b The main types of algae that krill eat are diatoms, including those of the genus Nitzschia.
Chapter 4
Missing Angels
In the sixteenth century, when European explorers set sail for the Americas, many of them encountered big sharks for the first time. When they returned home, they brought with them new words for the huge, unfamiliar animals they had seen.
“Above all there is an infinite number of some very large fish that they call tiburones,” wrote Spanish monk and traveller Juan Gonzáles de Mendoza in 1585. The word tiburón originated in the indigenous language of the Kalina people from the coasts of South America, and it’s still in use in Spanish today. The English adopted a term that has hazier origins and more multiplicitous meanings. It seems nobody can quite decide on the true origins of the word shark.
Both words originally applied to large sharks, distinguishing them from the smaller cazón, or dogfish, which Europeans in medieval times knew from fishing in waters closer to shore. Venturing beyond the horizon, sailors came across various pelagic sharks—those that roam across open seas—and no doubt included plenty of oceanic whitetip sharks. These bold, inquisitive sharks swim at the surface but only over water that’s more than five hundred feet deep, and they have a strong habit of following ships. Anyone watching over the side of those medieval vessels would have spotted this shark’s tall, rounded dorsal and tail fins, and long pectorals, all speckled in white at the ends as if they had been clumsily dipped in bleach. Even in those early days of European exploration, it was obvious that the ocean was teeming with oceanic whitetip sharks.
Shift forwards to the twentieth century, and there were still huge populations of this species cruising across the planet, through every ocean basin except the Arctic Ocean and the Southern Ocean, where the waters are too cold for them. A 1969 book about sharks described oceanic whitetips as “extraordinarily abundant, perhaps the most abundant large animal, large being over 100 pounds, on the face of the earth.” The 1971 documentary Blue Water, White Death set out to capture great white sharks for the first time on underwater movie cameras. The bait was a dead sperm whale, shot with an explosive harpoon by a whaling ship off the coast of South Africa. Divers climbed inside a metal cage and had themselves lowered into the sea to see what animals showed up to scavenge the floating whale carcass. There were no great whites, but almost immediately, dozens of oceanic whitetip sharks appeared, swimming in circles, lunging in and biting off chunks of blubber. “There must be twenty tons of sharks down there,” one of the divers said after making his way back to the ship.
A decade later, oceanic whitetip sharks were still reported to be superabundant. A 1984 report named them as one of the world’s most numerous large marine animals. Since then, however, their situation has taken a dramatic turn for the worse.
For every hundred oceanic whitetip sharks that roamed the Pacific Ocean in the 1980s, fewer than five are alive today.a In some places, young whitetips have become vanishingly rare, and over time mature individuals have been shrinking in size, which could mean females are having trouble reproducing; the smaller a mother shark, the smaller the energy reserves she can draw on, and the fewer offspring she produces. Data on the whitetips’ status aren’t available throughout their global range; the situation in the Indian Ocean is especially vague. But wherever there is good information, a similar story is being told. These large sharks that were once so plentiful are disappearing from the ocean. In 2019, experts at the International Union for the Conservation of Nature (IUCN) considered all the available facts and declared the oceanic whitetip shark to be Critically Endangered—the most imperilled category in the IUCN’s Red List of Threatened Species, a go-to guide to species endangerment.b Whitetips face a high chance of going extinct globally, and they’re not the only ones. In total, more than one-third of all known shark species and their close relatives the skates and rays, collectively known as elasmobranchs, are threatened with extinction. They are the second most endangered group of animals on the planet after amphibians. Close to four hundred species of elasmobranchs face an uncertain future in the Anthropocene ocean.c
Cautionary tales tell of once superabundant species that humans nevertheless managed to extirpate. Passenger pigeons are a classic example. When Europeans began to settle in North America in the sixteenth and seventeenth centuries, somewhere between three and five billion of these graceful pigeons were living in mixed hardwood forests in the east. Settlers told stories of enormous flocks blotting out the sun for hours and days as they passed overhead, of tree branches breaking off from the sheer weight of birds roosting in them, and of their droppings lying inches thick on the ground. That all came to an end when people cut down forests and hunted the pigeons as a source of cheap meat. By the turn of the twentieth century, there were no more passenger pigeons in the wild. On September 1, 1914, the very last passenger pigeon, named Martha after First Lady Martha Washington, died in the Cincinnati Zoo.
