What is the size of a great white in proportion to the size of it's tail? I was diving the other day and only got a glimpse of the tail because visibility was bad. I am curious to how big he/she was.

Theresa Lange
Alameda, CA
directseal@aol.com

The distance from the notch at the base of the tail to the tip of the upper lobe varies from 23.5% of the shark's total length (i.e., snout to tip of tail) in juveniles to 21.8% of the overall length in adults. These percentages are based upon measurements performed on 14 males and 19 females, ranging from 170 (5.5) to 391 cm (12.8 ft) long. The information here was taken from Table 15 in the following reference:

Bass, A.J., D. d'Aubrey, and N. Kistnasamy. 1975. Sharks of the east coast of southern Africa. IV. The families Odontaspididae, Scapanorhynchidae, Isuridae, Cetorhinidae, Alopiidae, Orectolobidae, and Rhiniodontidae. Investigational Report 39, Oceanographic Research Institute, Durban, South Africa, 102 pp.

How do you make sure you don't get bitten?

Lyal Holder
Berthoud, CO
lyalholder@aol.com

The best method of avoiding shark attack or deterring a shark once attacked has always been a topic of disagreement among, bathers, divers, and even shark biologists. There can be very different motivations that lead a shark to attack a diver.

One motivation for attacks on humans is defense. The majority of sharks, small and large, will bite divers when they unknowingly approach too closely, violating the species' individual space. The sharks perform a threatening behavior. It is best described for the gray reef shark (Carcharhinus amblyrhynchos). A shark will swim rapidly in tight loops in front of the diver and lower its pectoral fins, arch its back, lower its tail, and point its snout upward. The mouth may be opened and closed repeatedly to reveal the shark's teeth. If approached too closely, the shark may bite the diver, but will usually remove no flesh. This behavior is analogous to a cat's arching its back, bristling its hair, and baring its teeth and hissing.

The best way to avoid a defensive attack is to recognize the threat behavior and slowly move away from the shark in the opposite direction. When diving in waters with sharks, it is essential to wear underwater goggles so that you can see the shark and act accordingly.

A second motivation for attack is feeding. The white shark (Carcharodon carcharias), the tiger shark (Galeocerdo cuvieri), and bull shark (Carcharhinus leucas) are sufficiently large to feed upon humans. The white shark dwells in cold water and, for that reason must maintain a warm body to stay active. Individuals of this species may spit humans out because they are low in nutritional value in comparison to seals and seal lions. The fat making up the insulating coat of seals has twice the energy content of human muscle. The tiger and bull sharks are inhabitants of warm water and on occasion eat humans.

It is best to avoid swimming and diving near concentrations of prey of these species. White sharks are most abundant near colonies of seals and sea lions. Tiger sharks are abundant near islands with large populations of sea birds.

If you are interested in learning more about how to avoid shark attack, I suggest that you read the following articles:

Baldridge, Jr., H.D. 1996. Comments on means for avoidance or deterrence of white shark attacks on humans. Pp. 477-479 in Klimley, A.P. and D.G. Ainley (Eds.), Great White Sharks: The Biology of Carchardon carcharias. Academic Press, San Diego, 513 pp.

Klimley, A.P. 1974. An inquiry into the causes of shark attacks. Sea Frontiers, 20(2):66-76.

How many people are attacked each year by sharks? Where does this happen most often? What are the numbers attributed to the Red Triangle of the California coast? What percent of these attacks result in death? Can you give statistics for the past 50 years?

John Hauser
San Dimas, CA
hauser52@earthlink.net

It is difficult to know how many humans are attacked worldwide by sharks each year as shark attacks in remote areas are rarely reported. You can contact Dr. George Burgess of the Florida Museum of Natural History, who keeps statistics on the number and locations of shark attacks in the International Shark Attack File. Seventy-six confirmed and 2 unconfirmed unprovoked attacks on humans have been recorded from Baja California, Mexico to Washington State from 1926-1993. The majority of these attacks occurred in Central California, an area encompassing the "Red Triangle." Only nine of these attacks were fatal. This number is small relative to the large number of human deaths each year in California due to automobile crashes. For information about shark attacks in various regions see the following articles:

Baldridge, H.D. 1974. Shark Attack. Droke House/Hallus, Anderson, North Carolina.

Burgess, G.H. and M. Callahan. 1996. Worldwide patterns of white shark attacks on humans. Pp. 457-469 in Klimley, A.P. and D.G. Ainley (Eds.), Great White Sharks: The Biology of Carcharodon carcharias. Academic Press, San Diego, 513 pp.

Levine, M. 1996. Unprovoked attacks by white sharks off the South Africa. Pp. 435—448 in Klimley and Ainley (Eds), Ibid.

McCosker, J.E. and R.N. Lea. 1998. White shark attacks in the Eastern Pacific Ocean: an update and analysis. Pp. 419-434 in Klimley and Ainley (Eds.), Ibid. (see Table I for statistics for last 50 years).

West, J. 1996. White shark attacks in Australian waters. Pp. 449-455 in Klimley and Ainley (Eds.), Ibid.

What are other ways to prevent shark attacks, aside from nets and sharkpod?

(name witheld by request)

One new approach to deter sharks is to generate electromagnetic fields near a diver. Sharks are very sensitive to these fields and use them to localize their prey and navigate. However, they may be startled by a sudden increase in an electrical field. Sharks also withdraw from sounds that have a sudden onset. Finally, the discovery of a small flatfish, the Moses sole or Pardachirus marmoratus, that prevented sharks from biting down on it by secreting a chemical—has led to recent research on chemical repellents. This chemical, termed pardaxin, possessed surfactant (or detergent) properties damaging cell membranes. Researchers conducted tests that showed that one commercially available surfactant, sodium dodecyl sulfate, repelled captive lemon sharks (Negaprion brevirostris). However, to produce a withdrawal response, the chemical had to be delivered into the mouth cavity. For more information on sound and chemical repellents, consult the following articles:

Klimley, A.P. and A.A. Myrberg, Jr. 1979. Acoustic stimuli underlying withdrawal from a sound source by adult lemon sharks, Negaprion brevirostris (Poey). Bulletin of Marine Science, 29:447-458.

Nelson, D.R. and W.R. Strong, Jr. 1996. Chemical repellent tests on white sharks, with comments on repellent delivery methods. Pp. 471-475 in Klimley and Ainley (Eds.), Ibid.