People More Dangerous to Sharks than Vice Versa
In the past 20 years, the numbers of many shark species have declined, says FWC scientist.

By Wendy B. Dial, FWC
Posted Tuesday, March 16, 2010

 
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As a top predator in the world's oceans, sharks deserve our cautious respect. However, the fear they instill among swimmers is disproportionate to the actual
number of negative encounters. Sharks prefer other prey. Attacks on humans are usually a case of mistaken identity.

According to International Shark Attack File statistics, the odds of a shark attack on a human are 1 in 11.5 million, and of a fatal shark attack, 0 in 264.1 million.
Humans are top predators too, and they pose more of a danger to sharks than sharks do to people. Some anglers love to catch and kill sharks just for the thrill, while
people harvest most sharks primarily for their meat, fins, hides and jaws. Brent Winner, a scientist at the Florida Fish and Wildlife Conservation Commission's Fish
and Wildlife Research Institute, noted that people harvest an estimated 100 million sharks annually worldwide.

Scientific data show that many shark populations have been dramatically reduced by almost 50 percent over the past 25 years," Winner said. The lemon shark, known to scientists as "Negaprion brevirostris," has an unhealthy
habit. This large, coastal shark gathers - aggregates - at specific locations off Florida's southeast coast from November through June for what scientists believe are "pre-breeding aggregations."

Clustering makes the lemon-colored sharks that reach 9 feet vulnerable to overfishing, so the FWC recently passed a rule protecting lemon sharks from recreational and commercial harvest in all Florida state
waters. (See "Lemon law adds teeth to shark protection.") Another factor against the recovery of the lemon shark, and many other shark species, is delayed maturity. Many species of shark don't reproduce until age 14 to
18, and then they have only a few pups. Some lay eggs, while others give live birth. Luckily, sharks have some built-in advantages to detect and eat food besides their
keen sense of smell and sight - and their teeth. Some have lateral lines on both sides of the body that are a series of fluid-filled canals equipped with cells sensitive to movement in the water. Another shark sensory system consists of
ampullae of Lorenzi (fluid-filled pores on the lower head and snout).

"The pores detect the presence of electric fields produced by all living creatures in the sea," Winner said. "This allows sharks to find prey at close range, without seeing it."
There are 350 different sharks - cartilaginous fish whose skeletons lack true bones.

Other large coastal sharks
If a school of Spanish mackerel swims by, schools of blacktip sharks (Carcharhinus limbatus) may follow, seeking dinner. They are usually to blame in most "bite-and-release" attacks on humans. They grow to 6-8 feet. A blacktip shark look-alike, the spinner shark (Carcharhinus brevipinna), named for its spinning, leaps out of the water, grows to about 6-8 feet, feeding on sardines,
herring, small sharks and rays.

The bull shark (Carcharhinus leucas) inhabits estuarine, near-shore and offshore waters of both the Gulf and Atlantic coasts of Florida.

"It is one of the few sharks that may inhabit fresh water," Winner said. "It sometimes ventures hundreds of miles inland via coastal river systems. "This is one of the more dangerous shark species, accounting for the third-highest
number of attacks on humans," Winner added. However, most Florida shark attacks are near shore and involve blacktips, spinners, bulls and hammerhead sharks. Bull sharks
are opportunistic feeders, feeding on a variety of bony fishes, marine invertebrates, sharks, rays, dolphins, sea turtles and even sea birds.

Nurse sharks (Ginglymostoma cirratum), which are sluggish swimmers and often rest on the bottom, can be dangerous to humans if provoked. They're usually 6-9 feet, but
they can reach 14 feet and weigh over 300 pounds. You can easily recognize nurse sharks by their unique profile and fleshy, catfish-like barbels that hang below their nose to help detect prey.

Smaller coastal sharks
Large sharks often prey on the blacknose shark (Carcharhinus acronotus). This small shark - reaching only 5 feet - swims in bays, lagoons and coastal waters looking for
small fish. Its name comes from the dusky smudge on its snout.

An even smaller shark, the 3- to 4-foot bonnethead (Sphyrna tiburo), also frequents near-shore Florida waters. It is the smallest of the hammerhead family. Crabs,
shrimp and small fishes dominate its diet.

Another small shark is the Atlantic sharpnose (Rhizoprionodon terraenovae), a 2- to 4-foot fish that frequents coastal waters throughout Florida and often ventures into
bays and estuaries.

"I've been in this state only 20 years, yet, in that short time, the numbers of many shark species have declined. In 1989, we didn't have harvest restrictions on any
species. Now, harvest is prohibited on more than 20 shark species in both state and federal waters," Winner said.

The ocean is a wild environment for humans (Homo sapiens), but it is home to sharks. As apex predators, sharks maintain the natural balance of life in the sea. For more
information on sharks, go to research.myfwc.com/features/<http://research.myfwc.com/features/category_main.asp?id=1867> and click on saltwater.