The Brains Of Fish
Part 1. The Basics.
Despite modern assumptions, fish haven’t always been seen as simple. Across history and culture, they’ve often been treated with reverence, curiosity, even fear.
In early Christian tradition, the word fish wasn’t just symbolic, it was a cipher for something hidden. The Greek word Ichthys (Ἰχθύς) served as an acrostic for Iēsous Christos Theou Yios Sōtēr: “Jesus Christ, God’s Son, Savior.” Fish appear again and again in scripture and religious iconography: the miracle of the loaves and fishes, Jonah’s three days inside the “great fish,” baptismal waters teeming with unseen life. In these stories, fish often stand in for faith, representing the presence of something deeper, just beyond understanding.
You see echoes of this in other traditions too. In Japanese folklore, the catfish Namazu is said to live beneath the earth, thrashing violently and causing earthquakes when stirred. And while the metaphor is myth, the pattern isn’t fiction. In the 1920s, seismologists Shinkishi Hatai and Noboru Abe studied aquarium catfish and found that the animals became unusually restless hours before earthquakes, jumping or swimming erratically. In their study, the catfish correctly signaled about 80% of 178 seismic events.
That symbolic weight may have roots in something more grounded. Long before angling was a sport, it was survival. Fishing meant casting into unknown waters without maps, sonar, or models, just a net and the belief that something would return. Without any real understanding of migration patterns or feeding cycles, the behavior of fish must have seemed arbitrary, even divine. Whether by coincidence or insight, those old associations mirror something we’re now beginning to grasp: fish perceive the world in ways we don’t. Their sensory reality is full of signals we overlook, minute shifts in pressure, tiny chemical trails, electric fields, polarized light. They operate in a domain we barely register. That’s why, even with all our data, barometric pressure, lunar phases, thermal maps, it can still feel like fish are breaking every rule. The variables we can track may look favorable. But what about the ones we can’t?
The more we study fish, the clearer it becomes: their world isn’t smaller than ours, it’s just different. If you want to understand fish behavior, whether out of curiosity or to become a better angler, you have to start by letting go of human assumptions. It’ll change the way you move through the water, how you choose flies or lures, and where you focus your attention. To fish well is to understand not just where fish are, but how they interpret the world. You’re not just watching an automaton that lives underwater. You’re watching a living creature that lives in a parallel sensory universe.
Understanding Sensory Processing: A Quick Human Example
When we think about our senses, we usually picture the organs that detect them, our tongue for taste, nose for smell, ears for hearing. But those organs aren’t where sensation actually happens. They’re part of the peripheral nervous system (PNS), which collects information from the environment and relays it to the central nervous system (CNS), the brain and spinal cord. A good analogy is a radio. The antenna picks up the signal, but the receiver inside the radio is what decodes it into sound. Your PNS works like the antenna, gathering input, while your CNS acts like the receiver, making sense of it. It’s in the CNS that raw input becomes something recognizable: hot, cold, sweet, sharp. Your tongue might detect a chemical, but it’s your brain that decides it’s “spicy.”
Some signals, though, don’t go all the way to the brain before triggering a response. For example, if you touch a hot stove, the signal travels from your skin to your spinal cord, which immediately sends a command back to pull your hand away. This reflex arc happens so fast your brain doesn’t have time to get involved, you move before you feel the burn. The pain will register a moment later, but the initial reaction is fast and automatic. These kinds of responses are called reflexes, and they’re mediated by the PNS and spinal cord without requiring higher brain involvement. Spinal reflexes aren’t just a human trait. Dogs lifting their paw from a thorn, horses twitching their skin, or frogs kicking when touched, all of these are processed entirely in the spinal cord, no brain required.
What about fish?
At a general level, Fish have both a central and peripheral nervous system, and the flow of information between them works much like it does in humans. The PNS detects environmental cues, pressure changes, chemical signals, physical contact, and relays them to the CNS, where that input is processed and translated into action. While the fish brain is smaller and less complex than ours, it’s sharply specialized. Distinct regions are dedicated to smell, vision, taste, balance, and motor control, and they handle these tasks with remarkable efficiency.
Reflexes play an even more central role in fish behavior than they do in ours. For many species, survival depends on reacting instantly, darting from a shadow, striking at movement, or adjusting to changing currents, without the luxury of pause or deliberation. While humans often override reflexes with conscious thought, many fish rely on fast, automatic responses to make it through their environment.
That said, not all fish behavior is purely reflexive. Some species, especially social or highly adaptive ones like minnows, carp, and cichlids, have demonstrated the ability to assess risk, form memories, and even learn by observation. Reflexes may be the first line of defense, but in many cases, fish can modify or override them based on experience.
