Seeing a school of fish hovering in place or a lone fish holding its position can be mystifying. If you’ve wondered why fish remain stationary instead of constantly swimming around, you’re not alone.
If you’re short on time, here’s the quick answer: Fish stay in one spot to conserve energy, avoid predators, feed, rest, and establish territory.
Conserving Energy
Hovering takes less energy than swimming
Fish are able to conserve energy by hovering in place for extended periods. This is because hovering takes far less effort than actively swimming. When a fish hovers, it uses its fins to maintain position while allowing the current to support its body.
This is much less strenuous than constantly swimming and flapping their fins and tail to propel themselves forward. Some fish like sharks and tuna are specially adapted to be able to hover nearly motionless while they wait to ambush prey.
This energy conserving behavior allows them to strike quickly when needed while avoiding wasting energy on unnecessary movement.
Studies have shown that hovering fish use up to 50% less energy than fish actively swimming. The exact energy savings depends on factors like the strength of the current, the size of the fish, and fin shape. However, the energy savings are quite substantial across most fish species.
Being able to hover in place gives fish a tremendous evolutionary advantage as they can patiently stalk prey or avoid predators with minimal exertion.
Currents allow effortless suspension
Another way fish are able to conserve energy is by allowing themselves to be suspended in place by ocean currents. Rather than having to constantly swim to maintain their depth and position, fish can passively drift in currents, expending little to no energy.
Areas of upwelling and convergence where currents meet provide ideal spots for fish to hang nearly motionless with minimal effort.
Species like jacks and mackerel are specially adapted to take advantage of currents and will congregate in areas where currents do the work of suspending them. Allowing oneself to be suspended in a current is similar to the energy savings of hovering, but with even less effort required on the part of the fish.
Drifting in a current can reduce a fish’s energy expenditure by up to 80% compared to active swimming. This ability gives fish accessing to rich feeding areas like upwellings while minimizing their energy output. It provides huge advantages for growth, reproduction, and survival.
Evading Predators
Blending into surroundings
Many fish have evolved camouflage coloration and patterns that help them blend into their surroundings and avoid detection by predators. This can include having colors and patterns that match the seabed substrate, aquatic plants, or coral that they live among.
For example, flounders have evolved flat bodies and the ability to change the color and patterns on their upper side to match the seabed that they lay on.
Other fish may have countershading, with dark colors on their topside and lighter colors below, to blend into the ocean lighting environment when viewed from above or below. Some species can even change color rapidly using specialized chromatophore cells in their skin to better match changing surroundings.
Safety in numbers
Fish that school in large groups may have better survival odds in the face of predators. If a predatory fish attacks a school, it is less likely to eat any single individual fish. Research has shown that a fish in a large school has upwards of a 95% chance of avoiding capture compared to a lone fish.
Staying in a group confuses the predator and makes it harder to single out and target any one fish.
In addition, schooling together allows fish to use the sensory capabilities of the entire group to more quickly detect an approaching predator via sight or changes in water movement. Herring, for example, flash a quick silver sheen across the school as they turn in unison upon sensing a threat.
This “safety in numbers” effect gives each fish better odds of spotting and reacting to an attack when they stay and swim together in one place.
Feeding
Ambushing prey
Many fish are ambush predators that lie still and wait for unsuspecting prey to swim close enough to strike. This stealthy feeding strategy allows them to conserve energy while still catching food. Some common ambush feeders include:
- Groupers – These large reef fish blend in with their surroundings and burst out to swallow fish and crustaceans whole when they wander too close.
- Frogfish – These funky-looking fish can change color to match their environment. They suck prey in with a sudden opening of their large mouth.
- Anglerfish – The anglerfish has a fleshy growth on its head that acts as a lure to draw in prey.
Staying in one spot allows ambush predators to remain perfectly camouflaged and strike their prey with lightning-fast speed. Moving around would make them more obvious to potential prey. Ambush feeding is most effective for fish that live among rocks or coral where they can hide and wait for food to come to them.
Filter feeding
Many fish are filter feeders, meaning they feed by straining tiny food particles out of the water. Staying stationary allows them to filter at high volumes without expending energy to swim around. Some common filter feeding fish include:
- Whale sharks – Despite their immense size, whale sharks feed on tiny plankton and small fish by swimming with their mouths open.
- Manta rays – These gentle giants flap their wings while stationary to channel water and food into their mouths.
- Basking sharks – Second only to whale sharks in size, basking sharks cruise with their mouths agape to catch plankton.
In a single day, a large filter feeder can strain over 2,000 gallons of water for food. Staying in place and filtering continuous mouthfuls of water is an efficient feeding strategy. Other reasons fish may filter feed in one area include:
- Abundant plankton – Fish congregate where currents concentrate dense swarms of plankton.
