Through A Glass, Darkly
Early attempts to document shark distribution patterns were based on opportunistic capture records and hampered by a general lack of identification guides. As a result, records were relatively few, heavily biased toward exceptionally large or otherwise unusual specimens, and were often based on misidentifications. Part of the problem is that 'allometry' the many and subtle proportional changes that occur as an animal grows was not well understood in sharks. A few specimens, from widely scattered locations and variously preserved, cannot reveal the full extent of shark allometry. As a result, each tiny difference was deemed reason to erect a whole new shark species.
The hit-and-miss nature of these early records created a very spotty and incomplete sense of the geographic range of any shark species. At the time, there was simply no way to determine whether a given capture represented the 'normal' range of a particular species or merely a stray captured far from its usual haunts. For example, in 1829 a young Tiger Shark (Galeocerdo cuvier) was captured off Iceland and given the name Squalus arcticus by Faber. Formally shifted to the genus Galeocerdo by Mόller and Henle in 1837, the name Galeocerdo arcticus was widely in use until the mid-20th Century and some older museum specimens still wear tags bearing that name. By many scholars' comparing specimens from many parts of the globe and representing the full spectrum of growth stages, we now know that G. arcticus is a junior synonym of G. cuvier . From oceanographic and other zoogeographic studies, we also know that young Tiger Sharks are occasionally carried to boreal parts of the eastern North Atlantic via the Gulf Stream. But so long as the Tiger Shark was recorded from only a few, widely scattered locations, we had no idea that it was primarily a tropical to warm temperate species.
The establishment of commercial shark fisheries in many parts of the globe did much to further the study of shark distribution. Spurred by the economic need for maximizing landings of targeted shark species, relatively complete data was gathered on capture date, location, and depth. These data allowed identification of seasonal centers of shark populations and revealed that at least some species segregate by size and sex. Because sharks were thought to be cannibalistic brutes, the fact that juveniles of a given species avoided their elders seemed to make sense. No one knew quite what to make of the segregation of males and females of the same species, raising the question of how they got together to mate. The process of dressing shark carcasses for market afforded opportunities to examine stomach contents, revealing something about what the targeted species ate. Because sharks were thought to be lowly scavengers, it did not surprise many that all sorts of inedible flotsam and jetsam from the fisheries ended up in sharks' stomachs. Although capture depths were scrupulously recorded, they revealed little about the preferred depths of targeted shark species. Fisheries scientists simply had no way of determining whether a given shark was caught at the resting depth of the fishing gear or was caught at shallower depths as the gear was being deployed.
Commercial shark fisheries also revealed that targeted species cannot long withstand concentrated fishing pressure. Shark fisheries generally experienced a pattern of "boom and bust", typically collapsing in two to five years as targeted stocks were depleted to a level below economic viability. We now know that sharks' slow maturation and low fecundity makes them especially vulnerable to commercial fisheries. But so long as the economic "boom" was big enough, there were always commercial fishermen willing to change location or target species every few years.
A rapid decline of shark populations were also noted by those tending mesh nets placed off popular bathing beaches to cull local shark populations. For more than 40 years, the Queensland Meshing Board of Australia and the Natal Sharks Board of South Africa have collected reams of data on shark species, length, weight, sex, maturity state, number of pups (if present), and stomach contents. The Natal Sharks Board also collected scores of detailed measurements of each shark specimen captured and preserved many of them in whole or in part. They also publish their results periodically, adding substantially to our understanding of the zoogeography and basic biology of South African sharks.
Commercial fishing and shark meshing have thus revealed a great deal about shark distribution patterns and life histories. It is ironic and more than a little sad that we learned so much about sharks as a living resource by systematically killing them.
Sport angling for sharks has long been the province of a privileged few. But increased affluence among the proletariat has enabled many more to enjoy the psychological thrill of 'conquering' a large, wildly snapping shark armed with rod and line an activity that reached epidemic proportions in the wake of JAWS. But there has always been a substantial cadre of sport anglers who do not wish to kill their quarry, merely subdue it. This distinction led to the "catch and release" ethic that has since become widespread among recreational fishermen. In response, marine biologists and fisheries scientists in Australia, New Zealand, and the United States began requesting that sport anglers tag and record date, location, length and sex of the shark before releasing it. To their delight, tens of thousands of sport anglers kindly and generously agreed to help.
One of the largest and most successful co-operative shark tagging programs is that conducted by the US National Marine Fisheries Service (NMFS) in the western North Atlantic. Over the past 40 years, more than 100 000 sharks of 33 species have been tagged by volunteer anglers. Of these, some 4 600 individuals (4%) of 29 species have been recaptured. Over 90% of recaptures came from just eight shark species the Blue (Prionace glauca), Sandbar (Carcharhinus plumbeus), Dusky (C. obscurus), Tiger Shark, Shortfin Mako (Isurus oxyrinchus), Blacktip (Carcharhinus limbatus), Scalloped Hammerhead (Sphyrna lewini), and Atlantic Sharpnose (Rhizoprionodon terraenovae) revealing much about their movement patterns in the western North Atlantic.
