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Winged swimmers

Jan 1, 2003

When the 32-year-old man waded with his family at a Mediterranean beach, he couldn't have suspected the finned hazard that awaited him. Suddenly, a trio of fish rocketed from the water. One of them collided with the man's head and right ear like a torpedo. The victim toppled. Severe nausea, vertigo and vomiting overwhelmed him. Conscious but dizzy, the man could not walk. Rescuers rushed him to the emergency room.

Doctors, whose findings were reported in The Journal of Laryngology and Otology, found his outer ear swollen and red, auditory canal lacerated and eardrum torn. Blood-tinged cerebral spinal fluid flowed out. A CT scan revealed a fractured right temporal bone, a skull base fracture and air invading his cranium. Fortunately, the patient's condition stabilized, and after one week he left the hospital. But a month later, the man continued to suffer hearing loss and occasional vertigo.

Typically, blunt trauma to the head is caused by car crash, assault or athletic injury. The culprit in this bizarre case was a flying fish. Ubiquitous in tropical and subtropical oceans, flying fishes generally grow less than 1 foot long, but can propel themselves at great speeds to soar high above the water's surface across impressive distances.

The Israeli doctors who wrote of the case in the journal reported, "To our knowledge this is the only documented or reported case of a temporal bone fracture caused by a flying fish." No information is forthcoming on the medical condition of the fish following its undoubtedly startling and painful collision with a human head.

While these living projectiles may inflict occasional injuries on swimmers and boaters, flying fish are widely appreciated and admired. Aeronautical engineers and physicists seeking to advance flight technology have studied these evolutionary wonders. Naturalists and artists for centuries have marveled at their vividly colored and distinctively shaped bodies. Many chefs and diners prize them, and they sustain valuable fisheries around the globe.

For mariners, a flock of flying fish can be enormously entertaining, and the charismatic creatures often make themselves familiar by flopping haplessly onto ship's decks, as high as 12 to 16 feet. Nothing beats a meal that practically leaps into your frying pan.

Some 52 flying-fish species live in the Atlantic, Indian and Pacific oceans. Most are restricted to waters above 20° to 23° C (68° to 73° F), possibly because cold inhibits the muscles used for flight. Some species chiefly inhabit coastal waters, while many dwell in the open ocean. The head and body are cigar-shaped, and the air bladder is unusually large, apparently to buoy the body's dense musculature. The biggest species is the California flying fish, growing as long as 15 inches.

Flying fishes can be quite abundant. In some places, their copious eggs sink the floating seaweeds to which they attach. In the eastern Caribbean Sea, scientists aboard the research vessel Provider in April and May 1988 counted 31,264 adult flying fish along a 950-nm route, ranging from 0.71 to 47.63 individuals per 0.5 nm. Yet, in such situations, only an estimated 20 percent of the flying fish near a boat may be airborne and consequently visible at any single moment. People can readily observe adults and juveniles from a boat because the approaching vessel incites the fishes to leap, apparently to escape the "predator." The fishes tend to swim and fly in schools of one species, but they do sometimes mingle. With practice, mariners can identify different species in flight.

For so-called wings, the flying fish employs its abnormally long horizontal fins. In some species, known collectively as four-wingers, both the front and rear pairs of fins are long and wing-like, while in other species, called two-wingers, only the front pair of fins is sufficiently elongated to function as wings. Typically, four-winger flying fish are larger than two-wingers, presumably because their greater wing surface area generates more lift and can accommodate a heavier body.

Despite its name, the marine flying fish does not actually fly with powered wing strokes like a bird, bat or insect. Rather it glides somewhat like a hang-glider, paper airplane or flying squirrel (another misnamed gliding animal). In the 1800s and early 1900s, scientists debated whether the flying fish flaps or glides while airborne. Complicating the situation, observers often hear a whirring noise that sounds like flapping; researchers now believe the sound results instead from muscles or the passive flag-like snapping of fin edges in the air.

