The Fastest Swimmers: How Fish Move Through Water

Fish locomotion is the result of interactions between segmental body muscles, flexible fins, and the surrounding water. The backward-traveling wave of lateral displacement that powers most fish is generated by the myotomal musculature, and the body and tail push against the water to produce forward thrust. This article explains the major swimming modes, the mechanical principles behind thrust production, the role of fins, and how schooling and environmental flows affect swimming performance. The content is written for students, researchers, life-science professionals, and informed general readers who want a practical understanding of fish swimming mechanics.

At a Glance: Fish Swimming Modes and Their Characteristics

The Mechanical Basis of Fish Swimming

Muscle Activation and Body Bending

Undulatory swimming in fish is powered by the segmental body musculature of the myotomes. Power generated by this muscle and the interactions between the fish and the water generate a backward-traveling wave of lateral displacement of the body and caudal fin. The body and tail push against the water, generating forward thrust. The muscle activation and strain patterns that underlie body bending and thrust generation have been described for a number of species and show considerable variation. This variation is due in large part to the complex interactions between muscle mechanical properties, fish body form, swimming mode, swimming speed, and phylogenetic relationships. Recent work in several laboratories has been directed at studying patterns of muscle power output in vitro under simulated swimming conditions. This work suggests that the way that fish generate muscle power and convert it into thrust through the body and caudal fin does indeed vary. However, despite the differences, several features appear to be common to virtually all species studied and suggest where future effort should be directed if muscle function in swimming fish is to be better understood [5].

The load against which the swimming muscles contract during undulatory swimming is composed principally of hydrodynamic pressure forces and body inertia. In the past this has been analyzed through an equation for bending moments for small-amplitude swimming, using Lighthill's elongated-body theory and a vortex-ring panel method to compute the hydrodynamic forces. Recent work on large-amplitude swimming has extended the bending moment equation to large amplitude, which involves the introduction of a new term, and developed a large-amplitude vortex-ring panel method. The latter requires computation of the wake, which rolls up into concentrated vortex rings and filaments and has a significant effect on the pressure on the body. Application is principally made to the saithe (Pollachius virens). The calculations confirm that the wave of muscle activation travels down the fish much more rapidly than the wave of bending [3].

Pressure Forces and Thrust Generation

The anterior body of many fishes is shaped like an airfoil turned on its side. With an oscillating angle to the swimming direction, such an airfoil experiences negative pressure due to both its shape and pitching movements. This negative pressure acts as thrust forces on the anterior body. A high-resolution, pressure-based approach applied to bluegill sunfish (Lepomis macrochirus) and brook trout (Salvelinus fontinalis) swimming in the carangiform mode, the most common fish swimming mode, demonstrated that these fish generate thrust on their anterior bodies using leading-edge suction mechanics, much like an airfoil. These mechanics contrast with those previously reported in lampreys, which are anguilliform swimmers that produce thrust with negative pressure but do so through undulatory mechanics. The thrust produced on the anterior bodies of these carangiform swimmers through negative pressure comprises 28% of the total thrust produced over the body and caudal fin, substantially decreasing the net drag on the anterior body. On the posterior region, subtle differences in body shape and kinematics allow trout to produce more thrust than bluegill, suggesting that they may swim more effectively. Despite the large phylogenetic distance between these species, and differences near the tail, the pressure profiles around the anterior body are similar. Such airfoil-like mechanics are highly efficient because they require very little movement and therefore relatively little active muscular energy, and may be used by a wide range of fishes since many species have appropriately shaped bodies [9].

Major Swimming Modes

Anguilliform Swimming

Anguilliform swimmers use their entire body for undulation. The whole body produces a sinusoidal wave that travels from head to tail, with the amplitude of the wave increasing toward the posterior end. Eels, lampreys, and hagfish are classic examples. These fish produce thrust with negative pressure through undulatory mechanics. The hagfish, in particular, has a unique defense mechanism involving slime that contains a network of long fibrous threads each approximately 10 cm in length. Hagfish release the threads in a condensed coiled state known as skeins, approximately 100 micrometers, which must unravel within a fraction of a second to thwart a predator attack. Viscous hydrodynamics can be responsible for this rapid unravelling, as opposed to chemical reaction kinetics alone. Under reasonable physiological conditions, unravelling due to viscous drag can occur within a few hundred milliseconds, and is accelerated if the skein is pinned at a surface such as the mouth of a predator [8].

Carangiform Swimming

Carangiform swimming is the most common fish swimming mode. In this mode, the undulation is confined primarily to the posterior third to half of the body, with the anterior body remaining relatively stiff. The caudal fin provides the main propulsive surface. Bluegill sunfish, brook trout, saithe, and lambari are representative carangiform swimmers. The anterior body of these fish acts like an airfoil, generating thrust through leading-edge suction mechanics. The pressure profiles around the anterior body are similar across species despite large phylogenetic distances and differences near the tail [9].

