2010年7月28日 星期三

The hammerheads of Layang

July' 07, 13 lucky divers ( including the dive master ) sought and found schools of Hammerheads in Layang, not once but twice !

Scalloped Hammerheads are found in depths ranging from 1-300 m in all tropical and warm temperate seas, from coastal areas near continents to oceanic islands far offshore . Eight species of hammerheads are known, each with a somewhat different hammer shape, so as to give each species the best adaptive advantage in its chosen habitat .
The hammer-shaped head is thought to have evolved to maximize the area of sensory organs such as the Ampullae of Lorenzini ( special sensors in sharks' skin ) to detect chemical, physical and thermal changes as well as the electrical fields of prey , including those buried in the ocean bottom—certain fishes, rays, crustaceans, etc. The hammer shape also allows Hammerheads to scan significantly larger areas of the bottom when hunting than other shark species.

" For this dive the maximum depth is 36m, " the dive master instructed us before the dive, " anyone who goes beyond this will be penalized " he added ominously.

We circled around in the middle of the ocean for what seemed an eternity, and were about to give up when the dive master suddenly spotted a " flashing " .

Individual Scalloped Hammerheads in schools sometimes 'flash' when the flickering underwater sunlight strikes their dermal denticles (scales) just right ; it is most often noticed when the hammerhead is twisting or looping in the water. Nobody knows whether the " flash" is an accidental effect , or part of the communication repertoire of the Hammerheads .

On cue, we plunged headlong into the deep in pursuit of the sharks , and found ourselves in the midst of a large school of Hammerheads .

During daylight hours, Scalloped Hammerheads gather near seamounts in schools of up to 500 individuals. Some researchers speculate that Hammerheads school as a prelude to mating, but many schools are composed mostly of adolescent females . The more likely reason Hammerheads school is "refuging" : the seamounts, which are usually near a rich food supply , serve as conspicuous undersea meeting places for the schooling fish to interact socially until nightfall, when the school breaks up and individual Hammerhead goes its own way to forage for food .

I was transfixed with the spinning Hammerheads and I couldn't stop myself from spinning along with them . I glanced at my depth gauge in passing .

" Shit ! " I swore into my regulator . I was 50m down ! I hurriedly looked around and saw a Swiss guy taking pictures below me , the guy had to be at least 55 m deep, give or take a m ! I felt instant relief : well , I'm not the worst offender of the depth rule !

I'm sad to say sharks of the world are in far deeper shit than I , as there are no laws protecting their populations.
Humans exploit all animals they can profit from , and the NOAA Fisheries estimate 100 million sharks are killed each year for their meat, skin, and oil . Great hammerhead meat is sold for human consumption ( fresh, fresh-frozen, dried-salted, and smoked) , fins for soup, hides for leather, and carcasses for fishmeal . Their jaws, teeth, and backbones are often sold as curios . During World War II this species was exploited for its liver , which is especially high in vitamin A , such that one gram of oil contains fifty thousand units of vitamin A (Gilbert 1967) . Not until after World War II when a synthetic form of vitamin A was created did the demand for the Hammerhead liver dwindle.

At this point in time about 50 percent of the sharks captured worldwide are part of what is called the bycatch - taken incidentally in high seas tuna and swordfish fisheries, A major factor contributing to this problem is the use of gear known as pelagic longlines , which consists of single-stranded fishing lines each stretching anywhere from 10 to 40 miles, carrying an average of 1,500 baited hooks . In some areas, the number of sharks caught accidentally in longlines reaches 90 percent of total captures. Sharks also die of entanglement in fishing nets as they need to continue swimming to breath .
Recently as stocks of bony fishes have been substantially reduced by large scale fishing, commercial fishermen have began to compensate by vastly increasing the capture of sharks. Today, most shark species are being overfished in many seas of the world ; Hammerhead sharks are among the species most likely to be finned as their fins are highly prized for the Asian delicacy, shark fin soup.
"Shark finning" is the process whereby the fishermen slice off the fins of the sharks and discard the rest of the body into the sea. This practice has recently been banned by Australia , Canada, Brazil, the United States and the European Union, but lenient standards and lack of enforcement hamper their effectiveness.