The mystery of how the flying fish flies was resolved by stroboscopic photography in the 1940s. The photographs revealed that a flying fish does not simply leap from the water, spread its wings and soar away. The actual pre-flight sequence is somewhat more complex and intriguing. First, the fish swims rapidly toward the surface, furling its fins against its body to reduce drag. During this underwater approach, a large individual may attain speeds of 22 miles per hour. The fish emerges from the water at an angle of approximately 30° to horizontal, and its flat underside acts like the bottom of a "flying boat" airplane to help produce lift. Leaping clear of the water, the fish immediately re-immerses only the lower lobe of its tail, which remains submerged for several more seconds, stroking 50 to 70 times per second.

This powers the fish as it taxis, wings spread, along the water's surface like a jet on an airport runway. The lower tail lobe is much larger than the upper lobe, enabling it to generate thrust during this taxi phase. Finally, after taxiing 15 to 80 feet, the fish becomes airborne, and the tail rises.

Covering some 150 feet in a single glide lasting one to 10 seconds, the flying fish eventually loses airspeed. Lowering its tail back into the water to resume beating, it can regain speed and launch itself back into free flight. Alternating taxiing and free flight in this manner several times, the airborne fish can traverse as much as a quarter-mile in 30 seconds. It travels farthest in calm to moderate winds.

As it glides at heights up to 25 feet, a flying fish can hit 30 to 45 miles per hour, ranking among the fastest creatures in the sea. A Pacific spotted dolphin, for example, swims 25 miles per hour, a shortfin mako 31 miles per hour, and a killer whale 35 miles per hour. While leaping, a yellowfin tuna travels 46 miles per hour, a swordfish 60 miles per hour, and a sailfish 68 miles per hour.

Most often, a flying fish makes a single hop per flight, but scientists have documented flights with up to 12 hops, like a skipping stone. Each taxi phase usually lasts about two seconds, while free flights typically last two seconds but can extend up to 13 seconds.

Flight habits

Aerodynamic studies of flying fish indicate that normal forward flights up to 33 feet high could theoretically occur with strong headwinds. Moreover, the analysis suggests that in certain high-wind conditions, a flying fish could reach altitudes approaching 80 feet — but the flight path would be a loop, and the animal would touch down far behind its launch site. Biologist John Davenport reports witnessing "occasional spectacular flights of this type, but only in very rough weather."

Waves also can influence the fishes' flights. According to Davenport, "Large waves often involve flying fish of all types leaping out of the water without taxiing, often to smash back into the water with little distance gained."

An ongoing scientific debate centers on whether the flying fish steers while in flight. Much of the evidence suggests that it cannot or does not. Most flights are relatively straight, and the body shape enhances aerodynamic stability rather than maneuverability. However, at least a few factors indicate some capability for adjusting the path of flight. First, scientists have examined the eyes of some flying fishes and found that instead of the curved cornea of most bony fishes, they feature a flat cornea that allows focusing above and below water. Second, some photographs seem to show a flying fish using its pelvic fins as air brakes to lower the tail into the water for additional taxiing. For example, the fish might visually monitor its height and then lower its tail at the appropriate time. Finally, in one study near Bermuda, flying fishes landed in areas of floating seaweed significantly more frequently than would be expected by chance. Possibly they were choosing to touch down in these food-rich, weedy habitats rather than the open waters.

The question of whether flying fishes steer while airborne remains unresolved, but studies indicate that the fishes do prefer to take off into the wind. One study found that flying fishes launching into the wind outnumbered three to one those taking off downwind. Likewise, in another study a researcher counted the number of flying fish taking off in each direction as a steamship approached. During one time period when the ship was steaming with the wind across its port bow, the scientist observed 182 flying fishes; all except four launched into the wind. Moreover, those four fish took off on the starboard side across the wind but generally glided around the bow and into the wind. During another time period when the wind was coming across the port quarter, 46 of 49 flying fishes launched into the wind. Finally, when the ship was heading directly downwind, approximately half of the 106 flying fishes observed took off to port and half to starboard. Apparently, because the fishes could not launch into the wind as the ship was approaching from that direction, they had no clear preference regarding their direction of takeoff. If flying fishes do choose to take off into the wind, how do they detect wind direction while still underwater? One expert suggests that they see from below clues on the sea surface, or they detect underwater pressure gradients generated by the wind.

