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  • Published: 3 April 2024
  • ISBN: 9780143776956
  • Imprint: Hamish Hamilton
  • Format: Trade Paperback
  • Pages: 464
  • RRP: $36.99

Deep Water

The world in the ocean


In early 2020 while on a field trip in the Cocos Islands, I took some time out and went snorkelling. I was in a shallow channel between two islets and the tide was running, so my only real option was to let the current carry me, which it did, and quickly, sending me shooting over an expanse of broken coral and sand. Although I had seen fish earlier in the day, there weren’t that many about in the water I was moving through, but after a few minutes a trevally came angling in towards me. It was a striking creature with a silvery, streamlined body perhaps 80 centimetres long and a vivid blue stripe that ran down its spine and back along its midline. It swam towards me, moving fast and without fear, until at the last moment it shot off to one side and swooped behind me. I turned, watching as it turned back to circle me again, and then again, occasionally swinging in close, then out once more.

It kept up this process for the next ten minutes or so, shadowing my path as the tide carried me through the channel and into the lagoon beyond. At first I was concerned it might be thinking of attacking me – certainly there was an edge of aggression to its constant motion – but what most struck me about its behaviour was its air of purpose, the sense I was being observed and assessed. There was no question that this was a creature with its own intentions and agenda, or that when I met its gaze it was looking back.

Anybody who spends time in the water will have had similar encounters. My brother sometimes feeds the blue groupers that live in Sydney’s rocky coves, cracking open sea urchins and holding the meat with his teeth, so the groupers must come close as if they are kissing him, their movements flighty, yet purposeful. I’m not sure I want to get that close to a grouper’s mouth, but I have handfed huge stingrays, delighting in their puppylike playfulness as their strange bottom-facing mouths suck and pull at my fingers like an affectionate sea squirt. Although their behaviour is quite different to that of the trevally, it is no less obvious that they are thinking and acting in the same way as the terrestrial animals most of us are more familiar with.

Despite such encounters, fish are often dismissed as little more than slimy automatons and – at least outside of Indigenous cultures – almost never considered as subjects of moral concern. When most humans do think of them it is as food or as pets of a purely ornamental kind. Pescatarians who would regard slaughtering a cow or a pig or a chicken as unutterably cruel and unethical happily consume fish as if they are little different to vegetables. Even our language often erases their particularity. In English we speak of one fish or many fish, as if they are so interchangeable it is not worth according them the dignity of a plural form. Likewise scientists and economists talk blithely of fish stocks, transforming the lives of millions of individual beings into a resource, little different to metal or timber.

These assumptions elide a world of astonishing complexity. Not only have fish inhabited our planet far longer than air-breathing animals such as ourselves, having first evolved over half a billion years ago, but they are also extraordinarily diverse. The more than 34,000 species of fish known to science make up 60 per cent of all vertebrate species, or more than mammals, birds and reptiles combined. Fish range in size from the minute Paedocypris progenetica, which is found in peat swamps and blackwater streams of Sumatra and Bintan and is less than 8 millimetres in length, to the immense whale shark, Rhincodon typus, which grows to almost 13 metres in length and can weigh well over 20 tonnes. I once swam with one whose tail was taller than I am; I have never forgotten the casual way it shot away with a leisurely flick of that tail.

Fish are also found in almost every watery environment on Earth. They make their homes below the ice of the polar oceans and the blood-warm rivers and seas of the tropics, on the mudflats and intertidal zones of mangroves and more than 8 kilometres below the surface in the darkness and bone-liquefying pressure of the ocean trenches. Some species lay eggs; others bear live young. More than 400 species are known to be hermaphroditic, and capable of changing sex. Eviota sigillata, a tiny reef fish found in the Indo-Pacific, lives a mere eight weeks, the shortest lifespan of any vertebrate. By contrast analysis of radioactive deposits in the eyes of the mysterious Greenland shark – which lives in the icy darkness of the North Atlantic and Arctic Sea and feeds on fish and the carcasses of seals, whales and even polar bears – has revealed the species lives for hundreds of years. One specimen was shown to be between 272 and 512 years of age, making it the longest-lived vertebrate in existence. The species lives so long that females are not believed to reach sexual maturity until they are a staggering 150 years old.

