Feared and respected by seafaring men, sharks continue to be a threat to seamen who have been forced to abandon ship, to downed airmen, and to the recreational swimmer.
Several means of shark protection have been devised, yet efforts fall short of total success. Numerous undersea research labs in the West Indies, Florida, Massachusetts, Washington, California, Australia, Monaco, and South Africa are systematically collecting data. More than a score of top echelon scientists are striving to unlock the secrets that pertain to the life, habits, and temperament of these cannibalistic scavengers of the sea.
Certain species of shark, notably the 40-foot whale shark (Rhineodon typus), do not normally attack man. Next in size, the basking shark (Cetorhinus maximus), is similarly docile. The teeth of both are undeveloped and virtually unused. This is not to certify, however, that humans will not be seriously injured by the flapping of a ponderous tail; or by having a boat capsize as a mammoth shark comes up under a small craft merely to scratch its back.
Several smaller species regard everything that moves within their domain as potential food. They will strike at virtually anything that sinks, swims, flaps, or floats. The notorious white shark (Carcharodin carcharias)—ranging to more than 19 feet in length and weighing about 2,400 pounds—the tiger shark (Stegostoma tigrinum), and the mako shark (Isurus glaucus) in particular, have been known to attack small boats in quest of human prey as well as man-caught fish. One case recorded by the Smithsonian Institution reveals that a shark hovered near a small boat for four hours and made an attack every time the operator rose up to make an observation.
Three points are agreed upon by shark authorities. Shark behavior cannot be predicted; the creatures can never be trusted; and they can be vicious. A feeding pack demonstrates a demonic fury not equaled by even a pack of wolves. Nevertheless, a knowledge of shark characteristics, plus a few defensive techniques, might well preserve the life of a shipwreck victim, a casual swimmer, a skin diver, or a downed airman.
Sharks range in size from greater than 40 feet down to six inches. Table 1 depicts the names, average adult lengths, and the relative degree of threat to man for the more dangerous of a total of more than 250 separate species.
Dangerous sharks abound in warm tropical waters, but some live in temperate and arctic regions. Predacious white sharks frequent the coastal waters of California, even when the mean temperature is colder than 50° Fahrenheit. Others have penetrated as far north as Alaska. A variety of dangerous sharks have also been sighted off New Jersey and Massachusetts where water temperatures are often too cold for comfortable summer swimming. Numerous small sharks die in Newfoundland breakers and are cast onto the beaches in droves. The Greenland shark (Somniosus microcephalus), some 20 feet in length, makes his abode in even more frigid seas—and his temperament is believed to be a threat to human life.
The fact that nearly all swimming and skin diving is performed in warm regions accounts for statistical data indicating that the vast majority of shark attacks on humans occur near tropical islands or in temperate zones during the summer months. Disaster survivors or skin divers in arctic waters may thus erroneously believe that their only danger lies in unfavorable climatic factors. However, no consolation is to be derived from such statistics. Man-eating sharks, it would seem, thoroughly enjoy impromptu meals of quick-chilled Homo sapiens.
While most sharks live near the surface, certain species prefer the deeper channels. Though they are not the really deep-diving creatures that inhabit the great trenches, sharks have been known to descend as deep as 9,000 feet.
Certain species of shark have also been observed in fresh waters of coastal rivers, estuaries and bays—notably the Chesapeake Bay and the Delaware River. Thus, no tidal waters are really safe or free from some degree of shark menace.
Feeds on plankton and small fish; cavernous mouth contains undeveloped teeth; powerful tail may be dangerous; dark gray or brown with white or yellow spots and white belly.
Feeds on plankton and small fish; undeveloped teeth; skin extremely rough; has attacked boats but not man; gray-brown to black.
Highly aggressive and vicious; ranges into cold, northern waters; travels alone; will attack boats.
Abides in Arctic; probably dangerous to life, though sluggish when captured; no attacks on man due to lack of human swimming and diving in the extremely cold waters near Greenland.
Abides in cold or deep water; probably dangerous.
