A sardine swims through the cold waters of the eastern Pacific, and it’s not alone. Like many fish, sardines don’t do well on their own and always swim with others. This one races along, surrounded by a swirling school of a thousand animals that seem to think as one, turning together, speeding up, slowing down. But the little fish isn’t a mindless part of a machine; it’s watching and thinking, feeling, listening and deciding what to do next. It somehow knows the unspoken rules that keep the school together.
Choosing which school to join is partly a matter of finding other fish that are a good size match. Rule one: don’t be the biggest or the smallest fish that stands out, or you’ll be the one a predator spies and eats first. How a fish sizes itself up against its schoolmates is not entirely clear, but they somehow figure out who is bigger and who is smaller.
The next rules ensure the sardine doesn’t bump into or get too far from the other fish. Rule two: if the fish behind gets too close – within two body lengths – then speed up. Rule three: if the fish in front gets closer than that, slow down.
In perfect synchrony the school seems to turn all at once, but the fish are not all equal; there are leaders and there are followers. Our particular fish is a follower. Its preferred spot isn’t at the front where the leaders swim, but further back in the midst of the pack. It decides which way to turn by watching its neighbours, perhaps just the closest few or all of the sardines in its field of vision. Pressure-sensitive pores along the sardine’s body (called the lateral line) let it feel the position of its nearest companions by sensing wrinkles they leave behind in the water as they swim through it.
Suddenly, a wave of panic rips through the school and the sardines all dive down and huddle together. The fish further back haven’t seen the sea lion but they get the message of imminent danger, written on the shining, turning bodies of the other fish around them. Like a Mexican wave travelling around a stadium, waves ripple through the school from fish to fish. The waves move much faster than the fish themselves and rapidly transmit vital survival information to the whole school.
As anxiety levels rise among the sardines, they begin to pay even closer attention to what’s going on around them and more precisely copy their neighbours’ movements. Now they’re under attack, it’s more important than ever to blend in and do exactly the same as everybody else. Any fish behaving differently could catch the sea lion’s eye and become its next target. The sardines blend together until it’s difficult to make out a single fish, and they all get lost in the crowd.
The sea lion launches another attack, this time splitting the school in two. The divided fish know they must stay together and like a cascading fountain they flow back to reform a single throng. The hunter hasn’t yet made a successful strike, but it has driven the sardines towards the shore, restricting their movements in a sandy cove. Again and again it swoops in, but the sardines seem to predict its every move. It’s as if the school can read the sea lion’s mind but in fact they’re just incredibly fast. Giant nerves transmit signals between the fish’s brains and muscles, and they respond within a fraction of a second.
The attacker hardens its resolve and pushes once more through the school. Tension mounts, and the sardines swim faster. The school turns in on itself and forms a tight, spinning sphere. Every fish tries desperately to get to the middle. Each wants to hide behind another and get as far as possible from the hunter’s snapping jaws. This selfish geometry shows the fish aren’t looking out for one other. They’re each just using the school to try and stay alive.
Finally, the sea lion picks off a sardine, then another. Those were the unlucky ones. Inside the school there is still safety in numbers. By sticking together, most of the sardines have escaped unharmed, a far greater proportion than if they were all solitary and roaming the oceans on their own, just lonely pairs of eyes watching for danger.
A life spent in water is not enough to distinguish fish from non-fish, despite the claims in ancient fish books. And yet the ways that fish move through their three liquid dimensions is a crucial part of being a fish.
Back in California, as my 15 year-old self watched those schooling sardines expertly avoiding the sea lion’s jaws, I was gazing into a world very different to my own. Fish inhabit a medium that’s 900 times denser and 80 times gloopier than air, factors that dominate everything they do. They have to push their bodies forwards to overcome the drag of water that tries to hold them back. On the plus side, though, all it takes is a gas-filled balloon, the swim bladder, and fish can shrug off gravity’s pull and float effortlessly in their buoyant surroundings. No bird, bat or insect can fly with such ease.
Other aquatic animals have their own ways of getting around underwater: squid and octopus squirt water jets to propel themselves along; some have flapping ‘ears’ on the sides of their heads; crabs and shrimp have flattened leg paddles; tinier creatures row through the water with flicks of antennae and hairs. None swim as fast, as furiously or as far as fish. Fish have evolved over hundreds of millions of years to artfully and efficiently swim through the life aquatic.
Different strokes
You can look at a fish and from its shape alone know a lot about how it moves. In Senegal in West Africa, I recently saw a Yellowfin Tuna for the first time. It was propped upright on a bed of crushed ice and staring at the ceiling in a market, and it was massive. If I’d hugged it I would have wrapped my arms maybe half way around its silver body. This fish was all about power and speed. Its torpedo-shaped torso was solid muscle. The tail had been cut off and sat next to it, as if another tuna was diving through the counter; its shape is an elegant crescent, not so good for steering but ideal for cutting down drag while the tuna is cruising. To reduce drag further, during long swims, the pair of pectoral fins would have retracted into slots on each side of the tuna’s body, to give it a smooth streamlined profile. When they hunt, the pectoral flips back out to steer and chase after prey. Two elongated fins, also bright yellow and shaped into curving sickles, would have helped prevent the tuna from rolling over as it swam. Rows of triangular, yellow spikes along its flanks probably channelled a streamlined flow towards the tail, helping it shove water sideways and backwards, creating forward thrust.
The man behind the fish counter tried picking up the yellowfin’s tail, but struggled with its weight and slippery skin and dropped it on the floor. He tried and failed to pick it up again until someone came and helped him. I wondered how many people it had taken to catch it and bring the fish on deck in the Senegalese fishing fleet out in the Atlantic.
Fish shaped like tuna, with a torpedo body and a forked or a crescent-shaped tail, have evolved for long-distance, endurance swimming: mackerel and swordfish, marlin and sailfish. Swordfish and sailfish have a reputation for being the fastest fish, able to sprint at 100km per hour (over 60mph), but recent studies suggest that this is an inflation of reality. Even so, these hunters are far from lazy. Sailfish can probably manage bursts of almost 32km per hour (20mph), far speedier than any of the smaller prey fish which they chase after, and that is surely the point. If fish swam any faster they’d begin to run the risk of damaging themselves with cavitation bubbles. Fluids under high pressure form air bubbles that then collapse, creating an intense shockwave. Pistol shrimp on coral reefs make cavitation bubbles when they snap their claws (this is where a reef’s crackling soundtrack largely comes from). The shrimp’s tough carapace can withstand the shocks; fish skin and scales probably can’t.
