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Shark Central

Shark! Just the name evokes powerful images of teeth, jaws and powerful attacks. But is this image truly warranted? The answer, quite simply, is no. Sharks are indeed successful predators, a testament to their longevity. Sharks have remained virtually unchanged for over 40 million years and inhabit every ocean on the planet. The images we have of sharks, often portrayed in movies, are only one aspect of this spectacular fish.

(View Quick Time video)
Small fish flash around gray reef sharks in Rangiroa.

Sharks are cartilaginous fishes, those without true bones, and are related to skates and rays. Although we often evoke an image of a great white shark that can be 6-7meters long and weigh 2-3 tons, sharks come in all shapes and sizes down to the tiny pygmy shark which is just 25 cm long! Even shark teeth, often sold as souvenirs, range in size and shape: from the small, needle-like teeth of the sand tiger shark made for capturing slippery fish to the large triangular serrated teeth used for grabbing and thrashing large prey to the sharks without teeth, the filter feeders such as basking and whale sharks.

This incredibly prolific and successful creature is found at virtually all depths within the ocean realm, some so deep their life histories are just being discovered. Their ability to swim at these variable depths is partially dependent on their amazing liver. This liver replaces the swim bladder found in bony fish and is oil-filled allowing for sharks to be slightly negatively buoyant. So the old myth about sharks having to swim constantly or die is not quite accurate: They must swim or sink. The sharks living at depth often feed on crabs, clams and sea urchins while the midwater and shallow feeding sharks often feed on fish, marine mammals and even other sharks.

It may be hard to believe by looking at the daunting jaws of this powerful predator but when it comes to having their young, sharks may partake in one of three delicate processes. Some sharks lay eggs, often in a pod, which are often referred to as mermaids purses, and can be found washed up on beaches. Other external egg-layers like the Port Jackson or Dogfish, lay eggs that are attached to kelp or other plants to help camouflage their valuable clutch. Internal egg-layers, sharks that lay eggs within their bodies, allow the young to gestate until fully developed. Still other sharks, the large ones in general, give live birth to fully developed young. Those early years are difficult to survive, as young sharks are often prey themselves.

Although sharks are found all over the globe, two particularly well known hot spots for sharks are Rangiroa Atoll near Tahiti and the Farallon Islands located off the coast of California. These two places are as far apart on the earth as you can get, yet sharks are found in both locations. The myriad species of sharks live very different lives at these two remote havens, especially when it comes to interacting with their fellow ocean creatures.  

RANGIROA

Rangiroa atoll, the second largest atoll on earth, lies 250 miles north of Tahiti in the Tuamotu Archipelago. "Rangi" was once an active volcano yet now all that remains is a circular ring that houses a stunning lagoon in it's center. Twice a day massive volumes of water fill the lagoon, on the high tide, flooding in through two narrow passes from the outside ocean. When the water flows back out to the awaiting deep the water can reach speeds exceeding five knots. This outflowing current collides with incoming ocean swells to create spectacular standing waves up to eight feet in height. Outside the narrow passes depths plunge hundreds of feet along the coral covered slopes of this ancient volcano. It is at the mouth of the pass and the surrounding volcanic walls that one of the largest concentration of sharks in the world is found.

Silvertips, silkies, gray reef, black tips, hammerheads, and tiger are all species that inhabit this remote Polynesian lagoon. Tens, hundreds, and even schools numbering in the thousands of sharks exist within Rangi. The sharks that inhabit the blue depths here all seem to have complex and differing strategies of feeding, mating, schooling and traveling. However, few of these strategies are well known because sharks can be elusive and research on these fascinating creatures is relatively new. It is clear, though, that the impressive schooling of hammerheads, in the hundreds these days, is only a remnant of what the schools once were before humans. The schooling may be a type of breeding aggregation or, alternatively, could be used in feeding strategies - there are no clear answers yet. Other sharks clearly do breed in schools, baby black tips are found literally in the middle of the Blue Lagoon!

Thanks to the work of dedicated scientists like Michael Pool and others, such as cameraman Bob Cranston, we have glimpsed some of the beautiful and mysterious behaviors and interactions between the sharks of Rangi. Yet, in addition to the many shark species, there are scores of other creatures that inhabit Rangi waters. The interactions with these other creatures would be assumed to be of the straightforward predator-prey variety. But at least with one animal, the relationship and coexistence with the sharks is surprisingly complex.

Dolphins and sharks are thought to be deadly enemies but the relationship is far more complicated and intricate than that. It starts with the dolphins. When the tunnels of water exit the Blue Lagoon and hit the ocean swells they create giant waves. Within these waves a resident school of bottlenose dolphins come to play. The dolphins both surf the waves and use them as a springboard to launch themselves into the air to heights of twenty feet and more. Beneath the dolphins swim aggregations of gray reef sharks numbering in the thousands.