And this remarkable diversity is only the beginning. A growing body of evidence makes it clear that fish not only think and feel, but exhibit complex social behaviours and sophisticated cognitive abilities, are capable of learning, problem solving and tool use, and possess culture and even the sort of self-awareness previously assumed to be restricted to primates, dolphins, elephants and a few species of birds. These discoveries do not just demand a rethink of assumptions about the cognitive capacities of our finned cousins, they challenge ideas about how to identify and measure intelligence in species so different from ourselves.


ONE OF THE LEADING FIGURES in this emerging space is Culum Brown, professor in Biological Sciences and Vertebrate Evolution at Macquarie University in Sydney. Brown’s early research was on rainbowfish, small river fish native to Australia, New Guinea and Indonesia. A popular aquarium species, rainbowfish possess the curious mixture of nervousness and glassy regard that tends to lead humans to dismiss the idea fish might be intelligent. Yet Brown’s work reveals they recognise each other, have complex social lives and hierarchies, and are capable not just of learning to avoid dangers such as predators and traps, but of passing these techniques on to other rainbowfish. Furthermore, these abilities are not rudimentary: experiments show rainbowfish learn to associate signals with food three times as fast as rats and twice as fast as dogs.

Brown’s work with rainbowfish demonstrates that contrary to the old joke about goldfish remembering nothing for longer than thirty seconds, many fish have excellent long-term memories. When tested again almost a year later, the rainbowfish immediately recalled the required responses and responded as if no time had passed. Similar results have been observed in many other species of fish: tilapia taught to associate a signal with netting, for instance, remembered the signal and responded accordingly seventy-five days later, while gobies, which form highly detailed mental maps of the tidal pools in which they live, can still remember the location of neighbouring pools for at least forty days after being removed from their own pools.

Even more importantly, Brown’s research makes it clear that not only are fish capable of remembering, they are capable of learning from each other through observation and interaction, meaning behaviours can be passed between individuals and, even more significantly, between generations. This capacity to acquire and transmit stable behaviours is the foundation of culture.

At one level these discoveries should not come as a surprise. As Brown points out, ‘We’ve known about social learning and cultural transmission in animals for sixty years. Everybody looked for it in chimps first because they’re so much like us. But since then the search has cascaded through nearly all the animal taxa, to the point where I think it would be fair to say social learning and cultural traditions are present in almost all animals.’


Much of this social learning relates to foraging and food acquisition. Archerfish, for instance, develop their ability to shoot insects from the air by firing water from their mouths by watching other fish, while sea bass can learn to press a lever to obtain food by observing other individuals doing the same. But fish are also capable of learning more complex information about migratory routes and where to find food from each other. Guppies pass on information about where to go to find food to other fish as they join the group.

Evidence of this sort of cultural transmission in fish is widespread, especially in long-lived species. French grunts, a yellow fish native to the West Atlantic, spend their days sheltering from predators among the spines of sea urchins; when they emerge at sunset they follow paths to their feeding grounds that are learned from older fish. Similarly on reefs off Panama young female bluehead wrasse learn the route to the best breeding spots from older females. As with all forms of cultural transmission this process is also highly vulnerable to disruption, with studies showing cultural traditions are quickly lost if the individuals that possess them are removed from the group.

As their various vocalisations suggest, fish also possess complex social lives. Although humans often struggle to tell them apart, fish have no trouble recognising and remembering each other. And, like other social animals, they often form attachments, preferring the company of shoalmates and fish they are familiar with over other members of their species. Brown and his team recently discovered that Port Jackson sharks – a gentle, bottom-dwelling species of bullhead shark whose heavy head and elegant brown markings will be familiar to many in southern Australia – have well-established social networks and prefer the company of individuals of the same age and sex. In other words they socialise with their peers, in much the way humans do (the same study revealed the sharks also perform remarkable feats of navigation, migrating thousands of kilometres from their homes on Australia’s east coast to Bass Strait before returning to the same reef they left from).

