors you see as a child are the same colors you see as an adult. Fish eyes are different. Fish retinas contain stem cells that allow their eyes to change throughout their life. Many species of fish occupy different habitats at different

life stages, and their eyes will change to adapt to these different habitats. For example, salmon start out life in a river and then move to the ocean. A fish’s vision may also change if its diet changes over time: If a fish’s diet shifts from crabs to fish, its vision can change to improve its ability to see fish.

The colors that tarpon can see change throughout their life. The colors that juvenile tarpon can see are somewhat different from the colors seen by adult tarpon. I define a juvenile tarpon as having a fork length of 40 inches or less (a fish roughly younger than eight years old). Juvenile tarpon primarily see dark blue and a range of green colors. This is because they inhabit turbid waters that are dominated by colors within those wavelengths, such as colors within the green spectrum. So if you’re targeting juvenile tarpon rolling by the mangroves or in the backcountry creeks, throw a green fly to increase the odds they’ll see it. When tarpon reach adulthood (greater than 40 inches fork length), their ability to see colors in the green spectrum decreases, and their vision in the wavelengths shorter than greens, such as purples and blues, increases.

Adult tarpon also develop cells to detect ultraviolet (UV) light. Humans cannot see within the UV spectrum. For simplicity, think of UV light as a very

dark purple. Tarpon develop cells to see UV light, because when they become adults they spend more time in clear water, where shorter-wavelength and UV light are more abundant. It’s unknown why tarpon see UV light, but there are a couple of hypotheses. One hypothesis is that since some fish species reflect UV light for communication, tarpon may use their UV vision to detect the UV light reflected from prey. Another hypothesis, and the one that I believe, is that with UV light so abundant in the clear, shallow waters adult tarpon inhabit, the UV light creates a background against

Tarpon vision is keenest in upwardand forward-looking regions. The best place to put a fly is above the tarpon and in front of its face.

Tarpon have a collection of “reflective tape” cells in their eyes to maximize the capture of light. This gives them a visual advantage when feeding at night or other low-light conditions, such as under mangroves, docks, bridges and in turbid waters.

These are the conditions in which tarpon can see their prey better than their prey can see them.

which tarpon detect the silhouettes of prey that would otherwise be camouflaged. This could explain why a 200-pound tarpon will hit a 2-inch fly: Their vision enables them to see the small fly highlighted against a UV background.

Tarpon have no vision in the longer wavelengths of light such as the reds. This is because red light is quickly absorbed in the marine environment, so red quickly turns to black the deeper you get in the water. Red flies and lures probably just look black to tarpon.

Visible light spectrum with the location of peak color vision for juvenile and adult tarpon. Each red circle represents where tarpon have peak vision. The change from juvenile to adult is around 40 inches fork length.

What I find interesting is that the research on tarpon vision was published about 10 years ago; however, I have seen guides use purple, blue, and green flies for adult tarpon for the past 30 years. Over the years, the guides must have done their own trial-and-error experiments with tarpon flies of different colors to see what flies worked best. It’s very interesting to see the guides’ tarpon fly research results match the research done on tarpon vision. So if you learn nothing else from this article, remember to always listen to your guide.

Mike Larkin received his doctorate from the

University of Miami’s Bonefish and Tarpon Conservation Program. Highlights of his research include the discovery that bonefish in Florida migrate to the Bahamas, bonefish live to 21 years, and the completion of the world’s first bonefish stock assessment. He currently works as a fisheries biologist in St. Petersburg, Florida.

TAIL FLY FISHING MAGAZINE 33