Prefers diet of small fish; elongated upper caudal (tail) fin; docile but has attacked boats.
Aggressive and dangerous; distinctive broad head.
Ferocious and dangerous; may attack boats; will enter fresh water rivers; uniform gray-brown color when adult; marked with brown bars when young.
Very active; frequently attacks boats; dark blue-gray or blue above, white below.
Prefers diet of small fish; may bite if angered.
Easily identified by two nearly equal dorsal fins, trim lines and yellow or brownish color; will enter fresh water; ranges New Jersey to Brazil, inshore.
Potentially dangerous; one variety seldom approaches coast; Pacific island inshore variety about seven feet in length; distinctive white tipped fins.
Lives mainly at sea (pelagic) but comes inshore at times; blue-grey above, white below.
Abundant in eastern Pacific; potentially dangerous; gray-black above, cream color below.
North Atlantic deep water habitat; seldom seen, but bites if aggravated.
Seldom attacks but bites if aggravated; jaws lock in tenacious grip; rounded fins; dangerous because it lies on bottom and is easily stepped on in shallow water—notably in Atlantic.
Prefers fish diet; may bite; has smooth skin unique among members of shark family.
Slow swimmer but dangerous and becomes active near food; distinctive indigo blue above, snow white underbelly.
Similar to blue whaler except gray or black in color.
Dangerous; will make unprovoked attacks; enters fresh water; frequent Australia and New Zealand; distinctive bronze coloration.
Very dangerous because it enters fresh water freely (reported 160 miles upriver in South America); inhabits lakes Nicaragua and Yzabal; found in ocean from New York, West Indies; gray above, white below; slow swimmer except when excited by food.
Found in cold or deep waters; probably dangerous; unique because of number of gill openings.
Prefers fish diet; may bite hand, feet, knees; will enter fresh water; distinctive fin tip coloring.
Pacific Island habitat; gray color fading toward white on belly; fins are dark but not blacktipped.
Lives in cold or deep water; may attack or bite.
Pacific specie dangerous—Atlantic more docile; very distinctive homely appearance due to fleshy lobes and feelers around head, as implied by name.
Prefers fish diet but will bite if provoked.
Large, semi-protruding teeth present moderate danger; will bite if tormented.
Moderately safe but will bite if angered.
Reasonably safe because of small size, but can inflict bad bites on hands and feet.
Reasonably safe due to small size, but will bite when teased; flat disc-like bodies similar to rays.
Known to be dangerous; distinctive cross-bar coloration; abides in Oregon and California waters; males smaller.
Reasonably safe but will bite when tormented.
Sharks smaller than the foregoing are considered to be too small or sluggish to be of any great threat to man; however, they have been known to inflict bites on fingers, hands, and feet of fishermen and swimmers, especially if teased. Their abrasive hide will also cause skin damage.
* Man-eating varieties.
For further data, the text, Sharks and Survival, edited by Dr. Peter W. Gilbert, is recommended.
The breathing mechanisms of both sharks and rays include five to seven gill openings (compared with only one for nearly all other aquatic life). Shark species that inhabit coastal estuaries or hover near the surface far out at sea are unable to “pump” water through their gills by direct muscle action. They must maintain forward motion to force water into the mouth and out through the gill openings, and thus obtain oxygen. Because of this characteristic, South African and Australian swimming beaches are effectively protected by means of vertically suspended gill nets that snag, and thus suffocate, marauding sharks. Only the deeper diving species are able to lie at rest and pump oxygen-bearing water as do the bony fishes.
Sharks do not have bony vertebrae in the same manner as mammals and bony fishes. The skeleton consists of a cartileginous substance that is elastic in nature; hence the zoological classification (along with rays) of Elasmobranchii. The scaly hide is made up of a similar substance, is very tough, and is highly abrasive to human skin.
Sharks possess a well developed acoustical-pressure “sonar” system that detects and tracks underwater sounds in much the same way as porpoises are known to communicate and follow a target. Sensors are located along both sides of the body, in the head and around the cavernous mouth. This system is capable of detecting waterborne vibrations from a considerable distance, and is probably the initial means of detecting and tracking prey.