The lesson of the passenger pigeon is proving difficult to transfer underwater because the myth lingers on that the inexhaustible seas are too big and animals living there far too abundant for humans to put a dent in their populations. But as the oceanic whitetip sharks show, existing in huge numbers over vast tracts of ocean is not enough to make a species immune to human destruction.
The chief explanation for the whitetip shark’s precipitous decline is linked to the very same habit that first brought the species to the attention of European sailors. As the seas have filled up with fishing boats, more whitetips have been swimming up to investigate and getting hooked and killed on fishing lines. The same goes for dozens of other ocean-roaming shark species. Great hammerhead sharks, up to twenty feet long, with their distinctive tool-shaped headgear, and their slightly smaller relatives the scalloped hammerheads are both now Critically Endangered. Great white sharks are classified as Vulnerable to extinction globally and as Critically Endangered in European seas. Pelagic thresher sharks, which stun fish in schools by whipping their immensely long tails over their heads, are Endangered. So are shortfin and longfin mako sharks, which race through the ocean faster than any other sharks. In all, since the 1970s, the global abundance of oceanic sharks and rays has declined by 71 per cent.
To wipe out so many animals that used to occupy hundreds of millions of square miles of ocean might seem like an improbable feat for fishing vessels alone to achieve. And yet, mounting evidence shows that fishing pressure is easily high enough to explain these multiple vanishings. In the past few decades, humans have massively expanded and intensified industrial fishing to the point that the collapse of once-abundant shark populations became inevitable.
A series of technological innovations made this possible, beginning with the switch from wind-powered to fossil-fuelled ships. In the late nineteenth century, steam-powered trawlers began plying the ocean, to be replaced in the twentieth century by diesel ships, which were even faster, more powerful, and capable of hauling larger fishing gear through the water and up on deck. At-sea freezing facilities allowed fishers to preserve their catches, stay out longer, and venture farther from shore. Wartime technologies heightened industrial fishing powers, with advances in sonar, radar, and navigational tools making it safer to be at sea and easier to locate fish schools. And in the 1950s, the American chemical company DuPont invented a new type of fishing line made from a single, long nylon fibre that was strong, lightweight, and difficult for fish to see in the water. Monofilament lines swiftly became a favourite of fishers around the world and since the 1980s have been used to make longlines, the gear used most widely through the ocean and that catches the most sharks.
The principle behind longline fishing is straightforward. A vessel pays out a single line, often using a mechanical line shooter that propels it into the water faster than the boat speed so it has a chance to sink down. Fixed on the longline at regular intervals are shorter branch lines, or snoods. At the end of each snood is a palm-size metal hook with a chunk of fish or squid attached as bait. The line can be set at a chosen depth, for instance, to catch fish near the seabed, such as cod and hake. More commonly, floats are attached so that the line drifts near the surface and catches tuna, marlin, and swordfish. Radio beacons are attached so fishers can find the ends of the lines after they have soaked in the sea for anywhere from a few hours to as long as a day. Then a hydraulic winch pulls the longline back in, and crew on deck unhook the catch by hand.
The hauling-in process alone can take between ten and twenty hours, which gives an idea of the scale of these operations. In the US pelagic fleet, the average length of longline set by an individual vessel is twenty-eight miles. Many other fleets use lines more than twice as long.
Sharks snagged on longlines are often referred to as by-catch, because the prime targets of these fishing vessels are other animals such as tuna. Even so, sharks make up a major part of catches and are economically important for fishers, who commonly keep the sharks’ meat and cut off their fins to sell in the trade for shark-fin soup. The precise number of sharks killed this way is not well known because most catches go unreported, and fewer than one in twenty tuna longliners have independent observers on board monitoring the catches. When observers are stationed on longliners, they witness just how intensive the shark slaughter can be. In 2018, observers on a Spanish longliner operating south of the Cape Verde Islands saw each longline catching an average of 7.6 oceanic whitetip sharks and as many as 54. During three months of fishing, that single vessel caught 416 whitetips, half of them dead by the time they were hauled in. Together all these living and dead sharks weighed more than twenty metric tons, an order of magnitude higher than the official reports for all the longliners operating that year across the entire Atlantic.
