River Monsters (22 page)

So when I go back, even if that’s not for a while, there is a chance that I might encounter one of the real monsters. Maybe they’ll even wait us out, like they did the dinosaurs.

11

EVEN IN PARADISE, I AM

Looking at the lake’s bright surface, I had difficulty reconciling what I saw with the newspaper reports. They reported that here, in December 2002, a twenty-three-year-old man went for a
cooling dip at night with a friend and then disappeared. His body was found three days later, showing the unmistakable marks of a shark attack. From what a lakeside resident told me, the two men
had set out to cross a narrow neck where the water is shallow, but in the dark they got separated and the victim strayed into the main body of the lake, where there is a steep drop-off close to the
edge. A few weeks later, while locals were still consoling themselves with the fact that this was a freak event, an eighty-four-year-old early-morning swimmer suffered a fatal shark bite to his leg
in nearby Burleigh Lake. The police brought in netsmen from Queensland’s offshore shark control programme, who captured three female bull sharks (
Carcharhinus leucas
) ranging from five
feet to seven feet, four inches, but none contained any human remains.

From Miami Lake to the sea, by way of the canals, is about ten miles. Although inland, the waterways are tidal, but sluice gates partially restrain the outgoing tide in order to prevent the
picturesque green borders from extending to mud. When the sluices are open, however, any fish with a body up to four feet deep could, in theory, pass through. Authorities reckon there are hundreds
of bull sharks in the canals. In fact, one man fishes for them from the balcony of his sixth-floor flat, overlooking Lake Orr. He says in eighteen months he has caught twelve and lost around
eighty. To land them, he sends his friend running downstairs with a gaff.

Normally we don’t need a physical barrier to prevent sea fish from moving inland. The transition from salt to fresh water is a barrier in itself, as secure as any metal grille. This is
because, in order to take up oxygen, fish have a convoluted expanse of thin membrane, the gills, in intimate contact with the surrounding water. But this semipermeable membrane (whose microscopic
pores allow the passage of small molecules but not large ones such as salts) also allows the passage of water. The direction of this movement depends on whether the surrounding water is more or
less salty than the fish’s tissue fluids. In fresh water, water molecules diffuse inwards; in sea water they diffuse outwards. For this reason, freshwater and sea fish have fundamentally
different strategies for keeping their body fluids at the correct concentration. Freshwater fish must constantly excrete the surplus water they take in; sea fish must constantly replace lost water
by drinking. (The salt contained in the sea water that sea fish drink is actively removed from the body by special cells in the gills.)

A freshwater fish dropped into the sea would, therefore, experience runaway water loss, whereas a sea fish placed in fresh water would become waterlogged, its cells exploding from increased
osmotic pressure.

Even brackish water, the in-between zone that is neither fresh nor salt, is uncomfortable for most fish. Most sea fish can’t tolerate the half-and-half mixture of salt and fresh that
sluices the inland waterways of the Gold Coast. So the presence of sharks here appears to offend the laws of nature as well as our sensibilities. In fact, sharks are the last sea fish you would
expect to find inland because their tissue fluids are even more concentrated than those of other fish.

This is because sharks retain certain waste products in the body, specifically urea and trimethylamine oxide (TMAO), to the point at which their bodies are marginally more ‘salty’
than the surrounding sea water. This allows water to gently diffuse in, rather than out, with low-level excretion removing any surplus. This increased internal saltiness is a near-perfect
adaptation to living in the sea, but a shark transferred to fresh water would experience an even greater shock to the system than its less well-adapted marine brethren: an inrush of water that
would demand very high-volume excretion to counteract it – like frantically baling a holed boat to stop it from going under.

So how do bull sharks manage to break the rules? And they’re not just in the Gold Coast canals but also in estuaries and rivers worldwide where these are accessible from the warm coastal
shallows where bull sharks are normally found.

In 1993 I wandered into a gloomy shop in Manaus, where the Rio Negro meets the Amazon, looking for rope and tarpaulins. Hanging from the ceiling were several stuffed fish, dark brown with age
and varnish, the biggest of which, thick bodied and seven feet long, I at first took to be a piraiba, the Amazon’s giant catfish. The general body shape was about right, but closer inspection
revealed an asymmetric tail and five parallel gill slits. The shop’s owner told me that one of his employees, no longer alive, caught it from a spot just downriver. This bull shark had been
nine hundred miles from the sea, and there are other reports from near Iquitos, in Peru, more than two thousand miles inland. A fisherman I met later told me he’d caught four or five in the
’60s and ’70s on long lines set for catfish. Another man told me he’d seen one in the ’80s when he was working at the fish-freezing plant in Iranduba near Manaus, where many
fishing boats unload their catch. ‘They called everyone to see it. It had no scales, like a catfish,’ he told me. ‘When you moved your hand down its body towards the tail, it was
smooth. But in the other direction it was rough, like a file.’

More recently, in a bar in Zambia, I saw some jaws with the same characteristic dentition: broad-based serrated teeth in the upper jaw (for cutting, like steak knives) and narrower, more pointed
teeth in the lower jaw (for gripping). People originally thought the ‘Zambezi shark’ was an endemic species, but now we recognise it as the same species as the inland shark of the
Ganges and Lake Nicaragua, not to mention the bull sharks that have been found in US rivers. These include an eighty-four-pounder that two commercial fishermen, Herbert Cope and Dudge Collins, from
Alton, Illinois, caught in 1937 1,750 miles up the Mississippi. Something had been chewing through their wood-and-mesh fish traps, so they set a wire-mesh trap baited with chicken guts. Although
some dismissed this as a hoax, this catch is now widely considered to be authentic, although bull sharks would now find dams blocking their way.