Numerical investigations of carangiform swimming have examined the most relevant dimensionless parameters in a fish swimming environment, including Reynolds number, Strouhal number, and slip number. A three-dimensional numerical study of the fish-like lambari, a body and caudal fin swimmer with carangiform kinematics, was conducted using the URANS approach with the k-omega-SST transition turbulence closure model. The study reported the equilibrium Strouhal number and its dependence on the Reynolds number, following a power-law relationship. The research also conducted a comprehensive analysis of the hydrodynamic forces and the effect of body undulation on the production of swimming drag and thrust. Propulsive and quasi-propulsive efficiencies were computed, and the influence of the Reynolds number and slip number on fish performance was examined. A vortex dynamics analysis revealed different wake configurations under variations of the dimensionless parameters, and the relationship between the generation of a leading-edge vortex via the caudal fin and the peak thrust production within the motion cycle was explored [13].

Median and Paired Fin Swimming

Some fish rely primarily on their median and paired fins instead of body undulation. The roles of fish median fins on the hydrodynamics and muscle actuation in carangiform swimming have been studied, showing that these fins contribute to stability and maneuverability during swimming [17]. Fins from ray-finned fishes do not contain muscles, yet fish can change the shape of their fins with high precision and speed while producing large hydrodynamic forces without collapsing. Fully instrumented micromechanical tests on individual rays from rainbow trout in both morphing and flexural deflection mode at large deflections revealed that the flexural stiffness of the mineralized layers in the rays, known as hemitrichs, is 5 to 6 times lower than their axial stiffness, an advantageous combination to produce stiff morphing. The collagenous core region can be modeled with spring elements which are 3 to 4 orders of magnitude more compliant than the hemitrichs. This fibrillar structure provides negligible resistance to shearing from the initial position, but it prevents buckling and collapse of the structure at large deformations [10].

Batoid Swimming

Batoid fish, such as rays and skates, swim using their enlarged pectoral fins. Flow around a tethered model of a swimming batoid fish studied using wall-modelled large-eddy simulation in conjunction with the immersed boundary method at a Reynolds number up to 148,000 provided the first evidence of hairpin vortical structures near the body surface using three-dimensional high-fidelity flow field data. These small-scale vortical structures are mainly formed through two mechanisms: the leading-edge vortex-secondary filament-hairpin vortex and leading-edge vortex-hairpin vortex transformations in different regions. The hairpin vortices create strong fluctuations in the pressure distribution and frequency spectrum. Simulations conducted at lower Reynolds numbers revealed that low-Reynolds-number simulations are meaningful when the focus is on the force variation tendency, whereas high-Reynolds-number simulations are needed when concerning flow fluctuations and turbulence mechanisms [14].

The Role of Fins in Swimming

Caudal Fin Function

The caudal fin is the primary propulsive surface for most fish. In carangiform and thunniform swimmers, the caudal fin generates the majority of thrust. The shape and stiffness of the caudal fin vary among species and affect swimming performance. The leading-edge vortex on the fin dominates the overall thrust production for these swimmers, and its enhancement provides an effective and robust means for harnessing fish-fish hydrodynamic interactions in a school [7].

Median Fins as Stabilizers

The median fins, including the dorsal, anal, and adipose fins, play important roles in stability and maneuverability. The roles of fish median fins on the hydrodynamics and muscle actuation in carangiform swimming have been investigated, showing that these fins contribute to the control of body position and orientation during swimming [17]. The question of where the rudder of a fish is located has been explored in studies of the mechanism of swimming and control of self-propelled fish schools [20].

Fin Ray Mechanics

The mechanical properties of fin rays determine how fish control fin shape. The hemitrichs, which are the mineralized layers in the rays, have a flexural stiffness that is 5 to 6 times lower than their axial stiffness. This combination allows for stiff morphing, meaning the fin can change shape while maintaining structural integrity. The collagenous core region provides negligible resistance to shearing from the initial position but prevents buckling and collapse of the structure at large deformations. These insights can serve as guidelines for the design of efficient bioinspired stiff morphing materials and structures at large deformations [10].