Until recently, it’s been nigh on impossible to calculate the worldwide impact of longlining on sharks. The global fleet of longliners is an unwieldy mix of vessels, some big, some smaller, most operating far from shore way out of sight from land. Now, though, thanks to another technological advance, the full extent of longlining is coming into view, and with it a clearer idea of just how deadly this form of fishing is for sharks.
International maritime law requires all large vessels to carry an identification system that broadcasts their position via satellite every few seconds, so vessels can avoid crashing into each other. Now researchers are using the publicly available data to map fishing activity. Computer algorithms recognise the characteristic patterns made by different types of fishing vessels as they move across the ocean. Trawlers take a wandering path. Purse seiners stay for longer in one spot while they gather in their circular curtain of nets, like a giant’s drawstring purse. And longliners tend to draw narrow V shapes as they motor slowly in one direction when setting out their fishing line, then turn around and trace another straight course while hauling it back in.
One year’s worth of satellite positioning data from seventy thousand vesselsd showed they collectively fished for forty million hours and covered more than 285 million miles—the same distance one would cover travelling to the moon and back six hundred times. In that year, 2016, more than half of the world’s ocean was industrially fished.e That’s over four times the global area of agricultural land.
Longliners alone operate over at least 45 per cent of the ocean. They lace lethal lines throughout the territories of pelagic sharks, as revealed by thousands of sharks with their own identification systems in the form of satellite transmitter tags that scientists clip onto their dorsal fins. Pelagic sharks cruise immense distances through the high seas; one tagged oceanic whitetip shark swam four thousand miles in three months, all the way across the Indian Ocean. These species tend to occupy particular parts of the ocean where they feed and breed, such as a hot spot off the coast of California where great white sharks congregate, and many of these places are intensively fished. Combining satellite data from fishing vessels and tagged sharks, researchers have calculated how much the two overlap in space and time. Overall, pelagic sharks are forced to share one-quarter of their ocean space with industrial longliners. For some species the intrusion is much higher. During an average month in the North Atlantic, 63 per cent of the shortfin mako shark’s domain falls under the footprint of longliners.
There are no accurate figures for the total number of sharks that industrial fisheries kill every year. The best available estimate for the annual global death toll ranges between 63 and 273 million sharks. Many more sharks are dying off the record. Still, the devastation that longlines cause is plain to see from the slaughter that a single vessel can inflict—and from the global dominance of the longlining industry. Unchecked industrial fishing undoubtedly has the power to empty the ocean of pelagic sharks. These magnificent animals that were once so abundant now have very few places left to hide.
Many other types of sharks are at risk of going extinct, not just the sleek, torpedo-shaped animals sliding across the wide, open ocean that are getting hammered by industrial fisheries. These others, often less well known and hidden away, face threats from smaller-scale coastal fisheries and loss of their habitats.
Daggernose sharks weave their way through mangrove forests at the mouth of the Amazon River and along the South American coast. They have tiny eyes and rely on electrosensory perception in their flattened, triangular snouts to detect twitching prey in the murk. They don’t see the fine-mesh gill nets fishers set to catch mackerel until it’s too late. In recent years, the number of daggernose sharks has collapsed.
There are endangered zebra sharks and catsharks, weasel sharks and nurse sharks. Night sharks have big eyes, all the better for seeing in the dark depths, two thousand feet down, where they spend their days before swimming to the surface in shoals at night, when fisheries most commonly catch them.
The puffadder shyshark lives on coasts around the tip of South Africa. They are small and slender, brown with rusty orange saddles resembling the markings on their namesake, the puffadder snake. When disturbed, these sharks curl up in a tight circle and hide their eyes with their tails. They’re at risk partly due to their small native range, which is getting even smaller as the ocean warms and the shysharks are being forced southwards, but for this coastal species, the Cape of Good Hope is a dead end. They’re getting stuck in a climate trap.
