Trout do not sleep in the way that humans or other mammals do. Since fish lack eyelids, they cannot close their eyes, which leads to the common misconception that they never rest. However, like all organisms, trout require a period of reduced activity to conserve energy and allow for physical restoration. This necessary state of inactivity, although different from mammalian sleep, satisfies the biological need for rest. Understanding this requires examining the definition of rest for a fish and the specific behaviors trout exhibit when they enter this state.
Defining Rest: Why Fish Don’t “Sleep” Like Mammals
The concept of “sleep” in vertebrates is often linked to complex neurobiological markers, such as the cycle between slow-wave sleep and rapid eye movement (REM) sleep. Mammals possess a neocortex, which plays a role in regulating these sleep stages, but fish do not have this structure and generally do not experience the REM stage.
Instead of the deep unconsciousness seen in land mammals, a fish’s rest is defined by behavioral criteria. This includes prolonged reduced movement, a species-specific resting posture, and decreased responsiveness to external stimuli. The ability to remain partially alert is a survival adaptation, allowing the fish to respond quickly to threats. Fish rest is a reversible state of reduced activity that serves the same restorative function as sleep.
Observable Resting Behaviors of Trout
When trout enter a resting state, their behavior changes in observable ways that allow for energy conservation. In river environments, trout actively seek out areas of minimal current, such as behind large rocks, under undercut banks, or along the inside of a river bend. This positioning reduces the energy they must expend to hold their place against the flow of water.
Trout adopt a specific resting posture, settling close to the substrate, sometimes touching the riverbed, with their fins mostly motionless. Their overall body movement is significantly reduced, appearing almost suspended in the water column. Fin movements become minimal, just enough to maintain balance and avoid drifting. This period of inactivity typically occurs during the night, as many trout species are more active and feed during the day.
Physiological State During Inactivity
The external behaviors observed during trout rest are driven by distinct internal physiological changes that prioritize restoration and energy savings. The most significant change is a reduction in metabolic rate, which directly lowers the overall energy expenditure.
During this state of reduced metabolism, the trout’s heart rate and ventilation rate—the speed at which they move water over their gills—also decrease. This lowered demand for oxygen means the fish can sustain itself with less effort. If a trout is disturbed during this period, studies indicate it will exhibit a “sleep rebound” effect, requiring compensatory rest afterward. This phenomenon confirms the physiological necessity of this inactive state for proper functioning and recovery.