Similar to tuna and sailfish, there are torpedo-shaped sharks with forked tails, like makos and Salmon Sharks, which also swim long distances. They don’t have swim bladders and even with oily, buoyant livers sharks do have a tendency to sink. Helping to make up for this, their large pectoral fins are shaped in cross section like aeroplane wings. As they move forwards, water travels faster over the top of their fins than underneath, creating an upward pressure. A 2016 study found that Great Hammerhead Sharks spend up to 90 per cent of their time swimming rolled over on their side, at angles of between 50° and 75° to the vertical, an awkward-looking pose, but one which boosts the lift they get from their tall dorsal fin.
In contrast to long-distance marathon swimmers, fish with wide, fan-shaped tails tend to be fast-start sprinters. Ambush predators, like pike, barracuda and groupers, have wide tails that push aside a lot of water. Big tails are hard work, with a lot of drag, but they’re effective over short distances when sudden speed and the element of surprise are all-important.
Eels swim with undulating waves that run along their entire, bendy bodies, usually from head to tail; by switching to waves in the other direction they can swim backwards. Knifefish hold their bodies stiff and swim with undulations of a long fin that runs along their underside; Bowfins do a similar thing, but with a dorsal fin along their backs.
Flatfish move in the same way as regular, upright fish, they simply do it lying on their sides. Plaice, sole and various others spend their first few weeks as hatchlings swimming in a typical, vertical fashion. Then the bones in their skull begin to bend and shift, their mouth changes shape and one eye moves across their face to join the other (either the left or the right eye moves, depending on the species). One side of their flat body becomes pale and white, the other dark and speckled. When they’re finally ready, the mature fish lean over and adopt a sideways stance, laying their pale side against the seabed, their dark camouflaged sides uppermost, and look skywards with two eyes on the same side of their head. Now, instead of sweeping their tails from side to side, they undulate up and down. Some elasmobranchs have also adopted this flattened lifestyle, to sit and wait for prey on the seabed, but they do it in a different way: skates and rays squash themselves from top to bottom, press their bellies to the seabed and swim by flapping large pectoral fins stretched out to the sides like wings.
Flying fish have fins that look even more like birds’ wings. Underwater they gather speed, then jump clear into the air, unfurling their huge pectoral fins and holding them still, without flapping, while they glide for tens or even hundreds of metres. In 2010, Hyungmin Park and Haecheon Choi from Seoul National University in Korea put dead, stuffed flying fish in wind tunnels and found they glide as efficiently as hawks. The reason fish learned to fly was probably to escape from predators. From below, the sea surface acts like a mirror, reflecting light back down, so a waterborne predator won’t see a flying fish unless it’s a still, sunny day and it casts a shadow. And they’ve been escaping hunters’ jaws in this way for a long time. Flying fish fossils have been found in the same 235-million-year-old rocks as giant ichthyosaurs, which were perhaps the hunters they were trying to avoid.
Then there are fish whose body shape reveals that they try not to swim at all, if they can possibly help it. Deep-sea anglerfish save their energy (there’s not much to eat down there) and drift about, only waggling their tails when danger or food is near. Frogfish sit on the seabed doing their best to blend into their surroundings, and if they need to go somewhere they’ll stroll ponderously along using their pectoral fins as legs; if it’s really urgent they’ll even break out into a gentle gallop. And then there are handfish, fingering their way slowly across the seabed around Australia, their pectoral and pelvic fins splayed out and looking for all the world like little hands and feet.1
When fish swim in groups, things begin to get complicated. Half of all fish species spend some of their time swimming together. One in four live permanently with other fish throughout their adult lives. Take a herring or a sardine or an anchovy away from its swimming mates and it immediately becomes agitated.
Fish congregate in two main formations. First there are shoals. These are loose social gatherings in which fish mill about together, without paying too much attention to each other. Then there are schools. Shoaling fish can transform into elegant, spiralling schools when, for some reason, all the fish suddenly decide to swim and turn in tight synchrony. In a school, everyone swims in the same direction, their bodies in parallel. A school might lose its orderly structure, though, and once again form an untidy shoal. For decades, scientists have studied shoaling and schooling fish to try to understand how and why fish behave like this, as they go from one fish, to two fish, to many.
In his later years Konrad Lorenz, who came up with the idea of eye-catching poster colours, devoted himself to studying the social lives of fish and how they hook up to form schools. Instead of fighting each other and arguing over territories, there comes a point when some fish begin to get along, and Lorenz wanted to watch this switch taking place.
In 1973, Lorenz won a Nobel Prize for his work on animal instincts.2 He spent his prize money building an enormous aquarium at his home just outside Vienna. It was four by four by two metres (1.2 x 1.2 x 0.6ft) and contained 32,000 litres (7,000 gallons) of seawater, enough to fill more than 300 bathtubs. He stocked it with various coral-reef fish, including dozens of young Moorish Idols, with distinctive white, black and yellow bands. Then, for several years, he spent most afternoons watching them.
After Lorenz’s death in 1989, an incomplete manuscript was found in a drawer in his study. It describes in immense detail what he saw during his long vigils watching fish, totalling more than a thousand hours. He gave names to all his fish, and watched as they performed a complex repertoire of gestures. The Moorish Idols would beat their tails at each other or lock jaws and wrestle; pairs would race around the tank side by side, or dart at each other before slowly retreating. Lorenz’s notes are filled with sketches he drew of the territories each fish carved out around the aquarium. In March 1977, he wrote in his notes, ‘Glub and Fris fused their individual territories, but still exclude Bajo, which selectively either attacks Glub or Fris, when either of the two appears in the “wrong” place ... Kuna is still confined to its shelter at the left lateral wall.’
Eventually Glub, Fris, Bajo, Kuna and the other Moorish Idols all worked out their differences and came together to form a single, permanent school, parading around the tank together. On coral reefs the fish perform similar switches from territoriality to schooling, but no one had ever witnessed it unfolding like this. Lorenz acknowledged that even his large aquarium was cramped, but he was convinced his microcosm provided important clues about what fish get up to in the wild, when no one’s watching.