These abilities are not restricted to recognition and attachment. Fish often employ cooperative behaviours, especially when hunting: yellowtail amberjacks off the Californian coast have been observed using U-shaped formations to separate out and trap smaller fish, behaviour that closely resembles the sophisticated hunting techniques of mammals such as wolves and dolphins. Some species of cichlids – a diverse family of fish native to lakes and rivers in Africa and the southern Americas – also rely upon cooperative techniques to protect and raise young. Conversely fish are also quite capable of excluding and rejecting individuals that do not behave appropriately. When approaching potential predators, sticklebacks use a distinctive stop-start swimming motion to share risk by taking turns at the front. If an individual is reluctant to take the lead, or cheats by hanging back, its shoalmates will refuse to cooperate with it in future, meaning the sticklebacks recall the identity of malingering individuals and remember they are not to be relied upon.

It seems likely these behaviours are only isolated examples of a much more widespread phenomenon – as Brown observes, the practical obstacles to detailed observation of the behaviour of fish means our perspective on their lives is extremely limited. Yet there is no question that as in other species this social complexity requires high levels of cognition, and suggests the tendency to regard fish as primitive, emotionless automata is fundamentally mistaken, and that fish inhabit social worlds at least as complex as those of many mammals and birds.

No less tantalisingly, there is evidence a number of fish species are capable of using tools. At one level this is surprising – unlike mammals and birds, fish lack grasping appendages, and the aqueous environment makes manipulating and striking difficult. But wrasse and tuskfish have been observed using rocks as anvils to break open shellfish, and it is tempting to regard the archerfish’s use of water to shoot prey as a form of tool use. Fish are also capable of developing innovative ways of manipulating their environment, as in the case of the group of Atlantic cod in an aquaculture facility in Norway that took to stealing food from a feeding device after they discovered tags attached to their bodies could be used to activate it.

Some scientists have dismissed the idea such behaviours should be regarded as tool use because they do not involve the use of one object to manipulate another. Yet Brown believes these objections are based on a deliberately restrictive notion of what constitutes a tool. ‘What’s interesting is that when Jane Goodall and others came up with a definition of tools it had nothing to do with appendages: it was all about using an object to achieve a goal. But then the whole concept got hijacked by the primatologists, who substituted a primate-centric definition.’ Instead Brown argues we should return to Goodall’s original definition, and its emphasis upon the intention of the animal to achieve a goal it could not otherwise realise.

This emphasis upon intentionality has the advantage of excluding many examples of seemingly innate behaviours resembling tool use that have been observed among insects. More intriguingly, however, it allows behaviours such as nest-building to be a form of tool use, permitting a broader approach to our understanding of the evolutionary origins of tool use. ‘There’s clearly a lot of overlap between building nests and using tools,’ says Brown. ‘Both are clearly about manipulating the environment in a way that enhances your fitness, so you’re either more reproductively successful, you’re getting more food, or you’re safer from predators and other environmental stresses.’

This is particularly relevant to fish, of which more than 9000 species build nests. Sometimes these structures are relatively simple: certain species of wrasse, for instance, create mucous cocoons in which to shelter while they sleep, and eels and other species produce bubble nests to protect their eggs. But many also construct surprisingly complex structures out of stones and coral, often on a daily basis. ‘It isn’t simple or boring behaviour. It can be seriously sophisticated,’ says Brown. ‘If you’re building an igloo out of coral rubble it has to be structurally complex enough that it won’t fall down around your ears.’

Nests also provoke fascinating questions about the inner lives of the creatures that construct them. As Helen Macdonald has observed in the context of birds, nests may be part of the phenotype of the animals that make them, yet they are also highly responsive to local conditions, suggesting the animals that build them are capable of innovation and adaptation. And, more deeply, they unsettle our assumptions about the difference between skill and instinct. When we consider the elegant concentric rings, rays and doodles pufferfish create in the sand it is difficult not to wonder, as Macdonald does of birds, whether the animals making these structures are merely following a sequence of cues hard-wired in their biology, or whether, like us, they begin with some kind of mental image of the nest or symbols they create, which they then bring into being, step by step, refining as they go. As human beings we are familiar with the interplay between plan and form, that sense that altering one detail or rearranging an element will result in a greater sense of unity or balance: is it possible fish feel something similar? And what might it mean for our sense of who and what they are if they do?

Deep Water James Bradley

Through history, science, nature writing, and environmentalism, Deep Water invites you to explore the deepest recesses of our natural world.

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