Many skates and rays, the sharks’ close relatives, are also highly endangered. Giant butterfly rays look as if they’ve been flattened by a steamroller until they are eight feet across, making them all too easy to catch in fishing nets. Diamond-shaped flapper skates, nicknamed the manta rays of the North Atlantic, likewise suffer for their size; from the moment they hatch from their foot-long egg cases, the newborns are at risk of getting snagged in fishing nets. Wedgefish are sharklike rays that look as if they were made by stitching the tail of a regular shark onto the body of a stingray. Like sharks, wedgefish have fins that are highly valued in the soup-fin trade and command some of the highest prices; as a consequence of intensive fishing, they are now Critically Endangered.
All these elasmobranchs are struggling in the Anthropocene because the ocean is so intensively fished, a situation made worse by the fact that they tend to share a certain pace of life that puts them in jeopardy. Before humans came along, elasmobranchs were used to living in an ocean where they were rarely hunted and eaten. Usually, they were the ones doing the eating, and they evolved to be predators playing the long game. Sharks don’t sprint through life at a hectic biological pace, unlike sardines or mackerel, which hurry to grow up fast and produce copious numbers of offspring because at any moment they could be eaten by something else. Many fast-living fish can have their whole lives done and dusted while sharks are still taking their sweet time to reach maturity. Female oceanic whitetip sharks start pupping in their teens, flapper skates in their twenties. Greenland sharks take it to an extreme and don’t start reproducing until way past their hundredth birthday.
When sharks and rays are finally ready to begin reproducing, things still happen at a slow pace. Female sharks are typically pregnant for a year or more; shortfin makos give birth after a year and a half, basking sharks after two and a half. Female greeneye spurdogs, which gaze through the deep waters off Australia with dazzling emerald eyes, are pregnant for between thirty-one and thirty-four months, one of the longest recorded gestations of any animal.
Egg-laying elasmobranchs also have a leisurely start in life. Small spotted catsharks from the coasts of Europe entwine tendrils of their egg cases among seaweeds and seagrasses, leaving the embryonic pups to feed off yolk for up to eleven months before they hatch. In 2019, an angler off Scotland caught a female flapper skate, and before there was time to put her back in the sea alive, the shock caused her to prematurely lay an egg on the boat’s deck. Luckily, the pup inside was at a late enough stage of development that it survived, and scientists at a nearby marine research station took good care of the egg case. Eventually, after 534 days, the eleven-inch male hatchling finally wriggled out of his egg case, wrapped up in his wings like a burrito. He was the first flapper skate born in captivity, and after swimming test laps of his aquarium tank, he was released into the sea.
Elasmobranchs generally pour a lot of time and energy into producing a small number of big babies, bestowing each one with as good a chance as possible of surviving in the wild. Oceanic whitetip sharks give birth to six pups, on average, only once every other year. After nearly three years of pregnancy, greeneye spurdog females have between four and fifteen pups. Blue sharks push the boat out and give birth annually to fifty and sometimes more than a hundred pups, but generally, sharks produce no more than a few dozen offspring in a year.
The least fecund are sand tiger sharks, which commonly give birth to twins, the winners of a cannibalistic battle that goes on inside their mother. Before they’re born, fertilised eggs hatch into a shoal of tiny sharks, which proceed to eat each other until there are just two left, one in each branch of the pregnant female’s two-pronged uterus. By the time the surviving pups emerge into the ocean, they are so big, between three and four feet long, they have few natural predators. Except now, of course: humans are changing the rules, and sharks are born into an ocean where being big and living a long, slow life is no longer an advantage.
Long before endangered shark species finally blink out and go extinct, their gradual diminishing leaves marks on the environment. Take them away, and it potentially changes the way whole ecosystems work, which is one major reason the future ocean needs more sharks.
The idea that sharks are important for a healthy, functioning ocean generally comes from their position at the top of food chains. A lot of shark species, especially the big ones, are apex predators, primed to have a disproportionate influence over other species—not only the animals they eat but also those further down on the food chain. The disappearance of sharks could set off ripples of change through the rest of the ecosystem.