Hydrodynamic Models of Fish Swimming

Dipole and Vortex Sheet Models

Elucidating the hydrodynamics of fish swimming is critical to identifying the processes underlying fish orientation and schooling. Due to their mathematical tractability, models based on potential flow are preferred in the study of bidirectional interactions of fish with their surroundings. Dipole-based models that assimilate fish to pairs of vortices are particularly enticing but have not been thoroughly validated. A computational fluid dynamics campaign informed by experimental data was undertaken to validate the accuracy of dipole-based models. The locomotory patterns of a fish undergoing carangiform swimming were reconstructed from existing experimental data and used as inputs to CFD simulations of a fish swimming in a channel flow. Dipole-based models are accurate in capturing key features of the fluid flow but cannot predict the elongated flow streamlines around the fish that are evident in CFD. An alternative model that replaces each vortex in the pair with a sheet along the fish length was proposed. Using a pair of vortex sheets that span approximately 80% of the fish body length with a separation distance of approximately 50% of the body width, the model is successful in predicting the fluid flow around the swimming fish for a range of background flow speeds and channel widths. The proposed model shows improved accuracy at the cost of a mildly increased computational effort, thereby constituting an ideal basis for research on fish hydrodynamics [6].

Large-Amplitude Swimming Models

For large-amplitude swimming, the bending moment equation has been extended to large amplitude, which involves the introduction of a new term. A large-amplitude vortex-ring panel method was developed that requires computation of the wake, which rolls up into concentrated vortex rings and filaments and has a significant effect on the pressure on the body. Application to the saithe confirmed that the wave of muscle activation travels down the fish much more rapidly than the wave of bending [3].

Swimming in Altered Flows

Turbulence and Flow Refuging

Fishes suspended in water are subject to the complex nature of three-dimensional flows. Often, these flows are the result of abiotic and biotic sources that alter otherwise uniform flows, which then have the potential to perturb the swimming motions of fishes. Most of our understanding of fish behaviour in turbulence comes from observations of natural conditions in the field and laboratory studies employing controlled perturbations, such as vortices generated in the wake behind simple geometric objects. Laboratory studies have employed motion analysis, flow visualization, electromyography, respirometry, and sensory deprecation techniques to evaluate the mechanisms and physiological costs of swimming in altered flows. Studies show that flows which display chaotic and wide fluctuations in velocity can repel fishes, while flows that have a component of predictability can attract fishes. The ability to maintain stability in three-dimensional flows, either actively with powered movements or passively using the posture and intrinsic compliance of the body and fins, plays a large role in whether fish seek out or avoid turbulence. Fish in schools or current-swept habitats can benefit from altered flows using two distinct though not mutually exclusive mechanisms: flow refuging, which is exploiting regions of reduced flow relative to the earth frame of reference, and vortex capture, which is harnessing the energy of environmental vortices. Integrating how the physical environment affects organismal biomechanics with the more complex issue of behavioural choice requires consideration beyond simple body motions or metabolic costs [4].

Oblique Flow Conditions

In confined and intricate aquatic environments, fish frequently encounter the need to propel themselves under oblique flow conditions. A numerical study using a ghost-cell immersed boundary method coupled with GPU acceleration technology simulated the propulsion dynamics of flexible biomimetic fish swimming in oblique flow environments. The research scrutinized diverse biomimetic fish fin morphologies, with particular emphasis on variations in the Strouhal number and angle of attack, to elucidate hydrodynamic performance and wake evolution. The results demonstrate that as the fin thickness increases, the propulsion efficiency decreases within the Strouhal number range of 0.2 to 0.4. Conversely, within the range of 0.6 to 1.0, the efficiency variations stabilize. For all three fin morphologies, an increase in the Strouhal number significantly augmented both the lift-to-drag ratio and thrust, concomitant with a transition in the wake structure from smaller vortices to a larger alternating vortex shedding pattern. As the angle of attack increases, the drag coefficient increases significantly, while the lift coefficient exhibits a diminishing rate of increase. An increased fin thickness adversely affects the hydrodynamic performance, but this effect attenuates at higher Strouhal numbers. Variations in the angle of attack manifest a more pronounced effect on hydrodynamic performance [12].

Schooling and Hydrodynamic Interactions

Leading-Edge Vortex Enhancement

The hydrodynamics of schooling fish has been the subject of continued investigation over the last 50 years. Fish schools exhibit a variety of arrangements, and several distinct mechanisms have been proposed to explain the hydrodynamic benefits of schooling. Direct numerical simulations show that a caudal fin swimmer trailing another similar swimmer can significantly improve its swimming performance by positioning itself such that the wake-induced flow of the leading fish enhances the leading-edge vortex on the fin of the trailing fish. Improvements of up to 12% in both the thrust and efficiency of the trailing fish are possible with this mechanism. The mechanisms underlying these interactional effects were quantitatively analyzed by applying the force partitioning method, a data-driven method that partitions the pressure forces on the fish into mechanistically distinct components. The analysis reveals that the leading-edge vortex on the fin dominates the overall thrust production for these swimmers, and its enhancement therefore provides an effective and robust means for harnessing fish-fish hydrodynamic interactions in a school. In addition to confirming the potential energetic benefits of schooling, the leading-edge vortex enhancement mechanism could be exploited in coordinated swimming of bioinspired multi-vehicle or multi-foil flapping foil propulsion systems [7].