A need for flight

Why do flying fish fly? Clearly these creatures are superbly adapted for airborne travel — although it's curious that no saltwater fish can flap its fins to achieve powered flight — but why have they evolved this capability when other fishes are content with a purely aquatic existence?

Unlike many freshwater fishes that leap into the air to catch insects, flying fishes do not glide for the purpose of feeding. Only a handful of insect species, called ocean skaters, live a marine existence (see Ocean Navigator Issue 110, Nov./Dec. 2000 on halobates), and they do not fly. The only flying insects at sea are vagrants from land. Scarce and limited to areas along coastlines, they would not represent a viable food source for flying fishes. (Instead, the fishes feed on the tiny floating organisms called plankton.) Although airborne flying fishes do not feed, the Bermuda floating seaweed study suggests that one reason for flight might be to travel from food-poor to food-rich areas. However, little data exists to support this notion.

An alternate explanation proposed by some scientists maintains that flying fishes conserve energy by gliding rather than swimming. For example, penguins and certain marine mammals leap out of the water while traveling long distances. This behavior, known as porpoising, reduces energy expenditure in these particular animals compared to swimming. However, in flying fish, white muscle, rather than red muscle, is used apparently to accelerate during takeoff. This type of tissue consumes so much oxygen it seems unlikely a flying fish would experience energetic savings over swimming.

Scientists widely agree on a different explanation for why flying fishes fly. Food does appear to play a key role. But rather than attempting to feed themselves, the flying fishes probably glide to avoid being eaten. Their predators include mahi-mahi, seabirds and tuna. In effect, flying fishes have radically improved on the "leap for life" strategy employed by many other types of fish when pursued by enemies. Presumably, by propelling themselves out of the water and soaring long distances, the flying fishes improve their odds of escape. What better way to escape than to glide as far as possible?

According to one biologist, when a mariner sees flying fishes, large predators often can be spotted swimming in the water below. Another expert recounts seeing schools of flying fishes becoming airborne to elude predatory tuna. Then the tuna themselves began leaping to avoid the hungry dolphins that had arrived on the scene to feed on the tuna. Of course, the tuna lacked the ability to glide like the flying fishes. Even so, flying fishes are not always fast and agile enough to escape their predators. Mahi-mahi reportedly can jump and catch flying fishes in the air and also track them from underwater.

On May 5, 1991, biologists aboard the sailing vessel Balaena in the Gulf of Mexico observed several dozen spotted dolphins feeding on flying fishes at night. The episode occurred over a period of three hours. Generally dolphins are believed not to feed at the surface at night, and for at least one species of spotted dolphin, apparently no previous records exist of predation on flying fishes. At approximately 2050 that night, the dolphins began bow riding. Using a hand-held spotlight, the scientists identified the marine mammals as spotted dolphins and also noticed in the light beam flying fishes up to 10 inches long, seemingly frenzied. Sometimes in groups of two or three, the dolphins acrobatically pursued the gliding fishes, deftly zigzagging and accelerating. They snared the prey from behind but also from the side, even though their echolocation hunting capabilities no doubt were compromised by the air-water interface. At approximately 2340, the unusual encounter ended, leaving the mystery of how the dolphins successfully hunted the flying fishes at night.

"Some fish seemed to be able to leap sufficiently far away that a chasing dolphin abandoned the pursuit," reported the biologists in Marine Mammal Science. "However, the splash of these leaps seemed often to catch the attention of other nearby dolphins."

This real-world observation corresponds well with a different study, titled Optimal Flight Path of Flying Fish, published in the Journal of Theoretical Biology. The authors used analytical methods and optimal control theory to determine the best possible flight behaviors for the fish to achieve either of two strategies: maximum flight range or maximum flight time. The former would reduce the probability that the predator would find the prey. The latter would increase the possibility that the predator would abandon the pursuit. Indeed, comparing the results of their mathematical analysis to actual flight paths of flying fishes, the scientists concluded that the real-world flights approximate the optimum for maximum flight range.