There are also silvertips and silky sharks, and the occasional great hammerhead and tiger shark. These dolphins live, play, hunt and raise their young in the midst of one of the greatest concentrations of sharks on earth. The relationship between the dolphins and the various species of shark is complex and distinct. It is not uncommon for the dolphins to chase off gray reef sharks if they grow intrusive, and they sometimes team up to tail-slap a silver tip that approaches too closely. Dolphins acting alone are vulnerable; acting in concert they are dominant over most sharks. Hammerheads and tiger sharks are another matter. The dolphins do not toy with these predators. They gather into a tight group and withdraw.

How can the dolphins coexist so with the sharks? That mystery is still under investigation. But we do know that the perceptions of sharks the public had no longer apply. The sharks of Rangi and the world are like any other animal on the planet. Social interactions, predation, birth and death rule the lives of sharks as they do for bottlenose dolphins, fish, and even us.

THE FARALLONS

Five thousand miles from Rangiroa, and twenty-eight miles west of San Francisco's Golden Gate Bridge, lie a group of 3 granite outcrops that rise above the ocean's surface. The Farallons, stretching no more than a mile long on the largest island, are home to a staggering number of animals. And, similar to Rangi, there are variable species of shark around the Farallons. Due to the island's close proximity to the continental shelf and adjacent deep waters, it is not uncommon to see shark species such as Blue and mako sharks; sharks that are typically considered pelagic in nature. Thresher, salmon and even the mysterious sleeper shark that skulks in the cold deep have been known to make an appearance in the surrounding waters.

But, it is the white shark that dominates the underwater world here. Just like sharks everywhere, white sharks have a much greater and complex life history than what we are typically exposed to in cinematic thrillers such as "Jaws" and other Hollywood horror films. However, unlike the Rangiroa sharks, there does not appear to be any complex or mysterious relationship between the sharks and marine mammals of the Farallons. The relationship the sharks have with any species here is that of top predator.

The Farallon Islands are home or at least a waypoint for hundreds of thousands of breeding seabirds, and significant numbers of seals, sea lions, dolphins and whales. As you approach the islands the fog is often so thick you can see nothing but the water surrounding your boat. Then, the fog lifts and the South East Farallon Island appears before you in all it's grandeur.

This island is one where groups of pinnipeds (the seals and sea lions) come to breed, feed and rest. And in the fall and winter it is also where the pinnipeds' greatest predator congregates, hoping for a new seal pup or even an adult elephant seal as it's next meal.

A shark attack is a spectacular display of force and stealth. As the pups leave the island to travel and feed they swim at the surface of the ocean.

From the murky depths below, visual cues and a keen sense of smell, coupled with a very sensitive lateral line system that helps sense vibrations in the water, allow the shark to surge upon the unsuspecting seal. An adult elephant seal can weigh in at over two tons and could be a worthy adversary for even an adult white shark. So, after grabbing their meal, with powerful jaws agape, white sharks take a massive first bite with such force that the mortally wounded seal is left to thrash about while the shark watches nearby. When the danger and energy of the writhing seal subsides, the shark can then approach and tear bites off at its leisure. However, the sharks here cannot afford to waste too much time, as other hungry sharks may come and usurp a prize kill. It is not uncommon for four or five other white sharks to appear at the kill shortly afterward and take turns devouring the carcass.

The white sharks of the Farallon Islands have been studied intensively for the last 15 years. As part of a larger collaborative research project investigating long range movements, the white sharks of the Farallons and of nearby Ano Nuevo, 60 miles south of San Francisco, have been fitted with satellite tags. These tags are giving researchers like Scott Davis (University of California at Santa Cruz, UCSC) and fellow collaborators from the Point Reyes Bird Observatory, Hopkins Marine Laboratory, and UCSC integral information about their life history and movements throughout central California. Data from the satellite tags have yet to be retrieved but preliminary observations suggest that the white sharks found at Ano Nuevo Island and the SE Farallons don't overlap much between the two sites.

This is incredible considering the variety and abundance of prey readily available throughout these areas and the fact that sharks can travel great distances, so 60 miles between the two islands would be an easy trip. Research like Scott's and others leads to the conservation and preservation of these prehistoric champions. For example in the summer of 1997, California state law was changed to permanently prohibit commercial and sportfishermen from capturing great whites - a move that was widely supported by surfers and divers of the area. Although this may seem a small and long-overdue step, this and other legislation is finally giving a voice to shark conservation.

As research continues, conservation laws increase and public awareness grows, places like Rangiroa and the Farallon Islands are fast becoming shark hot spots. With this new awareness we must remember sharks are powerful and prolific top predators, worthy of great respect. Cage diving, chumming, and shark fishing still continue. How will you view the mighty prehistoric shark now that you know they are a whole lot more than Jaws?

 

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