Q&A on Shark Safety

With swimmers warned away from the water last week in Southern California due to white shark sightings, there has been heightened concern about the safety of sharing ocean waters with these notorious predators.

While white sharks are found randomly year-round, we still don't know too much about their migration patterns. Over the past few years we've seen more juveniles and sub-adults show up in this general area, but scientists can only speculate as to the reason.

It's likely that these youngsters are still just preying on fish and have not developed the more mature teeth patterns or desire for larger marine mammals yet. Whether or not these animals may pose a threat to humans encountering them accidently in their waters, though, is something that no one can or will say for sure.

But if you look at the statistics, the bottom line is you're taking a much bigger risk by getting into your car each day than by swimming, surfing, diving or snorkeling in the Pacific Ocean.

Below are answers to some common questions about white sharks.

How common are shark attacks on humans? While they don't typically prey upon humans, white sharks do pose an extreme threat if you meet them on their "turf," or, maybe in this case, "surf." Between 1950 and 2004, there were 93 white shark attacks on humans in California. Of those, 10 were fatal.

It has been nearly 10 years since a fatal white shark attack occurred in California. In December 1994, a diver was attacked and killed off San Miguel Island in southern California.

It is important to note that even though human use of the water over the years has greatly increased due to the growing human population and the popularity of surfing, swimming and scuba diving, white shark incidents have not increased in a parallel manner.

What do white sharks typically eat? Juveniles typically feed on fishes, small sharks and rays. Both adults and juveniles are ambush predators. Adults have a wider menu, which includes fishes, seals, sea lions, dolphins, whale blubber (scavenged), seabirds, marine turtles, rays and other sharks.

How large do white sharks get to be? Adult white sharks grow to about 21 feet and dominate their domain as one of the top-level predators of the ocean. A female white shark was captured off Point Vicente, Calif., in September 1986 that measured 17.6 feet and weighed 4,140 pounds.

Where are white sharks found? White sharks are widely distributed around the world, mostly in cold, temperate seas and only occasionally frequenting tropical seas. They prefer waters with sea surface temperatures of 50 to 72 degrees Fahrenheit. White sharks have been known to swim as deep as 6,150 feet.

How far do sharks travel in a day? I've heard estimates of up to about 50 miles in a day, but we don't know what distance white sharks are capable of traveling. Scientists have documented them traveling from California to Hawaii and to the Guadalupe Islands in Baja.

What is the white shark's role in the marine ecosystem? They play a crucial role in the marine ecosystem by helping to suppress pinniped (seals and sea lions) populations. White sharks, killer whales and disease are about the only factors limiting seal and sea lion populations in California.

How large is the white shark population?

Currently, there are no population estimates available, however, there is agreement between shark biologists that their numbers are very low. I've heard estimates for California of around 100.

Why are their numbers so low? Typical of top-level apex predators, they are long-lived, slow-growing, late-maturing and have a low reproductive potential.

Male white sharks don't become sexually mature until about nine to 10 years of age. Females become mature at around 14 to 16 years of age, and can produce two to 14 "pups" per litter. Birthing is thought to occur in the spring and summer months. Each pup is around 4 to 5 feet long at birth, and comes equipped with a full set of teeth. White sharks, as with most long-lived ocean creatures, suffer from high mortality rates during their first year of life.

Scientists believe white shark gestation periods last about 12 months, which means female white sharks may breed only once every two years. This slow rate of reproduction indicates that it would take a long time for white shark populations to recover if they became severely depleted.

How are white shark populations assessed? Since there is no fishing data available (they are protected), scientists must use other means. One method is to collect tissue samples and extract DNA samples for analysis. These studies are used to determine whether there are multiple populations of animals or just one white shark population that's spread out over large areas. This research on white sharks is still in its infancy.

Scientists at California's Farallon Islands visually document each shark based on unique external characteristics such as scars, marks, fin shapes and unique coloration patterns. They then note any re-sightings of each recorded animal.

It may soon be possible to use mark-and-recapture statistical sets to estimate the actual numbers of animals occupying that area. Evidence so far supports the notion that the number of adult white sharks around the Farallon Islands and, very possibly all of California, is less than 100.