Into the frying pan

Predator avoidance seems the clear explanation of why flying fishes become airborne when approached by a sail or motor vessel, which they presumably perceive as a large, hungry animal. But their escape doesn't always succeed. Unfortunately for the fishes, they see poorly at night and often land accidentally on the ship's deck, rather than in safe waters. Yet, fortunately for mariners, flying fishes taste good. The main ingredients for a delicious seafood breakfast often can be gathered simply by walking around the deck at daybreak and collecting the flying fishes that were stranded there overnight.

"Occasionally flying fish land on the side deck and get scooped up, gutted and end up in the frying pan with garlic and lemon," reports the LG Flatron online journal on May 27, 2001, during Leg 6 of the BT Global Challenge. LG Flatron went on to be crowned the overall victor of "The World's Toughest Yacht Race." Did a diet of flying fish provide a winning edge?

In Barbados, flying fish isn't an oddity, but a highly regarded staple of the island's cuisine. Fishermen travel as far as 30 miles offshore and use long nets to catch up to 3,000 flying fishes per day. Chum and coconut palms, which provide shelter and egg-laying sites, attract the prized cod-flavored creatures. Back on shore, the islanders slice off the head and fins, butterfly the fish and remove the bones. Flying fish seasoned, battered, fried and served with chips is a highly popular Bajan dish at restaurants and in homes. Menu options include flying fish pie, stuffed flying fish, and melts and roe (male organs and roe). Similarly, flying fishes support important fisheries elsewhere, including Indonesia, Korea, the Pacific islands, India, West Africa and Brazil.

Compared to other marine fishes, flying fishes are relatively easy for scientists to survey because adults can be counted from a ship's deck. Yet researchers have conducted few detailed studies of the fishes' distribution and abundance. One of the most comprehensive occurred in the waters surrounding Barbados, Martinique and Tobago. A primary goal was to assess the feasibility of expanding commercial fishing of flying fishes farther offshore, but the study also provided an interesting picture of where the adults, juveniles, larvae and eggs of different flying fish species live. In the study, Hazel Oxenford, Robin Mahon and Wayne Hunte, of McGill University in Montreal and the University of the West Indies, used a host of methods to count them in the eastern Caribbean.

Of the 15,346 adult flying fishes identified to species, the scientists found 52 percent were Parexocoetus brachypterus (a smaller species), 47 percent Hirundichthys affinis (the chief commercial species), and 1 percent Cypselurus cyanopterus (a larger species). The first two species were most abundant east of Tobago and Barbados and west of the Lesser Antilles, and fewer lived between those island groups. The larger species was most common northeast of Barbados and quite rare elsewhere. The researchers speculate that the peak abundance of flying fishes to the west of the islands could relate to the turbulence and eddies that frequently form to leeward of islands. These oceanographic features tend to increase food supply by carrying nutrients from the depths and concentrating the plankton on which flying fishes feed.

The scientists surveyed juvenile fish at night by attracting them with a light and then scooping them into a net. Of 2,211 juvenile fish collected, nearly three-quarters were flying fishes, indicating that this family represents a prominent component of the fish community. The three most common juvenile flying fishes were P. brachypterus, Exocoetus volitans (a primarily oceanic species), and H. affinis. Results of surveying flotsam for flying fish eggs revealed a surprising scarcity throughout the region, despite the abundance of adults. Furthermore, nearly all the eggs were oceanic species, rather than H. affinis and fellow species found in the region as adults. The scientists suggest that these species may not spawn on flotsam, as generally believed, but rather on the seafloor. Alternatively, their eggs on flotsam are so profuse that it sinks, and therefore eggs were not found.

Flying fishes are not the only ocean creatures capable of air travel. Flying squid launch themselves into the air and glide on wing-like fins, reportedly attaining speeds of 30 miles per hour and traveling up to 160 feet. They reportedly have been found on decks as high as 12 feet. One sailor tells of discovering a tiny flying squid — only 1 inch long — on a deck 6 feet above the water's surface. Meanwhile, in freshwater habitats, the hatchetfishes of South America and the butterfly fishes of Africa can flap their fins while airborne.

Fortunately they are smaller than saltwater flying fishes — and less likely to inflict cranial fractures, lacerated auditory canals and torn eardrums on unsuspecting boaters and swimmers. n

Pete Taylor is a freelance writer living in Portland, Maine. He has a graduate degree in marine ecology.