Why Do Sharks Strand?
Inspired by recent posts on this list about strandings of white and whale sharks, I have been pondering the matter of shark strandings in general. Following is a synopsis of my thoughts on this topic, which I hope will be of interest to users of this list and will spark an exchange of ideas about this most intriguing topic:
Shark strandings or beaching events are something of a mystery. For fishes that are generally regarded as being negatively buoyant in seawater, these events occur with surprising frequency yet with little or no apparent regularity.
Classic studies by Bone & Roberts (1969) and Baldridge (1970, 1972) on tissue densities and buoyancy in sharks revealed that due largely to the accumulation of low-density oils in the large liver, sharks are only slightly heavier than the medium through which they swim. Baldridge (1974) noted that a 1 015-pound tiger shark (Galeocerdo cuvier) tested at the Mote Marine Laboratory had, when immersed in sea water, an apparent weight of only 7.3 pounds (that works out to about 0.72% of its weight in air). Thus, sharks must invest very little energy in order to prevent sinking.
In cool temperate zones of both Hemispheres, basking sharks (Cetorhinus maximus) wash ashore with surprising frequency, and — in various stages of decay — have frequently been misinterpreted as 'sea monsters' (see Heuvelmans 1965 for a discussion of this matter). Although phylogenetically allied with the lamnids (the family which includes the white, makos, porbeagle and salmon sharks) the basking shark shares many hydrostatic features with deep-sea squaloids and hexanchoids (the orders which embrace the dogfishes and the cow sharks, respectively), including an exceptionally long body cavity filled with an enormous liver (up to 25% of the total body weight in C. maximus; up to 35% in certain deep-sea squaloids) which is low in vitamin content but rich in low-density oils (870-880 kg/m3 compared with about 1028kg/m3 for seawater), including a high percentage (70-98%) of squalene. Since the gastrointestinal tract (rich in autochthonous bacteria and other microbes) is typically one of the first organ systems to break down upon death of a host animal, it is tempting to speculate that the gases liberated through the processes of decomposition may be sufficient to tip the hydrostatic balance, rendering the carcass as a whole positively buoyant. Time and tide may eventually carry the carcass to shore, where it may be reported by a terrestrial primate with limited swimming capability but boundless curiosity.
Compared with their elephantine cousin, the basking shark, lamnids have a relatively short body cavity and smaller liver (about 15% of the total body weight). Yet these sharks, too, occasionally wash ashore — sometimes in moribund or freshly expired yet apparently uninjured condition. Users of this list will no doubt recall recent reports from South Africa of large white sharks (Carcharodon carcharias) washed ashore — in one disgusting case, to be beaten and fairly torn asunder by irrational and unsympathetic 'beach apes'. From near my own base of operations, in British Columbia, Canada, no fewer than six white sharks have been found stranded or beached in the province since 1962, mostly from the western coasts of the Queen Charlotte Islands. The most recent of these was a 5.2-metre-long male beached at Long Inlet, Graham Island, BC, on 16 December 1987. List-user and frequent contributor Ian Fergusson can, no doubt, provide information on white shark strandings from elsewhere, particularly from the Mediterranean and off southern Africa. In recent years, researchers have noticed that each spring a small number of salmon sharks (Lamna ditropis) beach themselves in central and southern California. This phenomenon is poorly understood and is being studied. This species is most common in continental offshore and epipelagic waters, from the surface down to a depth of at least 152 metres, but it has been known to come inshore — sometimes just beyond the breaker zone — which may contribute to this phenomenon. List-user Sean Van Sommeran can perhaps favor us with more information about this intriguing mystery.
Some time ago, a list-user (who's name escapes me at the moment), asked whether shark strandings may be somehow similar to those of whales. At the time, I thought the notion was charmingly naive, but have since had time to reconsider.