One of the clearest examples of this is the C-start reflex, a rapid escape response triggered when a fish senses sudden danger. You've seen it: a flash of the tail, a burst of motion, gone in an instant. This reflex isn’t driven by the forebrain, but by specialized neurons in the hindbrain, allowing the fish to turn and flee within milliseconds. It’s highly tuned and can be triggered by sudden sights, sounds, or pressure changes in the water.
So While humans often override reflexes with conscious thought, fish tend to rely more on fast, automatic responses, though some species are capable of learning when and how to override those responses. From finding food to avoiding predators to returning to natal waters to spawn, their behavior may be instinctive, but it’s anything but simple. Understanding how these systems work gives us a clearer picture of what fish are capable of, and how much we still don’t fully understand.
The brains of fish are complex, and certain aspects can vary by species, but they can largely be broken down into three main parts.
The brains of fish are complex, and certain aspects can vary by species, but they can largely be broken down into three main parts.
1. Olfactory Bulb
The olfactory bulb is the primary structure responsible for processing chemical cues in the water. The development of the olfactory bulb varies widely across species and is shaped by life history and habitat. Species that live in dark environments, such as caves, turbid rivers, or deep ocean zones, often have highly developed olfactory bulbs, relying on scent in the absence of vision. Anadromous fish provide another striking example. These species migrate from the ocean back to the exact freshwater location where they were born, often traveling thousands of miles guided by the chemical signature of their natal stream. Studies on coho salmon have shown that damage to this region can eliminate or severely disrupt their ability to home, even when other sensory systems such as vision remain intact.
2. Pituitary Gland
The pituitary gland plays a central role in regulating hormones tied to growth, reproduction, and seasonal transitions. Located just below the brain in a small cavity called the sella turcica, it operates in close communication with the hypothalamus. Experimental removal or disruption of the pituitary in goldfish results in the loss of gonadal development, unless supplemented with external hormones, highlighting its essential role in reproductive function. In 2018, ecologists in Arizona found a ten-inch-long tadpole with the girth of a soda can, an extreme case of disrupted metamorphosis likely tied to pituitary dysfunction. While not a fish, it illustrates how dramatically this gland can shape growth and development when hormonal signaling goes awry.
The pituitary glands structure and function are shaped by the ecological demands of each species' life history. In migratory fish, the pituitary helps coordinate smoltification, the physiological shift that allows juveniles to move from freshwater to saltwater. In seasonal spawners like trout and salmon, it regulates the timing of reproductive development, migration, and even shifts in coloration. The gland’s sensitivity to environmental cues, such as temperature, photoperiod, and water chemistry, enables fish to respond precisely to seasonal changes. The next time you hook into a fall-run brown trout with fire-orange flanks and a hooked jaw, that’s the pituitary at work. And when you accidentally snag a half-rotten zombie salmon on its last legs, that’s the pituitary too.
3. Medulla Oblongata
The medulla oblongata is a critical brain structure that regulates basic, life-sustaining functions such as heart rate, gill ventilation, digestion, and reflexive motor control. It sits at the base of the brainstem, forming the connection between the brain and spinal cord. Lesions in this region, documented in zebrafish and trout, can lead to loss of balance, erratic swimming, or respiratory arrest.
Although it doesn't handle complex decisions, the medulla processes input from the lateral line system, which detects vibrations and movement in the water. This is particularly important in fast-flowing rivers, low-visibility environments, and among species that rely on schooling or fine-tuned mechanosensory feedback. The medulla also governs many reflexes that enable fish to orient in the water column, maintain posture, and swim effectively. In short, it handles the behind-the-scenes work of keeping the fish alive, autonomously coordinating the essentials without conscious control.
Conclusion
We want answers. Something clean. A map, a seminar, a secret. The right fly, the right current seam, the right time of day. Despite the dozens of people who make their living claiming they understand fish, and that you can too if you pay them enough, there is no system that will let you think like a fish. Not unless you grow a lateral line and a swim bladder. Not unless your eyes split light differently.
Because fish don’t live in our world. They live in gradients. They read pressure the way we read signs. They see in polarized light. They smell in layers we’ll never parse. They feel vibrations you’d never notice. They’re not driven by logic. They’re tuned to a kind of perception we weren’t built to share. So when every condition appears to line up, right flow, right fly, perfect drift; and the water still feels empty, don’t blame the fish. They’re not breaking the rules. They’re just following rules you can’t see.
And that’s not something to resent. That’s something to respect. To chase.
Because every so often, just for a moment, all those layers align, and all things merge into one.
And that’s enough.