In addition, three of the five Jersey Shore attacks of 1916, which inspired Peter Benchley’s 1974 novel
Jaws
and the subsequent Spielberg film, were probably the work of bull
sharks, not a rogue great white as originally concluded. Even though the attacks stopped after the capture of an eight-foot great white in Raritan Bay, reportedly with human remains in its stomach,
the last three attacks were up the narrow Matawan Creek, which would be a very unusual habitat for a great white. And later newspaper stories refer to other sharks seen and captured in the creek
after the alleged culprit was killed. More recently, a reported attack in Lake Michigan was almost certainly a hoax, but they have been reliably identified in inland Louisiana.

The fact that the very shark species that swims into rivers just happens to be the one that’s the most dangerous to humans, more so than hammerheads, tiger sharks, and even great whites,
appears to be a very cruel irony. But this is no coincidence: the reason bull sharks have attacked more people than other sharks is precisely because they live so close to humans in shallow coastal
waters, estuaries, and sometimes rivers. Also, unlike where other sharks live, this water is often murky, so identifying objects or other animals is difficult for a bull shark’s small eyes.
Not having hands, fish often feel with their mouths. Rory McGuinness, our cameraman in Australia, once had his kayak bumped by a bull shark while he was paddling in the Gold Coast canals. So the
bull shark’s ‘aggression’ could just be curiosity or even self-defence when a swimmer inadvertently gets too close, although this doesn’t make them any less dangerous.

The bull shark’s ability to penetrate fresh water is thanks to a quirk of physiology that is unique among large sharks. When entering fresh water, bull sharks are able to reduce the
saltiness of their tissue fluids by about a third. They do this partly by excreting salt via their rectal gland. At the same time, the concentration of urea in their tissues drops by more than 50
per cent. Despite this, however, their body fluids are still more than twice as salty as those of typical freshwater fish, so they must rapidly get rid of surplus water. Experiments suggest that
when a bull shark enters a river, the rate at which it excretes water goes up more than twenty-fold. This increased workload on the kidneys requires the expenditure of more energy, so by the
mathematics of biological accounting, which all creatures unconsciously carry out, there must be some benefit to offset this cost in order to make moving into fresh water worthwhile. The question
is: what?

One advantage of fresh water is fewer predators, especially other sharks, to prey on bull shark pups. This tallies with the fact that river mouths and estuaries tend to be used as bull shark
nurseries. The choice of estuaries, rather than completely fresh water, is interesting and is probably a compromise based on a weighing of pros and cons. Despite the bull shark’s tolerance of
fresh water, small bull sharks have a harder time in a river, water balance-wise, than large bull sharks. This is thanks to the geometry of growth: small animals have a higher surface area to
volume ratio than their larger brethren. And with more surface area per unit of body mass, they take in water more readily and hence need to get rid of it faster. So stick to the estuary, where
predators are few and energy costs are not too high – the ideal combination.

But why adult bull sharks, apart from gravid females, are in rivers is less certain. Maybe it’s simply because they can, and they wander back and forth on a whim. The biggest bull sharks
I’d heard of in this part of Australia were in the Brisbane River, fifty miles north of the Gold Coast. In 2007 Terry Hessey caught a nine-foot, six-inch specimen at the mouth of the river,
which pulled 440-pound scales to their limit and was estimated at well over 500 pounds. This was identified as a gravid female by virtue of its distended abdomen (adding to the general stockiness,
which gives the bull shark its name) and the absence of penis-like claspers on the underside of the body.

The bull shark pups, up to a dozen of them, are born a year after fertilisation. Shark scientist Dr Richard Pillans estimates that there are two to five thousand juvenile sharks in the Brisbane
River – so many, in fact, that river anglers hold an annual shark-fishing competition. Terry caught his monster during this event. And although I’d seen a picture of this fish, beached
in tidal shallows, I needed to see one in the flesh. Despite knowing about the bull shark’s osmoregulatory bag of tricks, the idea of a shark in a river was still something to get used to, as
it is for most people.

So I met up with Terry and his fishing buddy Ben Cole at a place called Luggage Point, which even at night has to be one of the least picturesque fishing spots I’ve ever been to, although
this in itself makes it stand out in the memory. The place is known informally as the ‘poop shoot’ because it is the site of a huge sewage treatment plant that perfumes the air and the
intertidal mud. But fish have different sensibilities from humans, and the peculiar water quality here probably attracts them, from small bottom feeders right up the food chain. Terry had acquired
a couple of good-sized freshwater eels, about as thick as my wrist and three feet long, which are a favourite meal of bull sharks, along with stingrays. Bull sharks are mainly bottom-feeders, and
because they often feed in cloudy water, they don’t rely much on eyesight to find food – hence their small eyes. But they are far from blind in the water thanks to a sixth sense that
humans don’t possess. On a bull shark’s snout are pores known as the ampullae of Lorenzini, which are sensitive to minute electrical currents, such as those produced within the body of
a prey animal. The effectiveness of this sense has been most clearly demonstrated in hammerheads, which sweep their ampullae-rich heads over the bottom like a metal detector, locating buried
stingrays that are otherwise invisible.