Klinowska (1988) compared records of mass cetacean strandings in Britain and the United States against geomagnetic maps (which plot variations in the intensity of magnetic fields at the Earth's surface caused by differences in the underlying rock; these variations are represented as contour lines, so that areas of high magnetism appear as 'hills' and areas of low magnetism as 'valleys'). Klinowska's analysis revealed that most mass strandings and virtually all repeated strandings occurred where the magnetic valleys were oriented perpendicular to the shore. This sensational finding suggests that at least some whales navigate by following a magnetic map of the ocean floor. On land, magnetic variations are very irregular and there are many visual cues to guide navigation. There are no such landmarks in the vast, dark ocean. But there are regular magnetic variations. Magnetic hills and valleys stretch for huge distances across the ocean floor, and toothed whales seem to use the magnetic contour lines as invisible 'roads'. These magnetic freeways often follow continental margins, but not always. Klinowska theorizes that whales may strand when they follow these magnetic roads onto shore. Klinowska has also suggested that the daily pattern of variation in the total geomagnetic field may function as a biological 'travel clock' for whales; solar activity can affect this pattern, possibly causing irregular fluctuations which disturb the clock. Therefore, whale mass strandings may be the magnetic equivalent of traffic accidents.
It is not yet clear how whales sense Earth's magnetic field. Evidence is accumulating from studies carried out in Germany which suggest that cetacean retinas (which contain magnetite) are sensitive to magnetic fields of an intensity consistent with geomagnetism. Whether toothed whales actually navigate by geomagnetic cues has not yet been demonstrated. Mass strandings usually involve toothed whales which migrate over long distances. The more sedentary dolphins and porpoises seldom run into these difficulties. They presumably become familiar with all the small local anomalies and so build up a much more detailed magnetic map of the area in which they live. How toothed whales detect and respond to such minute local variations in geomagnetism remains a mystery, but Klinowska's magnetic highway theory is one of the best models we have to explain why these whales mass strand.
But sharks do not seem to mass strand. Baleen whales, many species of which appear to be migratory, occasionally strand, but usually as lone individuals or at most a few animals at a time. (This may be related to their less rigorously coastal habitat and/or looser social organization compared with odontocetes). As such, they may serve as a better model for shark strandings. List-user Richard Ellis, who has written a number of excellent popular books on whales, may be willing to shed some light on this matter.
In any case, thanks to Kalmijn's classic (many papers since 1969, but perhaps most notably his 1982 paper in Science) and on-going studies, it has long been known that elasmobranchs (sharks and rays) posses an acute sensitivity (as little as 0,05 nanovolts/cm2) to artificial electromagnetic fields and can be trained to orient to them under experimental conditions. In theory, Earth's magnetic field induces an electric current to flow through anything moving across it which can conduct electricity. This effect, called 'induction', is used in the construction of dynamos. In a simple dynamo, an electric current is produced in a coil of wire by rotating the coil between opposite magnetic poles. A living organism may be thought of as a bag of water throughout which various ions are distributed. When an organism moves across the lines of force of Earth's magnetic field, these ions are induced to move and tiny electric currents flow in its body. However, when an organism moves along the lines of this magnetic field, no current flows. The ocean is a visually concealing medium, making line-of-sight piloting and celestial navigation of little use to marine creatures. If a marine creature is able to detect the tiny currents induced in its body by geomagnetism, they would provide magnetic landmarks to help it navigate through the local environment.
Earth's magnetic field may provide several types of cues useful for navigation. Because the magnetic poles are displaced from the geographic rotational poles, there is usually a difference between the true north-south meridian and magnetic north. This 'magnetic variation', which may be large in some places and times, changes slowly over a period of years as the magnetic poles drift. In addition to these horizontal and vertical directional components, the geomagnetic field also has a third dimension, intensity. Geomagnetic field strength increases with latitude, from about 30 000 nanoTeslas (a Tesla is the magnetic induction equal to 1 volt-second per square metre) at the equator to about 70 000 nanoTeslas at the poles. The field intensity also decreases, as might be expected, with distance from the Earth's surface. In addition to these global features, there are low-amplitude, localized magnetic irregularities in both space and time called 'geomagnetic anomalies'. Theoretically, all these features could provide magnetotaxic creatures with a richly textured magnetic 'map'.
