It has been suggested that the greatest danger from sharks occurs in warm tropical seas, [however] there are records of sharks attacking people in the distinctly chilly seas of high latitudes, such as the fisherman at Wick, northernmost Scotland...who was bitten on the arm. - Rodney Steel, "Sharks of the World"
We've already mentioned the three most aggressive and dangerous shark species: great white, tiger and bull sharks. These species are the most deadly for several reasons:
- They are widespread.
- They are large enough that humans can look like prey to them.
- They are so powerful that the initial bite can cause fatal damage.
- They are at the top of the food chain, which means they're not instinctively afraid of anything.
However, other shark species aren't completely innocent. Sand tigers, hammerheads, and makos are also responsible for some attacks, while a third of shark attacks are made by lesser known species, such as black tips, nurse sharks, and various reef sharks. Overall, the bull shark may be the most dangerous species because of its aggressive attack patterns and its preferred habitat - shallow coastal waters.
Statistically, there are between 30 and 50 unprovoked shark attacks reported worldwide each year, with five to ten of them proving fatal. Florida has the most attacks in the United States, with numbers since 1990 ranging from 10 to 37 per year. The United States tops the list for attacks worldwide. [ref]
The vast majority of attacks occur within a few hundred yards of shore, simply because that's where most people enter the ocean. The number of attacks worldwide and in the U.S. have been increasing in recent years for a similar reason -- more people are taking coastal vacations and participating in ocean activities. There is no indication that sharks are actually becoming more aggressive.
Government protection of aquatic mammals has led to thriving populations of seals, sea lions, and sea otters off the west coast of the U.S. All of these animals are prey for great white sharks. As a result, coastal areas near San Francisco -- particularly places that are obviously inhabited by large ocean mammal populations -- have increased numbers of great white sharks. There hasn't been a spike in great white attacks in these areas because for the most part people know better than to go swimming with sea lions when they know great whites are around.
Although shark attacks do tend to be clustered in certain areas, sharks travel great distances and frequently break out of their range. Great whites in particular have no problem with cold water -- they can be found as far north as Oregon on the west coast and New England coastal waters in the east. Bull sharks are noted for their ability to tolerate fresh water, and they have been found swimming in rivers thousands of miles from the ocean. However, they generally prefer a tropical climate.