Surprising Factors That Can Attract Sharks While Swimming

Sharks are some of the oldest creatures on Earth, so it makes sense that they would have evolved ways to detect their prey in the water. Because of these visual cues, certain types of clothing and accessories can cause sharks to occasionally mistake a human for a fish and attack. Surprising features such as shiny jewelry, brightly colored nail polish, tattoos, and tanned skin may catch the eye of a hungry shark, prompting it to take a bite out of a body part sporting one of these decorations.

It turns out that sharks don't see in the same way that we do. Sharks have a high concentration of rods in their retinas, allowing them to see very well in low light conditions. This ability likely explains why sharks prefer to hunt at dawn, dusk, or night. However, shark retinas also contain fewer cones, so they don't see colors as well as us. In fact, it's possible that sharks see mainly in shades of blue and green. In the absence of most colors, sharks rely on the perception of contrast between light and dark shades to detect potential prey. The higher concentration of rods also makes sharks excellent motion detectors. Thus, a person swimming in the sea while wearing a brightly colored swimsuit or shiny piece of jewelry (which can look like light reflecting off fish scales) has a higher risk of being seen by a shark and mistaken as a delectable fish or seal.

Sharks have excellent vision in dim light, allowing them to see prey from far away. However, when they close in on the prey, some sharks (like the great white shark) use the contrast between the prey and nearby objects to detect the prey's silhouette. Because sharks can't use a wide spectrum of colors to distinguish prey, the key to their hunting vision is contrast, contrast, contrast.

This is where bright swimsuits and uneven tans come in as potential factors that attract sharks. The contrast between, say, a yellow swimsuit and skin or between a tanned patch of skin and paler skin next to it can catch a shark's eye and make it think it has spied some prey.

Researchers in South Africa, which has a large population of great white sharks, showed that breaking up the silhouette with bright lights can prevent sharks from attacking. According to the researchers, they placed lights in vertical stripes on the underside of a seal-shaped decoy, they reduced shark attacks to zero. The thought is that the lights broke up the seal silhouette, preventing sharks from recognizing it as prey. Sharks don't seem to like the bright lights themselves, but the pattern in which the lights are arranged is important as well. If only seals, such as this couldn't-be-cuter Ross seal that scientists finally spotted, had a way to put vertical light strips on themselves, they may just escape becoming lunch for a ravenous shark.

Part of why sharks are attracted to certain stimuli can be attributed to their unique vision and other senses. The structure of a shark's eye is similar to that of a human eye, but it has several adaptations for seeing underwater. 

The shark's eye consists of the following parts: the cornea, lens, retina, and tapetum lucidum. The cornea is the outermost layer of the eye and the first point of contact in the path of light through the eye. In humans, the cornea plays a major role in focusing light, but sharks may rely more on moving the lens to focus light. That lens lies just behind the cornea. Unlike humans, the lens in a shark's eye doesn't change shape. Instead, the shark has muscles that it uses to move the lens back and forth to help focus light. 

The next layer of the shark's eye is the retina, which contains photoreceptor cells (called cones and rods) that detect light and transmit information to the brain. (Strangely, these cells continue to transmit information postmortem, which is one of the surprising things that the human eye does after death.) Sharks tend to have a higher concentration of rods in their retinas, causing them to see better in dim light; they also have fewer cones, making it harder for them to see color. Finally, sharks have an extra layer behind the retina (which humans lack) called the tapetum lucidum. This layer reflects light back to the retina, optimizing the light absorbed by the eye and enhancing vision in low light conditions.

Even so, sharks also use other senses, including a keen sense of smell, hearing, electroreceptors (which detect electrical fields), and the lateral line system (which detects vibrations in the water) in addition to their unique sight.

Environmental factors also play a role in attracting sharks. Areas of warm water in the ocean are very attractive to bull sharks and great white sharks, which are species frequently involved in attacks on humans. They are also considered "threatened" sharks on the endangered species list. In addition, sharks like spending time between sandbars and steep drop-offs to take a break from strong waves. Add to this the fact that fish like to hang out in these spots, too, and you get the appeal. The predominance of fish at river mouths is also why sharks gravitate to these locations. Likewise, fishing piers have a large concentration of fish that are thrashing around in the water after biting on a hook, which is a siren call for sharks.

Certain times of day — and certain events — are prime hunting times for sharks. These are times of low light, such as dawn, dusk, and night. During storms, large waves may be generated in the water, churning up the nutrients in the ocean and bringing more fish from connected streams. The abundance of fish attracts sharks. 

To reduce the risk of shark attacks, don't swim in areas of warm water, sandbars, river mouths, or near fishing piers. It's also wise to avoid swimming at dawn, dusk, or night, or after a thunderstorm. That said, while it's smart to take precautions, your chance of getting bitten by a shark while swimming is quite low. As Roger Rulifson, a shark researcher at Eastern Carolina University says, "You stand a better chance of getting into a car wreck on the way to the beach than you do being bitten by a shark at the beach" (via Coastwatch).