Sharks habitually stalk crippled or wounded fish and other distressed sea life. They have been known to ignore the steady rhythm of the normal sounds emanating from a school of fish and pursue the spasmodic, sometimes frantic, sounds generated by a sick, frightened, or wounded fish. This characteristic is a factor in favoring the survival of the fittest and maintaining the ecological balance of marine life. It also demonstrates why distressed mariners must not become panicky or excessively weary while in shark waters.
The shark’s very keen olfactory sense is capable of “smelling” blood and waste odors from long distances downstream. At shorter ranges the visual sense is employed for selecting and attacking a victim.
The eye of the shark is unusual in that it is highly sensitive to light and dark patterns, yet has a low visual acuity. Observed objects are poorly defined as compared with human vision, but light and dark patterns are readily detected in the very dim light typical of murky or deep ocean waters.
The contrast between dark and light colors presents an attractive target to a shark’s eye. Consequently, numerous bites are directed at moving arms or legs, or shiny pieces of equipment. Scuba divers in dark suits at the surface are easily observed against the aura of light that penetrates the water from the sky.
The neural system of many sharks is also believed to provide olfactory (smell) and gustatory (taste) responses from barbels, fins, and skin surface areas as well as from the more human-like glands located in the anterior regions of the head and mouth. This physiological phenomenon may account for the violent frenzy observed among a mob of sharks.
When he is alone, the shark tracks his target to within visual range, then circles his quarry cautiously. While visual and olfactory characteristics are checked, he spirals inward, finally bumping the target with his snout. If satisfied that he is not about to be stricken dead (which is seldom), and that the quarry is edible (though it may not be), the shark bites vigorously with tremendous—in the range of several tons per square inch—force, accompanied by a violent shaking of the head. The victim’s flesh and bone are ripped away in jagged 10-to-20-pound chunks.
The cautious approach is typical when a predatory shark is alone and does not taste or smell blood; or, when several of them are randomly probing for food. Conversely, traces of fresh blood may drive them into a frenzy. This violent action is partly because of the stimulating effect upon the digestive system, and partly because of an instinctive knowledge that fellow carnivores are capable of sensing blood at great downstream distances, hence will swiftly arrive to compete for the victim of the moment.
In the presence of a large number of sharks, an otherwise “genteel” scavenger may attack swiftly, becoming ferocious and viciously bold. As with wolves and wild dogs, the pack instinct drives them into fierce competition; often resulting in attacks upon each other and thence mayhem, as an accidentally wounded shark becomes a victim and is torn apart within seconds.
The shark’s mouth may contain as many as 19 rows of teeth. The active row, or rows, are located well forward in the biting position. The auxiliary, unused rows are hinged back against the roof of the mouth. Individual teeth, or an entire second row, will move downward and forward, sometimes significantly within 24 hours, to replace those accidentally torn away.
In some species, such as the infamous white shark the upper jaw is moved forward as well as upward until the biting demon resembles a many-fanged open tube. Thus, he is capable of taking in a large dolphin or a human body. The mobility of the jaw also permits attack near the surface without the need for inverting.
Many persons erroneously believe that sharks “characteristically” roll into an inverted position to strike at objects on the surface. Though they are beautifully agile swimmers, fully capable of taking food while on their side or totally inverted, there is only meager evidence to indicate that they have a habitual pattern for doing so.
The menu of these oceanic scavengers is endless in variety. White, mako, and bull sharks have taken bites out of boat planking, sometimes leaving behind one or more teeth. Tigers and others have been opened to reveal lumps of coal, chunks of rubber, tin cans, clothing, birds, and even a roll of roofing paper.
Many varieties may live as long as 40 days without food. Growth is arrested during these periods of famine, but they do not appear to suffer ill effects. Growth is continued when food is again available. However, a shark’s disposition, never congenial, is not improved by fasting.
