 | What a Fish Knows: The Inner Lives of Our Underwater Cousins presents fascinating scientific research and entertaining anecdotes to explore the personalities and umwelt of fish. Our oceans and waterways are the largest habitat on earth (over 70% of the planet) and most animals live in water—yet until recently, these aquatic citizens were little studied or understood. The author provides ample evidence that “fishes are individuals with minds and memories, able to plan [and] to learn from experience. [They] have social lives. They swim together; they recognize other individuals by sight, smell, voice … and they cooperate.” |
Fishes have an acute sense of smell. Migrating fishes use their sense of smell to detect home waters. They also memorize currents and turbulence patterns, and use the Earth’s magnetic field to find home.
 | “Despite the misconception that fishes are silent, they actually have more ways of producing sounds than any other group of vertebrate animals. … Fishes can rapidly contract a pair of vocal muscles to vibrate their swim bladders… [other options to create sound include] grating their teeth in their jaws, grinding additional teeth lining their throat, rubbing bones together, stridulating their gill covers, and …expelling bubbles from their anuses.” |
Fishes use touch for pleasure in courting rituals, and cleaner fishes “curry favor with their valued clients by caressing them with their fins [to] strengthen the cleaner-client relationship.”
 | Cleaner-client symbiosis of fishes “is one of the most complex and sophisticated social systems of any animals . . . One or two cleaner fishes signal that they are open for business. . . . Other fishes of various types congregate at the cleaning station, where they wait their turn to be serviced by the cleaners. [Cleaners] pick over their client’s bodies, removing [and eating] parasites, dead skin, algae, and other undesirable blemishes. Clients benefit by receiving a spa treatment, including parasite removal. Cleaners get fed.” |
“With dozens of clients per cleaner, cleaner fishes maintain an impressive mental database of clientele [and] the system encompasses long-term relationships built on trust [and] reputation . . . These social dynamics support a degree of awareness and social sophistication quite out of keeping with our cultural impression of fishes.”
Fishes are also capable social learners, readily learning by watching others.
Some fish use tools. An orange dotted turkfish was observed digging up clams and smashing them against a large rock (with its mouth) to crack the shells and eat the contents.
Some fish use tools. An orange dotted turkfish was observed digging up clams and smashing them against a large rock (with its mouth) to crack the shells and eat the contents.
 | Tigerfish and catfish, among others, leap from the water to catch birds. This behavior “has all the hallmarks of flexible, cognitive behavior: it is opportunistic, since it is unusual behavior for the species; it requires practice to develop, and skill to execute; it is almost certainly transmitted through observational learning; and different methods are used [ambush from behind or underneath].” |
 | Some fish are romance artists. Male pufferfish create elaborate mandala-like nests to attract females, decorating with “bits of small shells he cracks in his mouth before sprinkling them into the central grooves.” Another elaborate nest is the mating bower of the male stickleback. “They produce a sticky, mucus-like substance in their kidneys [to] bind together pieces of leaves, grass, and algae filaments to his nest.” |
“About one in four of all fish species—some 8,000 species—devote at least some form of caregiving to their offspring. The effort ranges from protecting eggs to looking after the young through their most vulnerable first few weeks of life.
Many fishes, including sharks, are viviparous, that is, giving birth to live young.”
Many fishes, including sharks, are viviparous, that is, giving birth to live young.”
 | The discus fish feeds its young with a special mucus “enriched with antimicrobial substances that help protect babies from infections.” Other fish carry their eggs or young in their mouths (called mouthbrooding), and male seahorses and pipefish brood the next generation in their abdomens. “The female releases her eggs into the male’s abdominal pouch, where he fertilizes them and carries them until they hatch. “Birthing” involves contractions and contortions to expel the young from the pouch.” |
Aquatic life is wonderfully diverse.
“More than one-quarter of all fishes on a reef can transition from male to female or vice versa . . . Sex-changing fishes behave appropriately, performing male-typical or female-typical behaviors according to their current gender assignment.” Other fish are unisex (both male and female), some mate daily (blue-headed wrasses), and Amazon mollies are an all-female species, reproducing by parthenogenesis.
“More than one-quarter of all fishes on a reef can transition from male to female or vice versa . . . Sex-changing fishes behave appropriately, performing male-typical or female-typical behaviors according to their current gender assignment.” Other fish are unisex (both male and female), some mate daily (blue-headed wrasses), and Amazon mollies are an all-female species, reproducing by parthenogenesis.
 | Deep sea anglerfish might be the most unique. Female anglerfish use a dangling lure filled with luminescent bacteria to attract prey. “Some lures resemble a worm, which the anglerfish causes to squirm appealingly by twitching muscles at the base of the stalk.” Tiny male anglerfish latch onto females and eventually become fused, “living off her blood supply and fertilizing her intravenously. One female can end up with three or more males sprouting from her body like vestigial limbs.” |
Cooperative hunting, requiring planning and teamwork, occurs in several fish species. For example, “shoals of barracuda will swim in a tight spiral, herding prey into shallow waters for easier hunting.”