In 1998, a team of NMFS researchers published an atlas summarizing 30 years of co-operative tagging data. This atlas plotted taggings and recaptures recorded from 1962 to 1993. These recaptures reveal that many western North Atlantic sharks routinely travel distances of hundreds or even thousands of miles (kilometres). The all-time champion among shark long-distance travelers is the Blue Shark one individual tagged off New York was recaptured 1.4 years later off Brazil, 3 740 miles (6 020 kilometres) away. A fundamental problem with interpreting shark tag recaptures is that they reveal only the end-points of the animal's journey where it was tagged and where it was recaptured. But if researchers have a great deal of tagging and recapture data with which to work, as do the scientists at NMFS, basic facts about long-term shark movements eventually do emerge.
A more direct way to explore patterns of shark movements is telemetry. Telemetry can encode as sonic pulses or magnetic patterns a wide range of sensor data such as shark location, depth, water temperature, body temperature, degree of jaw gape, and so on. This technique thus has enormous potential for revealing a great deal about the short-term activity patterns of free-swimming sharks. Telemetry has already demonstrated such things as the extent of shark home ranges, the astonishing predictability with which they move through their home ranges, and daily movement patterns. There is little doubt that, as technology improves, telemetry will reveal many new and unprecedented aspects of shark distribution and activity patterns.
The simplest kind of telemetry is called "sonic telemetry". This technique involves attaching an electronic pinger to an individual shark, either by bolting it to the dorsal fin, inserting it into the body cavity, or hiding it in bait which the shark then swallows. The pinger broadcasts sensor data in the form of encoded sound pulses*, which are then translated back into data by a receiver. How long a sonic telemetry device transmits data is largely a function of how frequently the data is broadcast and battery life. With the latest generation of electronic components and long-lasting energy cells, many of these devices can barring malfunction transmit for several months. Unfortunately, most electronic pingers have a very limited broadcast range usually less than 2 miles (3 kilometres) or so and thus a telemetered shark must often be followed via boat. As an inevitable consequence, this technique is subject to the limits of human tracking endurance and often reveals little more than how sharks behave when chased by a research vessel (usually, they simply head for deeper water).
A newer technique, which avoids many of the logistical problems of sonic telemetry, is called "archival tagging". Archival tags do not transmit their data periodically, but simply store it until the tag is recovered. Many archival tags are programmed to detach and float to the surface after a pre-determined time. Such tags are termed 'pop-up tags'. Pop-up tags typically emit a radio signal, facilitating their recovery. Because archival tags do not continually transmit data, they can be smaller, less obtrusive, and longer-lasting than sonic tags. They also do not require being actively tracked, avoiding observer interference in normal shark behavior.
Recently, an interesting variation on archival tagging has been developed to study shark movement patterns. This technique involves placing a series of stationary monitors on the bottom in areas known to harbor concentrated shark populations. These monitors lie dormant until an archival-tagged shark swims past. The archival tags emit a continuous, low-energy signal. When a tagged shark swims past a dormant monitor, it 'wakes up', records which individual shark swam by at which date and time, and then downloads all of the data accumulated in the archival tag up to that time. There is even the intriguing possibility of equipping such stationary monitors with pivoting video cameras inside clear, acrylic domes. In this way, when the monitor is activated, its camera could record what the archival-tagged shark is doing while in the area, shutting down after it has moved out of range. In this way, a wild, free-swimming shark could be 'observed' by proxy.
A great deal can be learned about shark activity patterns through direct observation. Scuba has proven itself a powerful tool for scientists wanting to observe sharks in the wild. In addition, many recreational scuba divers are fine and observant naturalists. With the addition of video or still camera, we can record where and how sharks go about their secret lives. But some species avoid scuba divers and are notoriously shy of scuba exhaust. In heavily dives areas, in fact, many of the larger, more mobile sharks simply move away.
Snorkeling is a low-tech way an in-water observer may avoid intimidating sharks. The simple expedient of mask, snorkel, and fins generally allows much closer observation of free-swimming sharks than is afforded by scuba. Encountering a shark while snorkelling is also somehow more 'intimate' than on scuba, because one is not separated from the environment by all that gear. There is only the ocean, the shark, and you. However, developing first-rate snorkelling skills requires a high level of physical fitness and a great deal of dedicated practice. Even the best snorkellers can only manage dives to 70 or 80 feet (21 or 24 metres) for underwater 'hang times' of 60 or 90 seconds.
However, an old military technology is breathing new life into diving with sharks. Called 'rebreathers', the most advanced of these devices feature a closed-circuit design. By re-using oxygen-enriched air (called Nitrox) and removing expelled carbon dioxide via chemical scrubbers, divers using rebreathers reduce their exhaust bubbles to almost nil. As a result, they are able to get much closer to free-swimming sharks for longer periods. Rebreather technology is very expensive, beyond the reach of most recreational divers, but its benefits are reaping tremendous rewards among military and rescue divers as well as diving scientists and film-makers.
Direct observation of sharks on their own terms has revealed many astonishing aspects of their lives of which we would otherwise not know. For example, diving scientists and naturalists have reported sharks actively exploring and selecting their habitats, natural predation events, courtship and even mating. Through my own on-going research, I have learned that there is an important social component to shark distribution and activity patterns, regulated by a complex and subtle body language.
The newest generation of manned deep-sea submersibles and robot cameras is opening up a vast frontier to exploration, enabling us to observe species formerly known only as lifeless specimens. With each new discovery about how a given shark species exploits the resources of its habitat, our understanding of its biological needs and limitations becomes ever more clear. But toward the larger goal of understanding which sharks live where and why we still have a long, long way to go.