By Kristin Thoms
You may not see them, but they are there - lurking in the watery depths, elusive and stealth-like, often passing by unnoticed. Their senses are among the most well developed in the animal kingdom, which is why some call them the "perfect predator"… and Tampa Bay is their nursery.
Sharks, found in a mosaic of shapes and sizes, are incredibly powerful and superbly efficient hunters that seem to enamor us as much as terrify us. They are epic creatures, surviving in our seas for over 400 million years and evolving into one of the most highly adapted animals on earth. As apex predators, they are perched at the top of the marine food web, and have the astonishing ability to occupy both marine and freshwater environments.
As spring turns to summer and the warm days of fall, Tampa Bay brims with activity both above and below the surface. Scores of people flock to the water to enjoy recreational activities, while newly born marine life whirls beneath the waves. The bay becomes particularly inviting to sharks as its warm waters serve up copious amounts of food and provide a safe haven for baby sharks called "pups."
"Sharks inhabit the bay year-round, but overwhelmingly in the spring and summer months through October," said Robert Heuter, director of the Center for Shark Research at Mote Marine Laboratory. "They utilize the bay as a feeding ground and as a place to give birth to their young away from larger predators."
As "top dogs" of the sea, however, sharks don't have much to worry about when it comes to natural predators - they fall prey only to larger sharks.
About a dozen shark species frequent Tampa Bay.
Tenants include hammerheads, bonnetheads, nurse sharks, blacktips, lemon sharks and bull sharks to name a few.
Sharks migrate in and out of Florida's waters each year. Their movements are usually associated with temperature change or the presence of prey such as small baitfish, mullet and menhaden. Expecting mothers often travel into shallow-water estuaries, give birth and then leave, Heuter said.
Pups are left to mature on their own. Experts speculate this behavior protects them from being preyed upon by their mother.
Although the bay attracts both a frenzy of shark and human activity during the summer, shark attacks are rare. The odds of being attacked by a shark in the U.S. are roughly 1 in 6 million, according to the International Shark Attack Files.
"Bull sharks are the most aggressive and typically the only species we need to be concerned about in terms of serious attacks," Heuter said. According to the National Safety Council, you are more likely to drown in the bathtub than be attacked by a shark.
"In the summer, just be aware that there are larger sharks swimming around. The beaches are generally safe, but don't swim at dusk or dawn, and it's probably not a good idea to swim in the open bay," Heuter warned.
Sharks use the entire bay; however, scientists have pinpointed certain "hot spots" or "pockets" that individual species prefer.
"The mouth of the Alafia River is a popular pupping ground, or nursery, for bull sharks," Heuter said. "They enjoy the freshwater outfalls as they are physiologically adapted to handle the lower salinity." Bull sharks are unique among all sharks because their body chemistry adjusts until it matches the salinity of the surrounding freshwater. A special gland near the tail helps retain salt as the kidneys help recycle salt already in the body.
Juvenile hammerheads have been spotted feeding in some parts of the bay while nurse sharks have been observed mating in others.
"In part, Tampa Bay is a big shared habitat," Heuter said. "Pinellas Point serves as both a feeding ground for bonnethead sharks and a pupping ground for lemon sharks."
Heuter explained that although shark activity varies across the bay, there are clear distinctions defining critical habitat areas for certain species.
Terra Ceia Bay, located at the mouth of Tampa Bay, is a well-known pupping ground for black tip sharks, one of the top targets for commercial fishermen. For this reason, Terra Ceia Bay has been the focus of extensive research by Mote scientists studying shark distribution patterns and habitat use.
"We know very little about the early part of their lives, such as how many pups survive or how long they stay in specific areas. It's of increasing importance to determine what the young need in order to determine if we need to protect specific habitats," said Dr. Michelle Heupel, staff scientist at Mote's shark center.