Sharks are much more vulnerable to overexploitation than bony fishes , for several reasons.
As most sharks tend to be long-lived (Scalloped Hammerheads are believed to live to about 25 years ), maturation is slow . Male Scalloped Hammerheads reach sexual maturity at about 10 years , females at about 15 -16 years. After giving birth, a mother Scalloped Hammerhead may need to take a year off between pregnancies to re-build her energy stores , so Scalloped Hammerheads only breed once every two years , and as gestation is long ( about 12 months) and litter size small with a high juvenile mortality rate, they are ill-equipped to withstand high levels of fishing. The practice of shark fishermen of targeting pregnant females, which are more profitable because they are larger , and exploitation of pups in a nursery area because they're easy prey , can be particularly devastating for their population . Thus shark populations recover from overexploitation far more slowly. By contrast, bony fishes lay numerous eggs at an early age, and they grow and reproduce much faster. "As a result," said Michael Sutton, director of the Monterey Bay Aquarium's Center for the Future of the Oceans, "there is no such thing as a sustainable shark fishery.”

Besides the direct threats humans pose to the shark , even more important are the indirect threats towards its habitat. The ocean that provides life for all on earth is being threatened by our actions .
Through our excessive use of fossil fuels we are heating the atmosphere at an alarming rate , and this is causing our oceans to heat up as well. The Hammerhead sharks migrate with the seasonal change of water temperature. The heating of the oceans will force the Hammerheads into waters that were previously too cold, while the waters that they are used to swimming in will become too warm , this plus the overfishing of their prey ( tunas, mackerels, pilchards, rays, squids, and crustaceans ) by the fishermen in order to feed the mindless explosive growth of human population, dwindles the food supply even more .
Ocean pollution poses another big problem . We humans have historically been using the ocean as a bottomless garbage dump : the runoffs from land deposit all sorts of oils , chemical wastes, sewage , even radioactive materials into the ocean. The amounts are hard to measure and even harder to control. The effects of this are equally difficult to assess, and we may not know how much harm we are doing until it is too late. If toxic chemicals are ingested by animals, they accumulate in each individual and are passed up the food web, from prey to predator. Consequently, apex predators such as sharks are at higher risk of receiving concentrated toxins from their prey.

The decline in shark population has been steep, as documented recently by scientists using technologies including satellite tracking and DNA analysis. In March this year , a team of Canadian and U.S. scientists calculated that between 1970 and 2005, the number of Scalloped Hammerhead and tiger sharks may have declined by more than 97 percent along the East Coast, the status of Scalloped Hammerhead shark was thus heightened from Near Threatened to Endangered . The population of Bull, Dusky and Smooth Hammerhead sharks have also dropped by more than 99 percent . Porbeagle shark numbers have crashed by 95 percent in the northeast Atlantic, prompting the World Conservation Union to classify that population as critically endangered.
Globally, 16 percent of 328 surveyed shark species ( there are about 400 known species of shark) are described by the World Conservation Union as threatened with extinction. Despite mounting threats and evidence of decline, there are no international catch limits for pelagic sharks, though the U.S. government did set catch limits for certain species, fishermen often exceed them with no impunity .

While it is true sharks are predators that are large and powerful enough to be capable of harming a person , most are inoffensive and only certain species pose a danger to people who venture into their habitat territory . On a worldwide scale, the number of shark attacks on humans amounts to about 100 per year, of which only 5 to 15 are fatal. In most cases, the attack ends after the initial contact and the shark does not kill or eat the victim. Scalloped Hammerheads in particular are not dangerous to humans unless they are provoked in some way, such as by chasing, spearing, or touching them.
By comparison, many more people die each year from water-related activities that do not involve sharks ; even the number of casualties from lightning strikes is much higher ! Unfortunately, each shark attack is sensationalized by the media, and these stories then shape public perception of sharks , which to my mind is grossly unjust .

I've loved sharks from the very moment I first swam with them , some 18 years ago . They are just so beautiful ! Sharks possess large brains and integrated sensory systems, they can learn and remember, display social complexity, predatory flexibility, curiosity and possibly even playfulness. Play as we know is characteristic of large-brained, social animals that most of us would refer to as 'intelligent', e.g. dogs , dolphins . I believe sharks to be intelligent , and I believe it is imperative that we ensure the survival of these magnificient creatures , if only for our own sakes . As top-level carnivores, sharks are extremely important in maintaining the ecological balance of the seas .
Dotted along the coastline of Layang are several conical shaped objects which I was told were installed by the Malaysian Fishery Department for research in the conservation of fishes , my fear is it just might be too late for the Hammerheads, too late for all other sea fairing creatures, and too late for the great Mother Ocean .