Further aquarium studies, often on small, compliant species like Mosquitofish and Zebrafish, are helping to decipher the choreography of shoals. By watching shoals and schools take shape and tracking the movements of individual fish, researchers are discovering how they adjust their positions relative to each other, not getting too far apart or too close; when predators attack, fish swim in tighter, more synchronous schools with various ways of evading capture: diving to the side, splitting apart and rejoining together in cascades.
Despite appearances, fish do not join to form egalitarian super-organisms, entities with a mind of their own and no leaders guiding the way. Studies are finding there are in fact bold pathfinders prepared to take on the greater risk of getting caught at the front of the school. Hungrier fish also tend to be the ones up front, where they have a better chance of finding food compared to followers, trailing further behind.
Shoaling studies show the benefits fish can reap from swimming together. Most obviously, they avoid predators by confusing them with a blur of identical bodies, and diluting the impact a single hunter can have on a large group. Fish will even take turns to briefly leave the safety of the shoal, swim up to a nearby predator and check out what they’re up to. They then return to the shoal and apparently inform their shoal mates if an attack seems imminent, so they can swim away, or if the predator is otherwise engaged. Shoaling fish also boost their chances of finding food, especially if it’s patchy and hard to come by; with more fish looking, they’ll do better at finding those patches.
Fish also save energy by swimming in shoals rather than on their own. Like cycling pelotons and cars driving in the slipstream of the vehicle ahead, fish further back in a school use less power to keep up. As they swish their tails, fish fling spinning vortices into the water behind them, which their schoolmates have to swim through. But instead of struggling against a turbulent wake, a fish puts itself behind and in between two other fish in just the right spot to get an extra push from these vortices. Even fish at the front of the school make energy savings by riding the bow wave pushed ahead by fish behind them. And as is so often the case, people are learning from nature and applying these fish movements to the human world. Placing wind turbines in similar positions as schooling fish can make them up to ten times more efficient.
To probe deeper into fish-school dynamics, researchers make their own shoals. Based on observations of living fish, they programme fish avatars with rules on how to move. They then release these computerised fish into virtual aquaria and watch them swim around. Virtual predators are released, too, to chase after them, with their own set of instructions on what to do.
Hundreds of computer models like these have been made and refined until, as far as statistical tests are concerned, they’re identical to the way real fish move. But is that enough to say a model is true to life?
It’s a question posed by James Herbert-Read and colleagues at Uppsala University in Sweden. They set out to see if people could tell the difference between real and computer-generated shoals. In 2015 they built a simple online game with pairs of videos showing green dots swirling around. One video is the two-dimensional trace of a real fish shoal, the other is the output from a model. Players were asked to pick out which they thought was the genuine shoal.
Academics who study fish motion were, perhaps not surprisingly, very good at the game (not to mention immensely competitive), mostly choosing the right shoal each time. Almost 2,000 members of the public also had a go. Although not quite as good as the experts, the gamers could tell that something was wrong and spotted a clear difference between the two shoals, but they weren’t always sure which was the simulation and which the real thing.
The computer model failed to pass this fishy version of the Turing Test.3 Herbert-Read’s games weren’t testing the intelligence of simulated fish, but their abilities to swim like real ones. Seeing shoals through many pairs of human eyes revealed that something wasn’t quite right with the models, even though statistically they were a good fit. The perfect computerised shoal is still some way off, and fish are still holding onto some of their secrets of how they swim and school and shoal.
Shoal searching
Many years ago, I set out to uncover a part of the picture of how fish move and shoal. It was an adventure that began one night as a small boat left the northern coast of Borneo. I was a PhD student, and part of a small research team heading to a remote island in the South China Sea. That first night I was too excited to sleep. I stayed on deck as the captain picked a route through the lights of oil rigs, and I watched as the dark shadow of the mainland sank behind the horizon. Then after two days and two nights of motoring slowly through heaving waves, I was too seasick to sleep. Feeling wobbly and ocean-weary, I finally saw the lights of a tiny island peep into view, and an unsettling feeling began to seep in. I’d been planning and anticipating this trip for months, but as I neared my destination I suspected I’d made a terrible mistake.
Swallow Reef is a coral atoll shaped like a teardrop. Above the water line there’s little more than a narrow strip of sand and concrete, 1,500m (1,640 yards) long, with a runway servicing a small dive resort and a Malaysian military outpost. The expedition team wouldn’t be staying at either but sleeping under the stars or, when it rained, inside a rusting shipping container perched beside the runway. Washing facilities consisted of buckets of water, and to go to the toilet we either crept into one of the sparse shrubs growing at the far end of the island or, preferably, jumped in the sea. There was no internet, no phone signal and little electricity.
Arriving in this place that I’d thought of and talked about for so long, I was excited but at the same time felt a deep shock of disconnection, suddenly realising how far I was from familiar people and places. I was supposed to stay on the island for three months, and I began to doubt if I could see it through.
The following day, things got worse when the team set off to dive for the first time. We left the calm lagoon, passing through a channel cut into the reef and out into the open rolling sea. Inelegantly, I clambered into my scuba gear while sliding from side to side on the tipping deck. It didn’t help when the boat’s captain announced he planned to keep the engine running. Normally, for safety’s sake, dive boats are put in neutral so there’s no chance a diver in the water will get lacerated by the whirling propeller.
‘It’s too dangerous to stop,’ he yelled.
The waves were pushing us closer to the sharp, boat-wrecking reef crest. It meant that my dive buddies and I would have to imitate Navy Seals and execute what’s technically known as a ‘negative entry’. I couldn’t linger on the sea surface to gather my thoughts and check my dive gear was working; just fall backwards off the side of the boat and sink straight down, hopefully beyond the propeller’s chop.
I was seconds from screaming back at the captain, giving up on the whole thing and finding a way to get back home. But I dredged up a final burst of resolve, scrambled over the high gunwale and dropped into the water.
Instantly, I passed from hell into heaven.
The water was so clear I barely noticed it was there. It was the closest I’ve ever come to flying. Below me, the reef spread into the distance. It looked like a garden blooming with flowers, mosses and lichens with scarcely a bare patch of empty seabed visible. This was the healthiest reef I had ever seen. Throngs of fish roamed all around me and among them I spied one of the animals I’d come all this way to see. And, just like that, my fears and worries blinked out.