Keystone predators, the ones that keep entire ecosystems in balance, were first identified in a pioneering experiment carried out in the 1960s. The keystone in question was Pisaster ochraceus, a stout, purple starfish that has a healthy appetite for shellfish. Marine biologist Robert Paine cordoned off areas of shoreline along the rocky northwest coast of Washington State, picked up all the starfish, and moved them elsewhere. Within a few months, in places that were missing the five-armed predators, the local mussel population exploded. Paine watched as the mussels swiftly hogged all the space and outcompeted everything else, mostly other molluscs such as limpets and chitons. He showed that by eating so many mussels and keeping their numbers in check, the predatory starfish allowed a greater diversity of other species to coexist. Lose the predators, and the ecosystem loses its biodiversity. These are the keystones keeping the rest of the ecosystem from collapsing. Paine had picked a good study system to detect these effects, with animals that can be easily moved about and then generally stay put.
Doing a similar experiment with pelagic sharks is impossible; thus, identifying their keystone role is much harder. No scientists have tried excluding sharks from areas of habitat to see what happens. Instead, they’ve tracked the changes that unfold when fisheries deplete shark populations in an area. Some studies have linked a decline in sharks to increases in their prey, including stingrays, octopuses, and moray eels. Whether ripples of change continue all the way through ecosystems—and hence just how important sharks are for the ocean—remains a matter of much debate among shark biologists.
The most controversial investigation into the issue began in 2007, with a study of the shark fisheries off the coast of North Carolina in the United States, where large, predatory sharks, including bull sharks, tiger sharks, and scalloped hammerheads, had been overfished for decades. As populations of these apex predators declined, scientists spotted an uptick in the numbers of smaller sharks and rays. Doing especially well were the three-foot-wide cownose rays, named for their domed heads and straight-lipped mouths, which when they’re crunching on clams bear a passing resemblance to their terrestrial namesake chewing its cud.
While making the most of their new-found freedom from the jaws of larger sharks, the cownose rays were munching their way through the local bay scallops. Here was a ripple of change, kicked off by the loss of apex sharks, cascading all the way down the food chain—just as the science of keystone predators predicts. Authors of that 2007 study made an explicit link between the proliferation of cownose rays and the collapse of a scallop fishery. They made no comment on what, if anything, should be done about this; their job was simply to investigate the ecological changes taking place. But when members of the public caught wind of the study, their reaction was perhaps predictable.
“Save the Bay, Eat a Ray” was the slogan for a campaign that launched after fishers in the Chesapeake Bay decided their oyster harvests were dwindling because cownose rays must be eating them as well. Cownose rays were renamed “Chesapeake rays” to persuade people to eat them; the state of Virginia gave away free ray meat to restaurants to stimulate an appetite for it, and recreational fishers held tournaments to kill as many rays as possible. Like the Florida lionfish, the cownose ray was branded as an invasive species, and people were all too willing to help get rid of it. The truth is, cownose rays are not invaders but are native to the region, and they had been unfairly blamed for changes that were already taking place.
All this was laid bare in a 2016 study that found flaws in the original sharks-eat-rays-eat-scallops story and pointed out some overlooked facts about the species involved. A drop in predation by large sharks couldn’t have triggered a sudden proliferation in cownose rays because the rays reproduce far too slowly; female rays take eight years to reach maturity and thereafter give birth to a single pup each year. Cownose rays just don’t have the same capacity for rapid growth as Robert Paine had seen among mussels on the rocky shore. More likely, rays increased off North Carolina because they swam in from elsewhere. Cownose rays are known for congregating in the tens of thousands and undertaking immense seasonal migrations. Also, the timing was off. A more careful look at the data showed that the scallop fishery had already been collapsing for a while before the cownose ray bonanza. And besides, cownose rays don’t even eat that many scallops or oysters. Tragically, the legacy of the original research lingers, and cownose rays are still being heavily fished and targeted in fishing tournaments.
Moving on from that sorry study, scientists are still untangling the ways sharks influence their environment. No simple story is emerging of what happens when sharks are gone. The bottom line is that ecology is complicated, especially in the ocean, where water flows and animals restlessly migrate, forming connections that reach far and wide.