Because they do not have buoyancy bladders, sharks are immune to the underwater explosions that often kill large numbers of fish by rupturing their bladders. By contrast, there is evidence to indicate that explosions and other odd noises attract sharks. Some are attracted toward the low frequency sounds emanating from a group of swimmers. Once interested, a marauding maneater usually directs his attention to the swimmer who is somewhat apart from the others. A totally isolated swimmer making smooth, rhythmic strokes, or floating quietly does not provide as great an attraction to shark sonar as does a cluster of swimmers creating a loud, irregular noise pattern.
Two sharks of the same species may exhibit vastly different attitudes toward man. One may prove to be docile and merely curious while his kin may pursue an aggressive path and make angry strikes at anything within his domain. This unpredictable pattern has been observed where two similar sharks were well cared for while in an aquarium. Still others have beaten themselves to death by ramming the sides of a tank.
These phenomena cannot be separated with respect to sex, age, habitat, or time of day; thus no sizable shark is to be trusted even after a period of docile behavior. They must be accorded all due respect as a formidable underwater enemy at all times.
In some cases, inquisitive sharks can be driven off by a solid rap across the snout with a stout club, a chunk of metal, a camera, or a solid shoe kick. A study pursued by Dr. L. P. Schultz of the Smithsonian Institution and his associates indicated that in approximately 61% of the attacks studied, the shark was discouraged by these measures. Other survivors reported that the shark returned for further attacks until physically damaged. In a few instances, the human efforts at offense only angered the shark and caused him to intensify his attack.
Controlled experiments in aquariums have demonstrated that some sharks will be turned away by a bubble screen; others will swim through it. Later, the shark that was originally deterred by the screen found it to be harmless and ventured through it.
Spear guns are generally ineffective because of the toughness of the scaly shark hide. Similarly, a hand knife is of little value against a sizable shark. Even the supposedly soft underbelly is difficult to penetrate except by a forceful blow from a muscular man; and getting set to deliver such a blow is most precarious while dodging a swift shark. Even if the shark were to be cut, his blood would be a serious attracting agent for other sharks. The “Bang Stick”—a shark-killing device which fires a 12-gauge shotgun shell or a .357 Magnum bullet—is touchy to handle in that the muzzle must be placed against the shark’s body.
The scales of even a small shark will quickly remove a large patch of human skin, and possibly the flesh beneath. Shipwreck and aircraft survivors should, therefore, retain shoes, trousers, and shirts, to protect themselves from even a casual encounter with the shark who is interested in nothing more than scratching his fins on their elbows. (This advice is based on the premise that distressed persons can remain afloat while thus encumbered.)
One of the better protective measures is to avoid provocation. Foolish skin divers invite injury by grasping a fin or tail fluke to hitch a ride, by jabbing a shark with a spear, or inadvertently tempting him with speared fish tied to the fisherman’s body. The death struggle of speared or hooked fish will attract sharks, who then may injure the fisherman when they go after his catch. The knowledgeable person will jettison his fish to divert an overly inquisitive shark, then leave the vicinity rather than risk an unpleasant encounter.
Wisdom in choosing recreational sites is also an excellent preventive measure against shark attack. The majority of those who have been killed or injured were attacked at or near the surface because that is the section of the shark’s domain most frequently occupied by human beings. There may be a relative degree of safety for divers at some depth. Statistics compiled by Dr. Schultz seem to suggest that the greater the depth, the less chance of attack.
Conversely, the doctor’s tabulation of diving incidents may only indicate a dearth of skin divers below the 20-foot level. Or, the data may indicate that the deeper divers are well beyond the inquisitive amateur stage, thus better disciplined, better trained, and more adequately prepared in the art of survival.
Bright, shiny objects are a temptation to sharks. Aggressive creatures will strike at belt buckles, tin cans, paper bags, spinning boat screws, shining fittings, dangling arms and legs, and even human faces peering over the gunwales. On a few occasions they have been known to leap into, or over, a small open craft.