Humphead Wrasse can be very difficult to find. Most of the time they live on their own, ranging over wide tracts of coral reef. There are a few places in the world, though, where encounters become more predictable.
The humphead I saw on that first dive was a female. I guessed she was about 50cm (20in) from nose to tail; if she’d let me, I could have tucked her neatly under my arm and carried her off. Her flanks were pale green-grey and her tail trimmed in yellow. Her forehead was not especially pronounced or humped; that would come later.
She was swimming intently along the reef towards a particular spot that I would visit the following day, and most days after that for the weeks and months that followed. There I saw not just one wrasse but dozens. Most were females of a similar size, plus one tremendous, presiding male. He was so big he would have had a hard time fitting into a bathtub. He was a similar colour to the females except for his bright blue face and lips, and had a big bump on his head. Occasionally he swam right up and looked me closely in the eye – wondering, I suspect, if I needed to be chased off like the lower-ranking males that milled nearby. This giant male humphead was the first fish I’d ever seen that made me feel I was being carefully and thoughtfully watched.
The humphead shoal formed every day for a week around new moon for a single purpose, an act that for each female would last approximately four seconds. The dominant male, on the other hand, was in it for the long haul. When he wasn’t chasing off intruding subordinates, he was busy doing his best to tempt females to join him in the open water above the reef. When she decided the time was right, each female swam upwards with the male trailing eagerly behind. Only then did the size difference between the genders become obvious; he was at least three times bigger than his petite partners. They swam side by side, the male brushing his chin against her body. Then with a swift shimmy she released a cloud of eggs into the water and he added a puff of sperm. The two fish then parted; the female peeled away and swam off back down the reef, and the male turned around to pick up the next member of his harem. One by one, he mated with them all until there were no more females left.
Lots of fish species meet up in shoals to spawn, often in precisely the same places and times each month and year. In the Northeast Atlantic, from the Barents Sea to Iceland and the Faroe Islands, slender cod called Blue Ling gather together to mate hundreds of metres beneath the waves. Orange Roughy congregate on underwater mountains for the same reason. And on coral reefs, many species aggregate to spawn – groupers, snappers, wrasse, surgeonfish – sometimes swimming for days and weeks, across hundreds of miles to reach the spawning site.
Mating shoals range in size. There are small, select groups like Regal Angelfish. A single male, adorned in striking yellow, white and blue stripes, gathers a harem of three or four females. Each evening, 15 minutes before the sun sets over the coral reef, he begins nuzzling the females and leads them one at a time in a spiralling, upwards dance. As they spawn the male flicks his tail, whipping the eggs and sperm into a toroidal vortex – a spiralling doughnut shape, like a smoke ring – which rises up in the water, away from the many mouths on the reef that would gladly feast on the nutritious cloud.
Fish can also reproduce on a truly spectacular scale. Thousands of millions of Atlantic Herring rendezvous on Georges Bank, a shallow sandy shelf between Cape Cod and Cape Sable Island off the northeast coast of the United States. Shoals form at sunset as scattered herring draw together. When a certain density is reached (one fish in roughly five cubic metres (175 cubic feet) of sea) a chain reaction sets off and the shoal grows outwards in waves, similar to the Mexican waves of panic that sweep through a hunted sardine school. The waves move at 60km per hour (37mph), much faster than the herring themselves can swim. A colossal shoal, some 40km (25 miles) across, then begins a slow procession to the southern reaches of the bank, led by smaller groups of fish that seem to know where they’re going. On reaching their destination spawning begins, and the water becomes cloudy and thick with the next generation of herring. When the morning comes, it’s all over and the shoal disbands.4
Fish gain various benefits from mating this way. Rather than simply hoping they’ll bump into a suitable mate somewhere in the wide ocean, it makes sense to arrange a certain place and time to meet. Doing so can also reduce the chance that all those precious eggs will be guzzled by predators. In the Persian Gulf, Mackerel Tuna aggregate and spawn under oil platforms and Whale Sharks show up, too, to sieve the water for their eggs. Even a herd of a hundred Whale Sharks can’t eat them all and when they swim off, bellies full, there are still plenty of eggs left to start the next generation of tuna.
Other hunters show up at spawning sites to eat not eggs but the spawning fish themselves. A lot of sharks live on Fakarava Atoll, in the Tuamoto Archipelago in the middle of the Pacific. Divers regularly encounter 600 Grey Reef Sharks in one small area of reef, by far the greatest density of reef sharks documented anywhere in the oceans. Johann Mourier from Macquarie University in Sydney goes with colleagues to study this shark jamboree and they’ve discovered the ecosystem is flipped upside down. Instead of having more animals towards the base of the food web – the usual way of things – the reef at Fakarava is top heavy with apex predators. Normally, sharks endlessly roam large areas to find enough food, but this unrivalled crowd stay put, for a while at least, because plenty of food comes to them in the form of large, mottled fish called Camouflage Groupers. In June and July each year tens of thousands of them congregate at Fakarava to spawn, and a lot of them end up getting eaten by sharks, although not so many that the grouper population dwindles. There was probably a time when there were many more great shoals of spawning fish, in other parts of the ocean, being hunted by packs of sharks. But away from remote atolls like Fakarava other hunters have got to them first.
Human fishers have learned to target spawning sites. It’s logical to visit the spots where fish congregate like clockwork, usually when the moon is full or new. But unlike sharks, people often take things too far and wipe out whole shoals. In the Caribbean, tens of thousands of Nassau Groupers used to spawn in huge aggregations, but they’ve been so heavily overfished the majority of the spawning sites no longer form. Similar stories play out for numerous species worldwide. The lost shoals never seem to return, perhaps because young fish learn from the older ones where to go. When those wise old fish have gone, memories of the spawning site are lost with them.5
It was with this in mind that I went to the South China Sea and Swallow Reef to find many Humphead Wrasse. I wanted to work out just how vulnerable they would be if fishers did target their spawning sites instead of catching them one by one across a reef.