Recently (1993), Klimley used sonic telemetry to study the movement patterns of scalloped hammerheads (Sphyrna lewini) relative to physical features of the marine environment in the southwestern Sea of Cortez. While directional swimming of the tagged sharks showed little or no correlation with subsurface irradiance, temperature, currents, or submarine topography, Klimley's study revealed that the tagged sharks most often swam along submarine ridges and valleys that correspond to maxima and minima of the local geomagnetic field. Previous work by Klimley and his co-workers (1981, 1984, 1985, and 1987) had revealed that scalloped hammerheads form polarized, socially active schools that aggregate over seamounts by day and that at night these schools break up to feed. From the sum total of all these studies, Klimley concluded that the scalloped hammerheads in that part of the Sea of Cortez locate seamounts using geomagnetic cues, moving back and forth between feeding and refuging areas by swimming along 'magnetic highways' (which seem to correspond to lithospheric cracks through which fresh magma had extruded relatively recently). Although Klimley's studies do not suggest that shark strandings or beachings may be due to errors in reading a geomagnetic map (as may be the case with some cetacean mass strandings), they do provide the clearest evidence to date of geomagnetic navigation of an elasmobranch in the wild.
As recently noted by Ian Fergusson on this list, the white shark appears to be migratory in at least some parts of its range (such as off the coast of California). The salmon shark may also be migratory, following schools of salmon around wide swaths of the North Pacific. It is quite conceivable that these sharks, like Klimley's scalloped hammerheads, use geomagnetic cues to navigate. There may or may not be a correlation between shark strandings and geomagnetic anomalies, and these correlations may or may not be causally linked. (The basking shark appears to be migratory, too, but it seems to follow plankton blooms — possibly keying in on the dimethylsulphide signature of copepod grazing rather than geomagnetic cues; the blue shark is almost certainly migratory — sometimes traversing entire ocean basins — but it is primarily an oceanic creature, so it may strand only rarely.) It would therefore be interesting to compare sites of shark strandings with local geomagnetic signatures, if only to categorically refute this idea.
Before leaving you to construct your learned (and hopefully tolerant and well mannered) response to all this, I'd like to close with one more potential similarity between shark and cetacean strandings. In marshy areas of the southeastern United States, bottlenose dolphins (Tursiops truncatus) have been documented apparently using bottom topography to concentrate and even strand small schooling fishes, sometimes actually beaching themselves temporarily in pursuit of their prey. Killer whales (Orcinus orca) have been known use their bodies to produce waves that wash seals off ice floes in Antarctica and I suspect most of us have seen that spectacular stock footage (shot by Maurice Rumboll, if memory serves) of killer whales gathering in the surf and actually pursuing sea lions onto a desolate beach in Argentina.
Sharks, too, apparently sometimes pursue their prey onto shore. Tiger sharks, apparently intent on capturing their prey, have been observed to swim out of the water and onto the sand in pursuit of sea turtles. Similarly, there is a report of a tiger shark actually chasing a dolphin out of the water; the shark slunk back into the sea, leaving the wounded mammal stranded on the beach. White sharks are reported to swim part way out of the water to pluck hauled-out pinnipeds off rocks, using their dexterous jaws with pincer-like efficacy. Even more incredible is the acrobatic behavior of the smooth-hound shark (Mustelus mustelus), which feeds on semi-terrestrial crabs that live on muddy banks of estuaries and bays. These sharks have been observed launching themselves out of the water and onto the mudflats in pursuit of their quarry and then wriggling back into the water. As I remarked in my book, Shark Smart, "That kind of dedication is certainly above and beyond the duties of an aquatic predator!"
Which raises another possible cause of some shark strandings or beachings: perhaps once in a while, an overly ambitious shark — made incautious by the flushed 'hot pursuit' of some evasive prey — makes an error of judgment and accidentally strands itself.
That's all I have on this subject for now. I hope you have found this little essay interesting, thought provoking, and fun.
— R. Aidan Martin