Tagging white sharks with special satellite tags is another method that enables scientists to determine the range and movements of the big beasts — whether there's just one population or multiple populations moving up and down the coast — and just how free-roaming they are. Recently, a shark tagged off of the California coast was tracked moving to Hawaii and back for two years in a row.

Is fishing for white sharks legal? No. White sharks are not legal to take and have been completely protected in California waters since Jan. 1, 1994.

Are white sharks protected all over the world? No. In areas of the world not yet protecting white sharks, they have become a popular trophy fish, with each bringing thousands of dollars for just their valuable fins and jaws alone.

Fear has motivated the targeting and persecution of white sharks, and all sharks, for decades. White sharks have been listed as "vulnerable" on the International Union for the Conservation of Nature and Natural Resources' "Red List" of threatened species.

They now are regionally protected in many areas of the world, including South Africa, Namibia, the Maldives, Malta, Australia, the U.S. Atlantic seaboard and Gulf Coast (including Florida), and the coast of California (since 1994). In these protected areas, it is illegal to pursue, capture or possess great whites in whole or in part.

Unfortunately, they have not been granted full international protection yet, which allows a sizable black market network to perpetuate the sale of white shark jaws and teeth.

How do you tell white sharks from other species of sharks? If you see a shark greater than 15 feet long in California waters, chances are it's a white shark. They have heavy spindle-shaped bodies with conical snouts and a narrow tail stalk supported by stout lateral keels. Their coloration reflects a sharp demarcation between dark upper surfaces and white lower surfaces. The pectoral fins have a white trailing edge, black tips on the undersides, and a black spot occurs at the pectoral axil ("armpit") in some individuals. Their jaws are loaded with large, triangular, serrated teeth.

How do white sharks find their prey? White sharks initially use vision to find their prey, but then switch to a special organ located in their snouts that senses electrical pulses from the body, such as those generated by muscle movement or a beating heart.

With their come-from-below, ambush-style of hunting, sharks typically attack by biting their victims quickly with enough force to cause major trauma, then leave the injured prey to go into shock or to bleed to death before returning to feed on the carcass.

Off California's coast, many people who have encountered a shark have been lucky, walking away virtually unscathed. White sharks don't typically target humans. Most shark-attack victims who have lived to tell about it say they never saw the animal coming. Most of the recorded incidents resulting in an injury have involved the person being able to receive medical attention soon after the attack.

How can people avoid white shark attacks? There is only one foolproof method for avoiding a white shark attack: stay out of the ocean. If this is not an option, try to avoid places known for white sharks, such as California's Farallon Islands, Ano Nuevo and Bird Rock near Point Reyes. You should also avoid swimming in areas where marine mammals are congregating. Don't swim in or near areas frequented by sea lions, harbor seals and elephant seals, or near their rookeries.

Some scientists believe that wearing a wetsuit and fins, or lying on a surfboard, creates the silhouette of a seal when viewed from below. Shark attacks often are thought to be cases of mistaken identity, with surfing or swimming humans mistaken for seals or sea lions. Times of reduced sunlight, such as foggy mornings or dusk, are ideal times to be mistaken for a seal.

What can divers do to avoid encounters? Spend as little time on the surface as possible. Since white sharks typically attack marine mammals from below in a rushing manner, plan to get to the bottom as soon as you can. Diving in kelp beds is safer than diving in open ocean environments, too, as white sharks are thought to seldom enter these areas.

Be aware of ocean conditions around you too when there's lots of bait in the water and you see birds and marine mammals actively working the bait balls. These feeding frenzies also may attract white sharks.

One more suggestion: If you do have seals and sea lions in the area, watch their behavior. If you see them suddenly start behaving erratically and frantically while trying to vanish from the scene, take that as a good clue that you should probably be doing the same.

For more information and statistics on sharks and attacks, visit the California Department of Fish and Game Web site at http://www.dfg.ca.gov/mrd/whiteshark.html.

Carrie Wilson is an associate marine biologist with the California Department of Fish and Game. Contact her at: cwilson@dfg.ca.gov.