- Cleaner stations – Some fish even set up “cleaning stations” where they wait for small cleaner fish to pick parasites off of them while they filter feed.
So in short, ambush predators stay still to remain hidden from prey, while filter feeders stay put to process huge volumes of water flowing through their gaping mouths and gills. Both feeding strategies allow fish to eat while expending minimal energy on movement.
Next time you see fish hovering in one place, they’re likely perfectly positioned to grab a bite!
Resting
Fish often stay motionless in one spot for a variety of reasons. Here are some of the main reasons why fish hold their position:
Conserving Energy
Staying still requires less energy than swimming around constantly. By remaining stationary, fish can conserve their energy stores. This is especially important for cold-blooded fish that rely on external warmth from the water to regulate their body temperature and metabolism.
Holding position allows them to limit unnecessary energy expenditure.
Certain species like nurse sharks and stingrays will rest on the seafloor or in crevices for hours to recover strength. Other fish like trout may park themselves in slower moving currents to avoid burning energy fighting stronger flows.
Ambushing Prey
Many predatory fish are stealth hunters. They will hover perfectly still in wait for prey to swim by. Common ambush predators include anglerfish, stonefish, scorpionfish, and camouflaged flatfish like flounder or sole. By remaining motionless, these hunters avoid detection until it’s time to strike.
The element of surprise gives ambush predators an important advantage over prey. Fish like the frogfish even employ lures to attract oblivious prey towards their mouths as they lay in wait.
Avoiding Predators
On the flip side, holding position also helps small prey fish avoid larger predators. By staying still, they become nearly invisible. Their camouflaged coloring causes them to blend seamlessly into surroundings like coral reefs.
Fish such as blennies and gobies will wedge themselves into tight spaces in the reef, barely moving a muscle. Motionlessness means predators swimming by are less likely to notice them.
Social Interactions
For schooling fish like herring or anchovy, holding formation requires individuals to stay in one place as the group moves around them. Each fish holds its position relative to others, thereby forming a cohesive unit that confuses predators.
In mating rituals, male fish like bettas will flair their fins and remain stationary to attract females. The motionless posturing allows potential mates to assess fitness and health.
Regulating Buoyancy
Some fish have specialized swim bladders that allow them to hover effortlessly in place without sinking or floating up. The swim bladder acts like a hydrostatic balloon, adjusting to precisely counter the fish’s weight.
Fish such as rainbow trout and kelp bass utilize their swim bladders to maintain neutral buoyancy at a given water depth. This helps them stay suspended sans any swimming motion.
Establishing Territory
Fish establish and defend territories for a variety of important reasons. Here are some of the main factors that drive territorial behavior in fish:
Access to Resources
Many fish stake out territories to gain exclusive access to resources like food, nesting sites, and mating partners. For example, male cichlids excavate pit nests in the lakebed and then aggressively defend their nest site from other males. This allows them to attract mates to lay eggs in their pit.
Similarly, lionfish may claim coral outcroppings or rocky crevices to have prime access to prey that passes by their territory. Defending a high-quality foraging area can greatly improve a fish’s growth, survival and reproductive success.
Reproduction
As mentioned above, some fish like cichlids and bettas establish breeding territories to control nesting sites. Others like damselfish claim mating territories where they can court and spawn with females. Territoriality allows them to monopolize mates and prevent sneak spawning by rivals.
Some species like surgeonfish even defend territories where females can safely hide from harassment when they are ready to mate. So controlling a territory is key for many fish to attract mates and breed successfully.
Shelter from Predators
Territories often contain shelters like caves, dense vegetation or rocky overhangs. Defending these shelters can protect territorial fish from predators. For example, tropical gobies claim coral crevices or holes in the sandy seabed.
Having a secure hiding place will improve their odds of evading predators like groupers and sharks. Some fish also establish territories in areas with fewer predators, allowing them to spend less time being vigilant and more time foraging and mating.
Reduced Conflict
Territoriality provides a form of social organization that leads to less fighting between neighboring fish. Once territories are established, fish engage in agonistic displays like charging or fin flaring to defend their borders.
But full-contact fighting is reduced since each fish retreats to its own delimited space. This allows fish communities to partition resources and coexist with less injury and wasted energy on constant skirmishes with neighbors. Stable social hierarchies form around territorial boundaries.
Conclusion
As we’ve explored, fish have several key reasons for staying put in the water instead of constantly swimming around. Conserving energy, hiding from predators, feeding, resting, and protecting territory all motivate fish to hold their position either alone or in groups.
