Creature Feature by Dr Gerald Goeden: July 2013

Wednesday, 31 July 2013

The Unbelievable Basking Shark by Dr. Gerald Goeden

While the Whale Shark is the largest living fish, the Basking Shark, Cetorhinus maximus, takes a close second place. Basking Sharks often grow to 6 metres and sometimes to over 8 metres. Before heavy commercial fishing, some Basking Sharks reached 9-10 metres. The largest Basking Shark ever measured was caught off the coast of Canada in 1851. Its total length was 12.3 metres, and it weighed an estimated 17-18 tonnes.

Unlike bony fish with air-bladders, all sharks have a problem staying ‘afloat’. Most solve this problem by constantly swimming and having large, light livers inside them. The Basking Shark's liver, makes up about 25% of its body weight, and takes up much of the abdominal cavity.

Basking Sharks are found in most of the cool to temperate waters of the world. It is a coastal surface dwelling shark common to the continental shelves of most continents. It gets its name from slowly cruising in the sunny surface waters but will descend to about 1000 m in search of food.

The Basking Shark is a passive filter feeder, relying only on the water that is pushed through its gills by swimming. It filters out zooplankton and small fish from up to 2,000 tonnes of water per hour. Unlike the Whale Shark, the Basking Shark does not actively search for its food. Ichthyologists (people who study fish) have found that they may have an acute sense of smell that can detect plankton up-current of them.

After its size, the next thing you notice when swimming with a Basking Shark is its huge mouth. Preferred feeding seems to be at the surface where the slow-moving sharks (about 4 km/h) open their mouths and spread out the gill rakers that filter the food from the water. Even though they look like weak swimmers there are reports of bursts of speed and leaps completely out of the water (breaching).

The sight of an approaching Basking Shark with its mouth open is awesome but they are harmless to humans. Unfortunately, humans are not harmless to Basking Sharks. It is fished mainly for its fins (for shark fin soup). The skeleton is sometimes used in traditional Chinese medicine and as an aphrodisiac in Japan.

Because of increasing demand for shark fin the giant Basking Shark is disappearing fast. Only a few countries (UK, Malta, New Zealand, and US Gulf and Atlantic waters) have protected the species and stopped trade in its products.

Many thanks to Wiki-commons for the excellent photos.

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Tuesday, 9 July 2013

SWIMMING WITH TIGERS by Dr. "Gerald Goeden"

There is much concern about shark fishing and shark fining. Ocean conservation is at stake as predators in all the world oceans are subjected to over-fishing. Caribbean and Andaman Sea fisheries (Malaysia) are especially stressed. Growing support for eco-tourism based experiences like swimming with sharks and swimming with dolphins promise to outstrip the economic benefit of over-exploitation. The following report looks at the experience of one of our scuba divers in assessing a shark conservation project off South Africa.

During a recent photo expedition Jeff had a surprise visit from a big Tiger Shark. Jeff and Gerry tell the story:

“All of a sudden something was holding on to my scuba gear and shaking me like a feather. I was so busy getting some great shots that I didn’t see this big one come up behind me.”

“I was pretty lucky that I wasn’t hurt, but it seemed like the Tiger was just trying to figure out what I was.”

The Tiger Shark, which generally hunts at night, has a reputation for eating anything and everything and for an aggressive nature. Most of the time, Tiger Sharks eat fish, squid, seabirds, sea mammals, other sharks, and turtles.

Out of more than 360 shark species, only four have been involved in a significant number of fatal attacks on humans over the last 60-70 years: the great white, tiger, oceanic white tip, and the bull shark. Culling Tiger Sharks a few days after an attack probably has little value since most are nomadic and the offending animal may well move on. Some governments ‘protect’ swimming beaches with shark nets that catch any sharks swimming along the coast.

Even small Tiger Sharks require a lot of watching and big ones can weigh up to 1000 kg and reach 6-7.5 metres. They get the name Tiger from the vertical stripes and not from their diet. Based on diet, they are more like ‘trash-can sharks’ since stomachs have been found to contain buckets, rolls of wire, gas cans, paint brushes, and baseballs. One even contained old tires.

So why did the shark bite Jeff’s metal air tank?

First, the shark was confused by the metal on Jeff’s back. Tiger sharks have small pits (ampullae of lorenzini) on their snout and along their upper bodies which can sense weak electrical fields. Normally these sensors detect fish when the water is too murky for their usually sharp eyes to see. But different kinds of metals generate weak electric currents in seawater and alert the shark that something is going on.

Secondly, the Tiger Shark’s keen sense of smell would not be sure if the smell of human meant the next meal or danger. You don’t grow up to be a big shark without a little caution!

Finally, sharks adopt the same strategy as children; if you are unsure what something is, just put it in your mouth and taste it.

“Diving with Tiger Sharks is awesome and something I will never forget. The footage will be great for documentary work and we always learn something about these incredible animals. We have to work fast as Tiger Sharks are disappearing because of the shark fin trade and sport fishing.”

Nature has equipped these sharks with all the gear to be great hunters: they can sense movement in the dark, feel water pressure waves from injured animals, smell traces of blood in the water, and have excellent vision. They are fearless and haven’t learned to stay away from boats and fishing gear.