Life beneath the water’s surface is a world of constant activity, dictated by the fundamental needs of survival, reproduction, and movement. Fish represent more than half of all vertebrate species on Earth, inhabiting environments from the crushing pressures of the deep ocean to the swift currents of freshwater streams. This incredible diversity ensures that no single answer exists for what fish do all day, as their routines are specialized for their surroundings. However, the daily lives of all fish revolve around common behaviors necessary for finding energy, resting safely, interacting with others, and navigating their aquatic homes.
How Fish Find Food
The pursuit of energy is a primary driver of a fish’s daily schedule, with species employing a wide array of strategies to acquire sustenance. Fish diets fall into general categories: herbivores that graze on plants or algae, carnivores that hunt other animals, and omnivores that eat both. Some species are scavengers or filter feeders, passively consuming organic matter or plankton suspended in the water column.
To locate food, fish utilize a sophisticated suite of sensory tools adapted for the underwater world. Their “sixth sense” is the lateral line system, a series of fluid-filled canals along the body that detect low-frequency vibrations and pressure changes. This allows a fish to sense the movement of nearby prey even in dark or murky conditions. Additionally, the senses of smell and taste are highly developed; some fish, like salmon, use scent to navigate thousands of miles back to their natal streams.
Foraging styles vary dramatically, contrasting active hunting with passive feeding. Predators like bass use keen vision to ambush prey concealed in submerged vegetation. Bottom-dwellers such as catfish rely on barbels, or whisker-like feelers equipped with taste buds, to search the substrate. Certain species, such as electric eels, even use specialized organs for electroreception, generating a low-level electric field to detect changes and locate food in the absence of light.
Resting and Sleep Cycles
Unlike mammals, fish do not experience sleep in the same way, as they lack eyelids and do not enter a state of Rapid Eye Movement (REM) sleep. Their version of rest is a state of reduced metabolism and awareness, defined as a period of stillness and lower brain activity. This state allows them to conserve energy, consolidate memory, and rejuvenate the body.
When resting, fish often become less responsive to external stimuli, remaining nearly motionless, sometimes hovering in the water column or settling on the substrate. Since they cannot close their eyes, they maintain a semi-alert state, which is necessary to remain vigilant against potential predators. Some species have developed specialized resting behaviors, such as the parrotfish, which secretes a protective mucus cocoon around itself at night to hide its scent.
The timing of rest depends on whether a species is diurnal or nocturnal. Most aquarium fish are diurnal, resting at night when the light diminishes. Others, like certain catfish and loaches, are nocturnal, spending the day hiding in crevices and becoming active after dark. Even while resting, certain species, like some sharks, must keep moving to force water over their gills for oxygenation.
Schooling and Social Life
Many fish interact with conspecifics, often in highly coordinated groups. Schooling is a behavior where fish align themselves to move in the same direction and at the same speed, a synchronized movement distinct from a looser aggregation called shoaling. Only about a quarter of all fish species engage in true schooling.
The primary advantage of schooling is safety, as being part of a large group dramatically reduces the chance of any single individual being targeted by a predator. The collective also offers improved foraging efficiency, as more eyes can locate food sources, and the group can work together to herd prey. Furthermore, swimming in a tight formation provides hydrodynamic benefits, allowing individual fish to conserve energy by reducing water resistance during long-distance movement.
Fish maintain this precise synchronization using a combination of senses, where vision is paramount. The lateral line system is also utilized to detect the precise movements of their neighbors. Communication within the school happens through subtle body language, and some species also use sound production or color changes to signal to one another. Social interactions are also driven by reproductive needs, with many species gathering in large groups for spawning to maximize fertilization rates.
Navigating Their Habitat
Movement and positioning within the aquatic environment are constant activities. A major factor governing a bony fish’s vertical movement is the swim bladder, an internal gas-filled sac that maintains neutral buoyancy. By precisely controlling the gas pressure in this organ, a fish can hover effortlessly at a specific depth without expending energy.
Fish also establish and defend their territory or home range, using visual cues or sometimes sound to mark boundaries. Territoriality involves defending a space that provides necessary resources, often against other members of the same species. This static activity contrasts with the massive, directed movements of migration, which some species undertake for better feeding or reproduction.
Migratory journeys can range from short movements to epic travels of thousands of miles, such as those undertaken by salmon or eel species. Fish navigate these long distances using magnetic fields, currents, and their highly sensitive sense of smell to identify specific waterways. When not moving or feeding, many species employ camouflage or hide in rocks and plants to blend in and avoid detection.