Throughout the Pacific, Humphead Wrasse have traditionally been a much cherished fish. In Micronesia and the Cook Islands, they used to be reserved for royal feasts. In the Carteret Islands in Papua New Guinea, only elders are allowed to eat them. Spearing Humphead Wrasse was once an important ritual for young boys reaching manhood on the island of Guam. More recently, though, traditional fishing has been replaced by regional commerce. Seafood enthusiasts in Asia have adopted Humphead Wrasse, and various groupers, as their species of choice. Fishers throughout the Indian and Pacific Oceans now target them, often diving down and breathing compressed air through hose pipes that snake to the surface. They take with them plastic bottles filled with cyanide solution, which they squeeze into holes in the reef to stun the hiding fish, incidentally killing other reef creatures, including corals. The idea is to keep the fish alive and send them to cities where people pay top dollar to eat them. The humpheads are displayed in aquarium tanks in restaurants, where affluent diners point to the fish they want to eat. In China, the males’ big blue lips are considered a delicacy. And with so much demand, it’s taken only a few decades for the Humphead Wrasse to become highly endangered.
At Swallow Reef I wanted to work out what happened when the humpheads met up to spawn. If individual females spawned just once and new fish formed the spawning shoal each day, then there must be a large group of them living around the reef. But if the same fish were showing up day after day, then it would mean a much smaller collection of hard-working females bear the duty of spawning the next generation. It would also mean there was a greater chance that if fishers were to target the spawning site, the entire adult population could quickly be gone.
My task was to learn to recognise individual humpheads. I couldn’t very easily round up these big, endangered fish and fix identifying tags on them; instead I kept my distance and took photographs of their natural markings. Their other common name, Maori Wrasse, is a nod to the labyrinthine motifs on their faces that some say resemble the moko tattoos of indigenous New Zealanders.6 The iridescent blue lines on a Maori Wrasse’s face weave in and out, hopping and jumping in dots and dashes, perhaps a form of poster colours signalling to other wrasse.
I wanted to know if these markings were unique to each fish. If they were, I could use their patterns to recognise individual fish, to track them at the spawning site, to decipher their mating rituals and see how frequently each female came back.
First, though, I’d have to spend many hours in the water with them, catching on camera their complex faceprints.
Throughout my study following the movements of those big spawning fish, I knew they wouldn’t roam too far. Adult Humphead Wrasse always need a reef beneath their fins, and won’t spend long in open water. Plenty of other fish are less devoted to one particular place, and routinely set off on epic journeys.
Until recent decades, the main way of finding out where fish go was to catch them, mark them in some way, usually with a numbered tag and a ‘Return to sender’ message, then let them go and hopefully someone will catch those same fish again, somewhere and some time later. Like sending a message in a bottle there’s no guarantee the tagged fish will get picked up, and even when they do this approach provides just two pieces of information: the beginning and end point of an otherwise mysterious route. Now, though, fish draw lines on digital maps, with electronic tags following their every move.
Tracking technologies have come a long way since the early days. A Basking Shark was tracked with a satellite tag for the first time in 1982. The shark towed a sizeable floating package behind it on a 10m (33ft) cable; the device relayed its position via satellite whenever the shark swam to the surface. For 17 days, scientists watched the shark’s progress from afar as it swam south through the Sound of Bute off the west coast of Scotland, past the Isle of Arran, along the Firth of Clyde and circled around the rocky islet of Ailsa Craig. Then the transmitter broke free, sooner than it was supposed to, and a local resident found it on an Ayrshire beach and sent it back to scientists at the University of Aberdeen. It was still in good working order.
Since then, many large sharks have been tagged with devices now the size of a smartphone, fixed directly to the dorsal fin. A 2017 study revealed the paths of 70 Basking Sharks tracked from Scotland and the North Atlantic on long wintertime migrations. Some hung around the UK and Faroe Islands; some swam into the Bay of Biscay and others swam for months to the coast of North Africa; most covered at least 3,600km (2,200miles).
Research teams have gazed at their computer screens as similar tags have revealed other immense fish migrations. Salmon Sharks from Alaska escape chilly waters to spend their winters in Hawaii. In 2003, a female Great White Shark was tracked 11,000km (6,900 miles) across the Indian Ocean, from South Africa to Western Australia. A photograph of the shark’s ragged dorsal fin showed it swam all the way back to South Africa six months later. Bluefin Tuna race east and west along an underwater highway across the North Pacific, between Japan where they breed and California where they feed and fatten up. One young tuna did this three times in 20 months, swimming 40,000km (25,000 miles), equal to the circumference of the Earth. In 2012, the US press stirred panic over health concerns of eating tuna that migrate to American waters from Japan and could have been contaminated by the devastated Fukushima nuclear power plant, even though radiation levels in these fish were so low that eating a normal banana was more dangerous than a tuna-steak dinner.
Besides knowing where a large fish is at any given time, electronic tags are uncovering a wealth of other details of journeying fish. When Great Whites swim across whole ocean basins they commonly pass through wide expanses where there’s little for them to eat. Tags that track not only horizontal movements but also depth suggest that, as migration proceeds, Great Whites begin to sink, probably because they’re using up the buoyant, fatty reserves in their livers that can make up a third of their body weight. A Great White Shark liver weighing almost half a tonne contains 400 litres (90 gallons) of oil, storing 2 million kcal of energy (roughly equivalent to 9,000 Mars Bars). Like a camel’s hump, Great Whites seem to use their livers as a food source to survive long treks through ocean deserts.
Satellite tags have also helped solve a mystery surrounding the brains of manta rays. In 1996, researchers unexpectedly found what seemed to be a brain-warming device in Giant Oceanic Mantas as well as in their relatives, the Chilean Devil Rays. Various sharks, marlin, sailfish and tuna have similar bundles of blood vessels – known as retia mirabilia, Latin for ‘wonderful nets’ – which transfer heat generated by powerful swimming muscles to the brain and eyes, raising their temperature by 10–15°C (18–27°F) above their surroundings and keeping them sharp as they go on hunting raids into deep, cold waters.
Most fish have cold bodies, because seawater saps their body heat as it courses through their gills. Except, that is, in strange-looking fish called Opah. These giant silvery discs have white spots, red fins, a golden ring around each eye and retia mirabilia in their gills. Cold blood flowing from the Opah’s gills is warmed by blood flowing back from the heart (a so-called counter-current exchange), making them the only known fully warm-blooded fish; these deep-diving predators are the only fish with warm hearts.