Yellow-saddle goatfishes take on different roles. “Chasers flush the prey out of their hiding crevices, and blockers prevent their escape.” Cross-species cooperation has been observed with coral groupers recruiting giant moray eels for team hunting.
Yellow-saddle goatfishes take on different roles. “Chasers flush the prey out of their hiding crevices, and blockers prevent their escape.” Cross-species cooperation has been observed with coral groupers recruiting giant moray eels for team hunting.
 | But schools of prey fish have their own survival strategies. “Fleeing schools will [split] into two clusters that quickly swim around each side of a predator fish and reassemble behind it. If the predator turns around, the maneuver is repeated. [Or] all the fishes in a school dart away from the center as the predator attacks. The spray of fishes may cover ten to twenty body-lengths in just one-fiftieth of a second. " |
Unfortunately for fishes, the anti-predator benefit of schooling backfires with human predation: "equipment has been developed to detect and then reap almost all fishes in a single school."
The final chapter of the book covers human devastation of our aquatic cousins. The number of fish killed each year by humans “is between 1 and 2.7 trillion. . . . if we lined them up end to end, they would stretch to the sun and back—a round trip of 186 million miles—with a couple hundred billion fishes to spare.”
 | “Unwanted fishes and other animals caught incidentally in the pursuit of targeted species are referred to as bycatch. In commercial fishing, bycatch includes all seven kinds of sea turtles; dozens of seabirds . . . practically every species of dolphin and whale; countless invertebrates; living corals; and of course, a huge range of fish species. . . . Try to visualize a pile of marine creatures, weighing 200 million pounds, most of them dead, almost all doomed to die. That’s the daily bycatch we reap from the seas.” |
Also troubling: “Fishing fleets abandon or lose many untold miles of synthetic fiber drift nets and bottom-set fill nets every year—about 640,000 tons of derelict gear in total [which] continue entangling animals. The primary victims—dolphins, seals, sea birds, and sea turtles—become bait for other marine life, some of whom also become ensnared, until finally, by the sheer weight of all the dead bodies, the nets sink to the bottom of the sea.”
 | Farmed fish fare no better. On trout farms, “densities as high as twenty-seven foot-long fishes per bathtub volume of water.” And paradoxically, “the production of factory-farmed fish does not relieve the pressure on wild fish populations. This is because the primary food fed to farmed fish [is] fish.” One marine activist said: “When I see a salmon farm, I see slavery and debasement of the spirit of the fish that West Coast First nations viewed as the buffalo of the sea.” |
It is ultimately human consumers who drive the fishing industry:
“When we eat fishes, we fund their capture.”
“When we eat fishes, we fund their capture.”
Recreational fishing causes damage to delicate facial tissues and abrasions to the protective layer of mucus surrounding the scales.
 | “We hear no screams and see no tears when their mouths are impaled and their bodies pulled from the water. Their unblinking eyes—constantly bathed in water and thus in need of no lids—amplify the illusion that they feel nothing. [But] the accumulating evidence indicates a range of emotions in at least some fishes, including fear, stress, playfulness, joy, and curiosity.” |
Fishes “show the hallmarks of pain both physiologically and behaviorally. They have the “same specialized nerve fibers that mammals and birds use to detect noxious stimuli.” In experiments, they seek out pain relief. And, like human addicts, “fishes are prone to abusing amphetamine and cocaine, unable to resist them when they are freely available.”
What a Fish Knows demonstrates the many commonalities we humans share with fish and gives us a glimpse of their captivating lives.
The book ends with this sobering thought: “feeding fishes to humans is unsustainable even at current levels of consumption.” Something consider when making choices about what to eat.
The book ends with this sobering thought: “feeding fishes to humans is unsustainable even at current levels of consumption.” Something consider when making choices about what to eat.
Read more about our aquatic neighbors:
Sounds Wild and Broken, If Nietzsche Were a Narwhal, The Underworld,
An Immense World, The Salmon Cannon and the Levitating Frog
Read more about nature’s diversity:
Plant-Animal Hybrids, Lesbian Lizards, Aliens Among Us, Animal Emotions
Sounds Wild and Broken, If Nietzsche Were a Narwhal, The Underworld,
An Immense World, The Salmon Cannon and the Levitating Frog
Read more about nature’s diversity:
Plant-Animal Hybrids, Lesbian Lizards, Aliens Among Us, Animal Emotions
