Fish Behavior

By Bob Lusk
Pond Boss Magazine

Professional anglers work diligently to locate where the fish are. That’s their primary task. Next, they try to determine what the fish might be inclined to eat at that specific time. Afterward, they attempt to entice enough fish to take their bait and catch a full limit, ideally including at least one large fish, before heading to the weigh-in station to compete for the prize money.

Sounds pretty simple, doesn’t it?

It’s not.

These professionals, nearly universally, possess an intuitive understanding of fish behavior, allowing them to almost "sense" it. They then need to determine where the fish might be at any given time so they can leverage that intuition to provoke a strike from enough quality fish, giving them a chance to win.

Fish behavior is such a simple concept wrapped in such complicated trappings.

Here’s the simple part.

One male bluegill defending its nest against an intruder

Fish lack the brain structures required for "thinking." They can't reason or make choices based on cognitive thought. Instead, they react instinctively and through conditioning. Their instincts revolve around several fundamental drives: breeding, feeding, defense, and offense, varying by species. Their behavior is guided by these instincts and the life experiences imprinted in their small brains.

Much of their behavior is based on conditioning, similar to Pavlov’s dogs, especially as they age. If a fish swims through an area and finds food, it is likely to return. Conversely, if a fish is attacked in a specific spot and it happens repeatedly, it is likely to avoid that area in the future.

Here’s where it gets complicated.

A fish's senses work in unison, allowing its instincts and conditioning to govern its behavior at any given moment.

Here’s how it works: Each fish species has multiple senses—vision, smell, touch, hearing, and lateral lines—that vary in sensitivity. These senses send stimuli to the brain, which instinctively influences the fish’s behavior.

For instance, when a bass is hungry, its stomach signals the brain to eat, putting the fish on high alert and heightening its sensory systems. The bass uses its sharp eyesight to search for food. While its poor sense of smell is not very helpful, its keen vision and lateral line combine to detect and target something tasty and nutritious.

Bass have lateral lines to detect movement

The lateral line, the long tube visible along the middle of a fish's body on both sides just beneath the skin, detects pressure changes in the water and sends impulses to the brain. Through repetitive conditioning, an older bass can instinctively pursue prey based on the type of pressure change it senses through its lateral line.

For instance, if a school of shad is moving thirty feet away, the bass' lateral line picks up the movement. The brain quickly interprets this movement. The lateral line's function is to detect movement through impulses and condition a reaction over time. A bass' nature is to investigate any water movement. When the bass investigates, its eyes become the primary sensory mechanism. Even though the bass couldn't initially see the school of shad, it moves toward the detected movement. Once it gets close enough to see the shad, it instinctively attacks and feeds, inhaling as many shad as possible.

What about those shad? They move in a school as a defense mechanism. By moving in unison, a school of shad can be perceived by potential predators from a distance as one large fish. Ideally, this movement will cause predators to instinctively interpret the school as a single, large fish and decide to leave it alone.

These bass were ‘trained’ to eat fish food, not their natural instincts

Here's where it gets really interesting. When a big bass moves in to investigate and sees all those little fish, it makes a burst to feed. The shad then respond instinctively. Next time you see a shad, take a close look. Gizzard shad, threadfins, and most other shad species have a black dot toward the back of their bodies. Scientists believe this black dot is also a defense mechanism.

Here’s why: When a predator fish feeds, it strikes at the eyes of its prey. Chasing a shad, the bass has a 50-50 chance of striking at the black dot, which resembles an eye. If the predator strikes toward the back of the fish, the shad has a better chance of escaping. Additionally, when a bass bursts into a school of shad, the school scatters in all directions, trying to flee. The bass, using its vision, is confused by all the movement and can’t decide which shad to chase. Instinctively, the bass chases what it sees, and the school of shad flees. Both species rely on their instincts to survive. The bass may get a meal, but the school of shad largely escapes, losing only a few members.

Channel catfish behave quite differently. They tend to be cautious, except for larger fish when they are actively feeding. A channel catfish's nature is to be wary. When something hits the water, channel catfish flee, unlike bass that instinctively investigate. However, the channel catfish will return after a few minutes, especially if the object that hit the water remains still on the bottom. An old river fisherman taught me this years ago.

Catfish conditioned to eat fish food

He said, "When you cast your bait, let it fall to the bottom and leave it there. After a few minutes, a catfish will come and check it out." He was right. Not only will they check it out, but his next piece of advice was also spot on: "Don't set the hook on a catfish immediately. Wait until it runs with the bait."

Here's why that’s good advice: Channel catfish have taste buds on their whiskers. They "bump" their food to taste it before eating it. If it tastes good, they eat it. As for the larger channel catfish, the bigger they grow, the more predatory they become. Channel catfish are commonly caught on artificial lures when they are actively feeding, but their nature is to be cautious and taste their food before eating it.

Redear sunfish behave differently than other sunfish

What about sunfish, like bluegills? fish rely heavily on their senses to navigate their surroundings. While most sunfish prefer to swim in groups, they are also carnivorous predators constrained by the size of their mouths. Their movements are largely instinctive responses to stimuli. For instance, when an insect lands on the water's surface, bluegills immediately pursue it. They use their keen eyesight to locate the prey, driven by the collective behavior of their peers. Despite not always knowing the exact nature of their prey, they join the pursuit because their companions are doing the same. If the prey proves unpalatable or unpleasant in their mouths, they quickly eject it, allowing another eager fish to attempt a capture.

Bluegills make beds and guard them during spawn

Studying fish behavior requires dedication, observation, and time. One of the most intriguing aspects is observing fish of various sizes and species, noting their behaviors and interactions within a community. These dynamics offer abundant enjoyment and are well worth your time, especially while relaxing by the pond with your favorite drink. Observe how the fish react when you toss them some food pellets. Witness their spawning rituals in the spring and the excitement of bass chasing shad in the fall. Seeing these events firsthand fosters belief, and belief fosters understanding. Understanding, in turn, enables us to become better custodians of our cherished aquatic environments. This deeper insight can enhance our understanding of the natural world around us and inform how we navigate our own lives.

Bob Lusk, the “Pond Boss” is in his 40th year as a private fisheries biologist, traveling the nation designing, building, stocking, and managing premier fishing lakes. He’s written hundreds of articles and three books on the topic. He can be reached at boblusk@outlook.com to help you make your dream pond come true.