It had been assumed that mantas and devil rays were tropical, shallow-water species with little need for warming their brains. One idea was that their retia were actually keeping their brains cool. The puzzle was at least partially solved in a 2014 study that began a long way off the coast of Portugal. Thirteen devil rays were tagged at Princess Alice Bank, an underwater mountain in the Azores. The tracked rays swam thousands of kilometres south and plunged thousands of metres beneath the waves, on deep forays that no one had previously known about. The rays reached a depth of almost two kilometres (1.2 miles), placing them among the deepest diving ocean animals.7 Again and again, the devils swam straight down, then for an hour or so slowly returned to the surface, probably feeding as they went on layers of plankton. Occasionally the devil rays stayed deep for 11 hours at a time. Why they do this is not exactly clear, but at least now their brain-warmers make sense, as they regularly spend time in waters colder than 4°C (39°F).
Although tagging has provided rich insights into the lives of fish, there are some who call for caution on the indiscriminate use of these technologies, and question who should have access to the information generated, including near real-time data on where fish are. In the American state of Minnesota, a group of anglers recently petitioned to be allowed to use radio-tracking data on Northern Pike, one of their favourite targets.8 Scientists collect these data using public funds and so, the fishers argued, the public should be allowed to use the data however they chose. That case proved unsuccessful but another, in Australia, saw shark-tagging data used for a very different purpose to the one originally intended. Following a series of fatal shark attacks on people swimming off the Western Australia coast in 2014, the government introduced a kill order. Satellite tags had been deployed in a study of shark ecology aiming to help protect these animals from going locally and globally extinct. However, as part of the permit system, all data gathered by tags have to be made available to the licensing agency, and those same data were used to locate and kill sharks.
But on the whole, despite rare instances of being co-opted for nefarious purposes, tagging studies are uncovering great feats of fish migration. Legions of electronic devices are showing that the world’s waterways and vast open oceans are thoroughly busy, criss-crossed with well-worn paths between popular places, where animals return year after year, to mate, to feed and to track the passing seasons.
To find their way around and undertake these journeys without getting lost, fish have at their disposal a toolkit of finely tuned senses. They have good vision, they can smell and hear and feel currents passing over their body, and some seem to have an extra sense that remains somewhat enigmatic.
It’s not known exactly how, but lots of animals including fish navigate the world with the aid of an inbuilt magnetic compass.9 Stretching across the Earth is a net of magnetic field lines that rise in the southern hemisphere and fall in the north and vary predictably from place to place. By detecting the intensity and inclination of local magnetic contours, it’s possible to work out where you are in the world. Newly hatched European Eels follow their magnetic sense as they swim from the Sargasso Sea, in the far west Atlantic, towards the Gulf Stream as it flows eastwards and pushes them all the way to Europe. American Eels begin the same journey, riding the Gulf Stream, but they leave sooner and swim west. Migrating salmon seem to remember the specific magnetic field they encounter when they leave their home river and taste salt water for the first time. Several years later, after growing up at sea, the salmon follow this mental magnetic map and return to that same coastal area, then press on inland and eventually smell their way back to spawn in their natal stream.
Small-scale magnetic anomalies can also provide local waypoints. In pioneering tracking studies in the 1980s, American ichthyologist Peter Klimley tracked Scalloped Hammerhead Sharks swimming repeatedly between an island and an underwater seamount off the coast of Baja California. The sharks swam at night through pitch-dark seas and followed dead straight lines. Klimey deduced the sharks were following magnetic gradients that flow around the seamounts, which are made of volcanic basalt and are mildly magnetised as a result.
It remains to be discovered how exactly sharks, salmon, eels and many other animals pick up magnetic fields. It’s possible that sharks use their electric senses. When seawater flows past the Earth’s magnetic fields it induces weak electric currents, which sharks may detect with electro-sensitive pits on their snouts called ampullae of Lorenzini.10 For other species a long-standing theory suggests that some sort of sensory cells, perhaps rich in iron, can detect magnetic fields and trigger nerve signals to the brain. A clue came in 2012, when researchers discovered what appeared to be magneto-receptor cells inside the noses of Rainbow Trout. In a rotating magnetic field these cells swivelled about, like a school of fish all turning together in the same direction.
Whatever tools they use, there’s no doubt fish are master navigators that can find their way not only across entire oceans but also whole continents. A swathe of South America, where the Amazon River and its tributaries percolate through dense rainforest, is home to thousands of freshwater fish species, including a giant catfish known in Portuguese as the Dourada. These enormous fish can grow to almost two metres (6.5ft) long; they have a wide mouth, long whiskers and smooth, shiny skin that makes them look as if they’ve been dipped in mercury. Together with several close relatives, these catfish support the biggest fisheries across the Amazon Basin, and fishers have long known there’s something special about them – these fish are great wanderers.
Compared to other more globally famous fish species, especially sharks, Amazonian catfish are rather overlooked, and scientists lack resources to study them. The catfish have never been fitted with electronic tags; instead, a few teams of dedicated researchers have turned to simpler but more labour-intensive methods to find out where catfish go. One group gathered decades of data from interviews with fishers and surveys of catfish, young and old, across the river basin. Another team bought fish from markets around Amazonia, in the cities of Manaus and Belém, and from inside their heads picked out small ear stones, otoliths, which help the fish balance and hear. As the fish grew, their otoliths became imprinted with chemicals from the waters they swam through. And because the water’s chemical composition differs from place to place, it’s possible to decode the layers of chemical traces and reveal where a fish has lived at different stages of its life.
These studies are piecing together the catfish’s story and confirming what fishers suspected. Dourada and other giant catfish venture on spectacular migrations. Their lives begin far to the west in the headwaters, high in the Andean mountains. The young larvae then drift with the currents eastwards, arriving perhaps a month later at the mouth of the Amazon River, close to the Atlantic on the opposite side of the continent. There they wait, hunting and growing, for at least three years. When the rains come and the river floods, the full-grown catfish gather in great shoals and set off for the west, swimming upstream through white waters and back to the mountains. Genetic studies also hint that, like salmon, Dourada could make their way to spawn in the very same rivers where they were born.
The return trip across Amazonia, between the mountains and estuary, is close to 12,000km (7,500 miles), as far as New York to London and back again, or John O’Groats to Cape Town. This is the longest swim of any animal through freshwaters alone. Why the catfish go to all that effort and swim so far is another fish mystery that, for now, remains unsolved.