Fast Facts About Sharks
Sharks are vulnerable to fishing pressure because they:
- Grow slowly
- Take many years to mature (12 to 18 years in some species)
- Often reproduce only every other year
- Have few young per brood (only two pups for some species)
- Have specific requirements for nursery areas (bays and estuaries)
- Are caught in many types of fishing gear (hook and line, gillnet, trawl)
Sharks have adaptations allowing them to be apex predators including:
- Teeth that are replaced throughout their life
- Sensitive smell receptors
- Eyes that adapt quickly to low light levels
- Lateral line receptors that sense movement in the water
- Electroreceptors that detect electrical fields caused by the presence of prey
Art: Diane Peebles
Bits & Bites
There are more than 350 species of sharks
A shark may grow and use over 20,000 teeth in its lifetime
On average, there are only about 100 shark attacks a year worldwide - only 10 of those result in human death
Two-thirds of the shark's brain is dedicated to its keenest sense - smell
Sharks can generate about 6.5 tons per square inch of biting force
Sharks have extremely sensitive hearing - some species can hear sounds more than 700 feet away
About 75 shark species are in danger of becoming extinct
The shark's average life span is less than 25 years
The shark's skeleton is made of cartilage, a type of strong but flexible tissue
The shark's average swimming speed is a yard per second
About 30% of sharks are oviparous (egg laying) and about 70% are live bearers
The shark is among the oldest species of fish to swim the seas
For more sharky information:
U.S. Fish and Wildlife Service
National Marine Fisheries Service
Florida Museum of Natural History
Mote Marine Laboratory
Shark Species Common to Tampa Bay and the Gulf Coast
Mote researchers use a variety of approaches to study habitat use inside the bay, including gill nets, banned by commercial fisherman but used by permit to quantify the abundance of smaller sharks; hook and line; and tagging techniques, such as the use of traditional, acoustic and satellite tags.
Mote scientists have tagged over 13,000 sharks along the Gulf coast.
"We tag and release every shark possible," Heuter said.
Acoustic and satellite telemetry allows scientists to monitor an animal continuously, without the need to recapture a tagged animal or rely on tag reports from other individuals.
Over the past four years, Mote has attached 127 acoustic tags to sharks in Terra Ceia Bay, gathering a tremendous amount of information.
"Our most interesting find was that the majority of animals stayed the entire summer until they needed to migrate in late October or November due to colder water temperatures," Heupel said.
Scientists believe that sharks migrate to winter feeding grounds such as Florida Bay or the Keys.
"Even more unexpected was that early in the summer, in May and June, the sharks were using a very restricted, northern part of the bay. In August, they suddenly changed what they were doing and expanded their area to the whole bay. We saw young sharks repeatedly going in and out of Terra Ceia and Tampa Bay" - a pattern completely different than Heupel and other scientists had ever seen before.
Scientists hypothesize that as sharks begin to grow and perfect their hunting skills, they become more courageous and begin to explore areas outside their immediate nursery grounds. Why blacktips use such a restricted part of the bay at the start of summer has scientists scratching their heads. erhaps they are less at risk of becoming a tasty snack for a larger shark.
"As they get a little bigger and a little faster, they get better at surviving," Heupel said.
Occasionally, scientists will keep animals for dissection to examine stomach content or to section vertebrae to determine age and growth. They will also collect live animals and study them under controlled conditions, conducting sensory, anatomical and physiological studies, Heuter said.
Back in the lab, scientists examine the biomedical aspects of shark biology. They also are interested in what sharks can do directly for people.
"Sharks have a natural resistance to disease, specifically to cancer," Heuter said. Why sharks don't get cancer is an ongoing puzzle, but shark sleuths at Mote hope that studying the shark's immune system will someday result in finding a cure.
Overall, there's a whole lot of research going on in Tampa Bay.
Mote provides data to agencies such as the National Marine Fisheries Service so fishery managers can implement the appropriate management strategies. Shark populations are overfished and severely depleted, which could have serious implications since sharks play a keystone role in the marine ecosystem.
"There aren't too many things that can replace them. Without them, the whole ecological balance would be out of whack, which is why it is important to focus on conservation and management issues revolving around their status," Heuter said.
An important question Mote scientists are addressing is whether sharks return to the same sites each year. Thirteen years of tagging have revealed a trend for individual sharks to return to the same habitats during some part of their lifecycle.
Heuter says that pregnant females will often come back to the same nursery they were born in to give birth.
If that's the case, it's important that we keep Terra Ceia and other habitats like it, healthy, Heupel explained.
"Every area seems to have a sub-population of sharks; thus, if you fish an area hard, you may knock off a whole group similar to the way overfishing salmon up north impacts a stream bed," Heuter said.
Most sharks reproduce slowly, giving birth every other year to a small litter of five or six pups. That's not nearly enough to keep up with the world's fisheries, which kill an estimated 30 to 100 million sharks a year.
Despite the length of time sharks have been around, scientists still have much to learn.
Heuter's goal is that continued research will eventually dispel myths surrounding these remarkable animals and that people will educate themselves and others, fostering greater support for fisheries management.