Even though a boat does not provide complete protection, survivors at sea are obviously safer in a lifeboat or sizable raft than if drifting aimlessly in life jackets. But, where raft or boat facilities are not adequate for all survivors, the injured should be placed on board and the able bodied directed to cling—or be tied—to the side in a floating position. A water-treading individual dangles legs and arms in a manner that may attract a circling shark. Thus the horizontal position is best, though not immune to shark attack.
Castaways who do not have access to a boat or raft are advised to lock arms back-to-back and form a “Seafarers’ Shark-Kicking Society.” Injured or incapacitated persons may be partially protected within such a circle of able bodied shipmates. Groups of survivors have been known to fend off marauding sharks for as long as three perilous days and nights by this means. The locked circle also affords the advantage of allowing its members to take turns between resting and maintaining an alert watch; particularly at night during the normal feeding hours for sharks—and when mankind is normally most distressed emotionally.
Abandoned garbage and body wastes are like engraved invitations to hungry sharks. When these waste products must be left in the ocean, it is mandatory that castaways leave the immediate vicinity if at all possible. Otherwise the refuse must be weighted and sunk.
Enclosing garbage and wastes in plastic bags is advisable if bags can be obtained, perhaps as an important item placed in a boat’s survival package. In this connection, plastic coatings from food supplies can be opened carefully and retained for disposal purposes.
Strangely, rescuers have seldom been attacked while retrieving the injured victim of an attack by a single shark—even though the shark continued to bite the original victim. This is particularly true when there are several men in the rescue team. Perhaps this is because of the native instinct to seek already injured or otherwise distressed sea life. Most of the injuries that occur to rescuers are principally incidental skin abrasions, bruises and lacerations from rigid, sharp-edged fins or the thrashing of the powerful tail as the shark vigorously continues his quest of the original victim.
The statistics also seem to indicate a severe shortage of volunteer rescuers.
But then there is the other side of the coin. The gourmand can become the main course. In a survival situation, fishing for small sharks will prove to be a valuable source of food and body moisture. Based on the premise that one can be caught and safely beached or boated, the flesh will probably be edible—even raw. In some areas of the world certain species are the basis of gourmet dishes. For example, sharkfin soup is fancied in Oriental regions. Many American housewives have unknowingly purchased shark flesh under the name of “Scallops” or “Steakfish.” Yet, in West Coast cities of the United States, shark meat is freely marketed as “Grayfish.”
Prior to the days of vitamin-enriched, machine-processed foods, shark’s liver provided a profitable enterprise for fishermen because of its high vitamin A content.
Sharkskin was once a useful medium for polishing wood cabinet work. More recently sharkskin leather has become highly valued as a durable material for luggage, wallets and ladies’ handbags. The market value of such items is increasing because of the inherent difficulties involved in night-time fishing operations during the shark’s normal feeding hours.
Resourceful individuals who can successfully land one or two fair-sized sharks will find their “survival” situation greatly alleviated by the food and raw materials available from the carcasses.
Even though he is a true carnivore and an uninhibited cannibal, a shark will not devour decayed shark flesh. This factor spawned development of a chemical dispersing agent known as “Shark Chaser.” The repellent portion of the chaser package is a combination of copper sulfate and acetic acid. The resulting chemical salt exhibits characteristics similar to the natural organic acids found in decomposing shark but, the effect, unfortunately, is not reliably the same. James R. Stewart, Diving Officer, Scripps Institute of Oceanography, comments:
“We have excellent movies of the shark swimming through U. S. Navy shark chasers. Most of us have considered it a great psychologic crutch but that is all. After all, it has “shark repellent” reprinted right on the packet. However, I have yet to meet a shark who could read.”
D. E. Holt, Chief of Oceanic Biology Programs in the Office of Naval Research, wrote the author:
“Certainly, it has been known for some years that the copper acetate “repellent” component of the standard World War II “Chaser” did not provide any significant degree of protection from shark attack. Tests by participating scientists in our shark research program have confirmed this . . . However, it should be pointed out that in order to be realistically tested, it would seem logical to allow the dye-repellent to be diffused in the water as in normal operational usage. Indeed, it is this heavy solution of the nigrosine dye, rather than the repellent which has been shown to offer protection to the person afloat . . . .