Big fish, past and present
Diving every day with Humphead Wrasse at Swallow Reef in the South China Sea, I did my best to pretend I wasn’t there, which was difficult when there was nothing to hide behind. At first the spawning fish had been wary of me and rushed away before I could get good pictures of their faces. I feigned disinterest, turning my camera away so the domed glass lens wouldn’t convince them they were being eyed by a giant predator. Gradually they grew less camera-shy, and I worked out where to position myself. Hovering at the edge of the male’s territory, females would pass me after they spawned as they headed back home. Best of all was when they swam head on, then turned at the last minute to avoid bumping into me, providing a perfect shot of one cheek, left or right.
Between dives I had a lot of spare time on my hands. I wrote long letters home and persuaded pilots of the dive resort’s light aircraft to post them for me back on the mainland. I thought a lot about my best friend who was off on his own adventures, in the dry forests of Madagascar, and had no clue that less than a year later we’d be planning our wedding. Out on the island I didn’t look back at the digital pictures I was shooting underwater. The electricity supply was patchy and I needed all the power I could get for recharging camera batteries rather than running my laptop. So it wasn’t until I was far from the ocean, back in the lab in England, that I began to gaze at hundreds of fish faces.
I sifted through the images, tracing shapes, often getting lost along the meandering facial patterns as I tried to decipher the details of the spawning shoal. Eventually I picked out two pictures, shot on two different days, and saw the same face. Both showed the same three dark lines running back from her eye, the same white speckles on her forehead and a maze of golden cheek-scribbles. This female wrasse had come back to spawn two days in a row. My fishy game of snap was going to work.
From my growing catalogue of fish portraits I discovered the same females regularly spawned day after day and I estimated there were roughly a hundred mature females on the reef. Given a chance, those fish would have continued to spawn at Swallow Reef for many years, flinging ever more eggs into the oceans. And in time they would have returned to that same site in a very different role.
Humphead Wrasse are one of many marine fish that undergo a spontaneous sex change. Many are born as females, then later in life, when they’re at least five years old, become males. Their ovaries shut down, sperm-producing testes stir into action and they grow a big bump on their heads. From then on, the fish’s task is not to add eggs to the mix but to find their place in the hierarchy of males, and do their best to one day take over the spawning territory. Until then, they’ll sneak in and mate with females when the main male’s back is turned.
A similar gender switch happens in other wrasse, in cichlids, parrotfish, groupers, gobies and bass. Some fish do things the other way round, starting as males and becoming females, including the Orange Clownfish, the famous fish otherwise known as Nemo. Had the Disney movie been biologically correct, when Nemo’s mother went missing, his father should have changed sex and taken on the role as dominant female among the anemone’s stinging tentacles (and Nemo himself wouldn’t have been living with his dad in the first place; after they’re born, young clownfish don’t stay at home but drift off to find another anemone).11
Some fish switch sex in both directions. Chalk Bass in the Caribbean have both male and female sex organs at the same time, and use both throughout life-long partnerships. Pairs of bass live together, often inside old conch shells, and swap gender roles, male to female then back again, up to 20 times a day.
But none of my female Humphead Wrasse transformed into males. After I left Swallow Reef things changed. I had hoped to go back but the research team didn’t re-form. Only last year, I saw this part of the South China Sea again, although this time from the air. On a flight from Singapore to Manila, the pilot pointed out that we were flying over one of the islands China has claimed as its own. ‘Today the air is clear and you can see the runway,’ he said. I looked down at an island very like the one I’d lived on for months, only with a flotilla of large, forbidding ships moored around it.
In the last few years, China has been aggressively expanding its presence into these long-disputed waters, pouring sand and concrete onto coral reefs to build artificial islands and military installations, reinforcing sweeping claims over most of the South China Sea. That’s despite competing claims among several other sovereign states whose shores are lapped by these waters, including Malaysia, Vietnam, Taiwan and the Philippines. The US is also deeply involved in this distant sea which is rapidly becoming a global security hotspot. It’s no great surprise, then, that amid the mounting geopolitical tension no one was going to miss a few fish.
I stopped in the Philippines for just long enough to catch a connecting flight and continued my journey eastwards another 400km (600 miles) to Palau, a cluster of forested islands out on their own in the western Pacific. There I met Lori and Pat Colin, founders of the Coral Reef Research Foundation, who’ve spent years studying Palau’s reefs, including a substantial population of Humphead Wrasse. Over dinner, they confirmed a rumour I’d heard. Lori and Pat had been to Swallow Reef a year after I finished my PhD, and they didn’t see a single humphead.
The atoll was never officially a marine reserve, although the Malaysian military had acted as de facto protectors of the reef, ensuring no other vessels came close. It was out of bounds except to a few divers and occasional researchers. Lori told me the story she’d heard, that fishermen had been allowed onto the reef. It was one of the fleets that roam Southeast Asia, hunting for the last of these valuable fish, and in Swallow Reef they had taken all the Humphead Wrasse they could find. Had the fishers gathered them from the spawning site? Perhaps. The humphead’s former abundance on the atoll had been common knowledge, long before our expedition and my PhD. Still, I couldn’t help wondering if our attention and presence had fuelled interest in those fish.
My efforts to study humpheads suddenly felt hollow. While I’d been focusing in on the minutiae of their lives, bigger forces were at work. All those fish I’d learned to recognise from their facial patterns had been doomed from the start because of the price tag on their heads. I had witnessed and documented a phenomenon that may never happen again, not there anyway.
Swallow Reef wasn’t the last place I saw Humphead Wrasse. Diving in Palau I saw them almost every time I went in the water. I saw huge males cruising around and usually two or three large females on most reefs. I saw hand-sized teenagers and even thumb-sized juveniles flitting through shallow lagoons. And at the end of one dive, as I was waiting out my safety stop at five metres (16ft), I looked across and saw a pair of humpheads just below the surface moving in a familiar way. They shivered their bodies together and released into the water a milky cloud, exactly what these big fish should be doing.
