The simple answer to whether fish float when they sleep is generally no. When fish enter a state of rest, they actively maintain neutral buoyancy or seek out the bottom or shelter to settle. The phenomenon of a fish floating uncontrollably is actually a sign of illness, injury, or death, not normal resting behavior. Understanding how fish manage their position in the water column while resting requires examining their unique physiological and behavioral adaptations.
Defining Rest in Fish
The concept of “sleep” in fish differs significantly from the deep, unconscious state experienced by mammals. Fish lack eyelids and a neocortex, the brain structure associated with complex human sleep patterns. Instead of true sleep, most fish enter a state of reduced metabolic activity, which is better described as a period of torpor or deep rest.
During this resting phase, a fish’s breathing and heart rate slow down to conserve energy. They become far less responsive to external stimuli, such as movement or light changes, which is the behavioral marker scientists use to identify this state. While they are inactive, they typically remain aware enough of their surroundings to react quickly to perceived danger. This period of reduced activity fulfills the same restorative functions as sleep.
Maintaining Position Without Floating
The primary mechanism preventing most bony fish from floating to the surface or sinking is the swim bladder, a gas-filled organ located beneath the spine. This organ functions as a buoyancy compensation device, allowing the fish to achieve neutral buoyancy at its desired depth. When the fish rests, the swim bladder is precisely regulated to match the fish’s overall density to the surrounding water.
For fish with a closed swim bladder, gas is secreted from the blood into the organ via a specialized gas gland to maintain volume and buoyancy. This active regulation ensures they can hover without the need for continuous swimming. While hovering, fish are not perfectly stable; their center of mass and center of buoyancy do not perfectly align, creating a tendency to tip or roll.
To counteract this instability, resting fish must make subtle, continuous adjustments using their pectoral and pelvic fins, even when appearing motionless. This constant effort to stabilize position, known as “hovering,” is not effortless. Many species choose to settle on the bottom or wedge themselves into a safe location to minimize this energetic cost while resting.
Variations in Resting Behavior
Not all fish rely on the swim bladder for buoyancy control. Cartilaginous fish, such as sharks and rays, lack a swim bladder, relying instead on a large, oily liver and dynamic lift to maintain depth. Without the ability to hover, many bottom-dwelling sharks, like nurse sharks, rest heavily on the ocean floor or inside caves.
Other species have evolved distinct methods for securing themselves during rest. For example, some species of wrasse and parrotfish secrete a cocoon of mucus around themselves before settling into crevices at night. Many bottom-dwelling bony fish, such as flounders and catfish, have either reduced or completely lost their swim bladders, spending their resting period lying directly on the substrate. These variations highlight that while the method of resting is species-specific, the goal remains the same: a secure, low-energy state that prevents uncontrolled floating or sinking.