“Your thought, therefore, that an opaque or black irregular dye “cloud” would provide a non-distinct and confusing pattern to the shark is valid and, while the presently used dye is effective in this regard, efforts are underway to improve the extent and duration of this opacity . . . .
“Both field and laboratory testing have indicated that the eye of the shark plays an important role in the close proximity inspection immediately preceding an attack. . . . The concealment and obfuscation of these (human) profile patterns, therefore, whether by a dye cloud or plastic shark screen, would reduce the probability of attack.”
Advantageous though the waterborne dye cloud may appear, it cannot attenuate waterborne acoustical vibrations generated by the concealed swimmer; nor can it adequately suppress odoriferous-olfactory stimuli emitted by the body. Nevertheless, its advantages cannot be indiscriminately abandoned on the premise that shark chaser does not provide 100% protection. The man in the water needs every advantage he can obtain. He is in the shark’s field of combat; he cannot outswim or outmaneuver the shark in its own realm. The preservation of his life demands the use of every sensible means at his disposal.
The Navy has sponsored continuous research in seeking practical ways to protect distressed castaways. The most significant and promising device thus far endorsed by the Office of Naval Research is the plastic shark screen. It is sometimes referred to as the “Johnson Screen” after its inventor, Dr. C. Scott Johnson, who tested it at the Lerner Marine Laboratory on North Bimini Island. It is, in effect, a bag attached to circular flotation rings at the top. In the Navy version, it is attached to a life vest. In use, the bag is unrolled, the rings inflated with a carbon dioxide (CO-2) cartridge in the same manner as the life vest. The user partially fills the floating sack with water, then rolls into it. Additional water shapes out the bag to conceal its contents.
Mr. Holt describes some of the important features of the screen:
“Further, it can readily be seen that the Johnson Shark Screen would also provide containment for the chemical olfactory stimuli which plays a role in the intermediate range detection of prey by sharks, as well as drastically damping the acoustic impulses created by the man floating in the buoyant inflated screen.
“This screen has been subjected to many experimental tests and proven effective to the extent that it has been endorsed by the Office of Naval Research and distributed in a much improved compact form to more than a hundred Navy airmen for actual fleet evaluation. If these final tests indicate a sufficient operational feasibility for actual field use, the screen packet will be made available on a large scale to the Fleet.”
From the practical aspect, Mr. Stewart of Scripps Institution comments:
“The plastic bag concept may be fine—the wrapping of one’s self in anything that cuts down on dangling parts is fine if you can stay afloat without moving. However, having watched a good many sharks swim up to an object, (can, log, paper, sack, etc.) and bite it out of curiosity, I would want a lot more data before I would wrap myself up as a plastic wrapped steak.”
Sharks are indeed a menace to human life in ocean seas; an environment in which Homo sapiens does not belong. Within its own environment, the Elasmobranchii play their designated role in maintaining the ecological balance of the seas. If man is to invade or interrupt that balance, whether by plan or accident, he must be prepared to protect himself during the journey. Otherwise, the pleasure of the return trip is forfeit.
Upon graduation from the University of Minnesota with a degree in Aeronautical Engineering, Lieutenant Commander Frasier was commissioned an ensign, U. S. Naval Reserve. As a lieutenant, he augmented to the U. S. Navy, ultimately retiring after 20 years active duty. His duty stations include NADC, Johnsville, Pennsylvania; FAETuLant, NAS, Norfolk, Virginia; NAAS, Sanford, Florida; U. S. Naval Station, Adak, Aleutian Islands; ComCVG/ComRCVW-12, NAS, Miramar, California; O-in-C Officer’s Electronic School, NATTC, Memphis, Tennessee; Aircraft Maintenance Officer, NavSta, Argentia, Newfoundland, Canada; and Electronics Officer, ComFair Norfolk staff. Since July 1968, he has been a Senior Engineer with the Boeing Company, Vertol Division, near Philadelphia, Pennsylvania.