As well as humpheads, I spotted several hundred other fish species in Palau (out of roughly 1,400 known to live there). There were big, old fish with wrinkles on their skin who’ve roamed these reefs for decades, and on every dive I saw sharks. It was inspiring to witness this spot in the ocean that’s being so carefully looked after. Almost half the waters close to shore are safeguarded inside a network of reserves, based on a local, centuries-old custom of halting fishing in certain areas to let stocks replenish. Offshore, away from the islands, 80 per cent of Palau’s maritime territory was set aside in 2015 as an ocean sanctuary. Industrial fishing fleets, mainly hunting tuna, are no longer allowed and Palauan fishers are the only ones who now dip into the remaining 20 per cent. The result is that a riot of fish is still here, living long lives in huge, healthy populations, and a lot them assemble in colossal shoals to mate. Studies in Palau are revealing much that wasn’t previously known about the ways these fish come together, when there’s enough of them to form spectacular spawning shoals.
In the office of the Coral Reef Research Foundation, Pat Colin shares with me stories of his life spent watching fish and deciphering their intricate mating rituals. With his snow-white beard and a glint in his eye he’s clearly, after all these years, still devoted to the underwater world. He browses through hundreds of computer files and picks out videos to show me of shoaling, mating fish.
Next Pat shows me a video shot at Blue Corner, one of Palau’s most famous and impressive dive sites. In the water are hundreds of Moorish Idols swimming synchronously just above the reef, all turning together with flicks of their trailing, ribbon fins. It makes me wonder what Konrad Lorenz would have made of them, following his studies of just a dozen of these fish in his Viennese aquarium tank. ‘There are a lot of good naturalists who send me videos and pictures,’ Pat tells me. More divers than ever are in the water in Palau. Dedicated dive tours have begun taking visitors to the right spots at the right times to watch the spawning pageants play out before them. ‘Things like this get noticed,’ Pat says, even though Moorish Idols only form shoals for a few days every year.
‘Do you want to see something really cool?’ Pat asks me. Of course I do. He clicks another video file, this one labelled Lutjanus fulvus, the Blacktail Snapper. ‘The fish are really incredibly shy,’ he says. ‘You can’t get close to them.’ And he doesn’t try to. Instead, he fixes GoPro cameras to the reef and programmes them to take one shot every minute for a week at a time. These tiny, tough, waterproof cameras were developed for extreme sports fans; the internet is full of videos shot by surfers, skydivers and skiers with cameras strapped to themselves. But they’ve also been adopted as scientific spy cameras. In this way, Pat has eavesdropped on the mating shoals of Humphead Wrasse and another local giant, the Bumphead Parrotfish, which come together in their hundreds to spawn.
On the computer screen I see a panoramic view across a coral-rich reef from Pat’s three cameras. He hits play and we watch as one or two fish are caught on camera, swimming by. Then, quite suddenly, first one then all three frames are filled with yellow stripes and dark tails. Snappers block the reef and each other from view until it’s impossible to count them. Now and then a big eye gazes close into the lens, then disappears again in the next shot. The time-lapse images scroll through and for a few moments, the equivalent of an hour in real time, fish cover the screen. ‘Then they’re gone,’ says Pat. And as fast as they appeared, the spawning snappers disperse and the reef goes back to the way it was.
Ancient Egypt, 2,400 years BP
Stories carved inside Ancient Egyptian pyramids tell of a great and wonderful king called Osiris who ruled over Egypt with his wife Isis. Everyone adored them both except for Seth, Osiris’s evil, jealous brother, who plotted to bring him down. One year, at the feast for Osiris’s birthday, Seth brought in a trunk inlaid with gold and jewels. ‘Whoever fits inside this chest,’ Seth announced, ‘will show themselves to be the most true and loyal person.’ The courtiers and partygoers all took their turn to climb into the chest, but they were either too big or too small. Osiris himself stepped forwards and showed that he was a perfect fit. As soon as Osiris was inside the trunk, Seth’s henchmen slammed the lid shut and nailed it down. Then they took the trunk and threw it in the River Nile, and Osiris drowned.
Lamenting her lost husband, Isis went to the river and searched for his body, finding that it was already breaking apart. With her magic she restored Osiris and later bore him a son, Horus, who became the god of the sky.
When Seth heard about what Isis had done, he feared Osiris would return to seek vengeance. So he went to the river and found the remains of his brother’s body, then cut them into fourteen pieces and scattered them across Egypt, to make sure Osiris would never return. Once again enraged by Seth, Isis travelled across the land to gather up all the pieces of her husband, but she only found thirteen. The fourteenth piece was his penis. She couldn’t find it because an elephantfish, known as Oxyrhynchus or Medjed, had eaten it.
From then on people worshipped the fish that had eaten such an important part of their great king. They believed the fish was a divine manifestation of Osiris, who also became the god of the afterlife, of death and life and resurrection. In temples dedicated to Osiris people left offerings of bronze fish figurines and mummified fish.12
Notes
1 Anglerfish, frogfish and handfish are all members of the same order, the Lophiiformes.
2 Lorenz was renowned for his studies on animals that bond with the first thing they see when they’re born, a phenomenon known as imprinting.
3 In 1950, mathematician Alan Turing developed a way of testing artificial intelligence based on whether a person can tell if another human or a computer is answering their questions, via a screen.
4 We know all this thanks to a new sonar setup called Ocean Acoustic Waveguide Remote Sensing, or OAWRS, which surveys a patch of sea 100km (62 miles) across every 75 seconds in three dimensions.
5 There are signs that Nassau Grouper spawning aggregations are very slowly recovering in the Caribbean, following their protection. In the US Virgin Islands, the depleted Nassau Groupers may be following the more abundant Yellowfin Groupers to their spawning sites.
6 Another common name for them is Napoleon Wrasse, and I’ve still not found out why.
7 The deep-diving world record is currently held by a Cuvier’s Beaked Whale, which reached 2,992m (9,816ft or 1.9 miles).
8 Another electronic tracking technique involves smaller, cheaper devices, compared to satellite tags, which send out radio signals that are detected by receivers fixed in areas of interest, e.g. along a river.
9 Other animals with a magneto-sense include sea turtles, nematode worms, spiny lobsters, homing pigeons, ants and honey bees.
10 Named after the Italian scientist Stefano Lorenzini, who first found them in 1678.
11 But maybe I’m just spoiling things.
12 Tilapia, another fish from the Nile, also came to be associated with Osiris. People saw tilapia spitting out hundreds of tiny fish from their mouths and believed they possessed great regenerative powers; so they became a symbol of rebirth and renewal. Women and children would wear fish pendants as protective amulets. And tilapia, people said, led the boat of the sun god, Ra, on his journey through the Egyptian underworld and back into the sky, where each morning he was reborn.