Robotic Models of Schooling

Fish display remarkable locomotor and social abilities, from efficient swimming to coordinated schooling, that have inspired the design of various robotic fish. While robotics has largely benefited from biology, fish-like robots are increasingly used as scientific tools to investigate fundamental questions in biomechanics, sensorimotor control, and collective behavior. Robotic models have been employed to study the neuromechanical basis of swimming, the role of sensory feedback in locomotion, and the mechanisms underlying social interactions in schools. Robotic approaches provide unique advantages: they enable repeatable experiments, systematic variation of body and control parameters, and direct measurement of otherwise inaccessible quantities such as internal forces or energy use. A literature analysis reveals that only a minority of robot-fish studies contribute to biological understanding, with most focusing on engineering design. Among biology-oriented studies, closed-loop robotic systems capable of real-time adaptation remain underrepresented but are essential for probing sensorimotor and social feedback mechanisms. Future directions combine robotics, simulations, and emerging experimental technologies to unravel the multi-scale feedback loops that shape fish locomotion and schooling [11].

Fish That Swim Under Sharks

Pilot fish and remoras are known to swim under or alongside sharks. These fish use the boundary layer and wake flows created by the shark to reduce their own swimming effort. The mechanisms of flow refuging and vortex capture described in altered flow studies apply to these associations. By positioning themselves in regions of reduced flow relative to the earth frame of reference, these fish can maintain position with less energy expenditure. The study of such associations provides insight into how fish exploit environmental vortices and pressure gradients created by larger swimmers [4].

Practical Assessment of Swimming Performance

Measuring Swimming Performance

Swimming performance assessment in fishes involves standardized protocols that measure critical swimming speed, endurance, and metabolic costs. These assessments are typically conducted in swim tunnels or flumes where water velocity can be controlled. The protocols allow researchers to evaluate the effects of body morphology, fin condition, and environmental factors on swimming ability. Standardized assessment methods are essential for comparing results across studies and species [18].

Predicting Muscle Properties

The prediction of mechanical properties of fish muscle in vivo during steady swimming is an active area of research. Models that combine muscle physiology with hydrodynamics can predict the forces generated by swimming muscles under different conditions. These predictions are important for understanding the limits of swimming performance and for designing bioinspired underwater vehicles [19].

Observing Swimming Behavior

Contour segmentation of fish body with neural network models and characteristic parameter analysis of zebrafish swimming represents a modern approach to quantifying swimming behavior. These methods allow researchers to track body position, tail beat frequency, and amplitude with high precision. The characteristic parameters derived from such analyses provide quantitative measures of swimming performance that can be compared across individuals and conditions [16].

Common Failure Patterns in Swimming Studies

Incomplete Consideration of Reynolds Number Effects

Studies that focus only on low Reynolds numbers may miss important flow features that appear at higher Reynolds numbers. The flow separation behaviour and local pressure vary with Reynolds number. Low-Reynolds-number simulations are meaningful when the focus is on the force variation tendency, whereas high-Reynolds-number simulations are needed when concerning flow fluctuations and turbulence mechanisms [14].

Oversimplification of Body Morphology

Models that treat the fish body as a simple undulating beam may miss the airfoil-like mechanics of the anterior body that contribute significantly to thrust production. The anterior body of carangiform swimmers generates approximately 28% of total thrust through negative pressure, a contribution that is absent in models that only consider the caudal fin [9].

Neglect of Fin Mechanics

The remarkable performance of fish fins, which can change shape with high precision and speed while producing large hydrodynamic forces without collapsing, is often overlooked in simplified models. The nonlinear mechanical behavior of fin rays under large deformations must be considered for accurate predictions of swimming performance [10].

Limitations of Current Knowledge

Variation Among Species

Muscle function varies among species due to complex interactions between muscle mechanical properties, fish body form, swimming mode, swimming speed, and phylogenetic relationships. Despite common features across species, the way that fish generate muscle power and convert it into thrust through the body and caudal fin varies considerably [5].

Behavioral Complexity

Integrating how the physical environment affects organismal biomechanics with the more complex issue of behavioural choice requires consideration beyond simple body motions or metabolic costs. The decision to seek out or avoid turbulence involves sensory inputs, cognitive processing, and energetic tradeoffs that are not fully understood [4].

Model Validation

Dipole-based models, while mathematically tractable, cannot predict the elongated flow streamlines around the fish that are evident in CFD. The proposed vortex sheet model shows improved accuracy at the cost of a mildly increased computational effort, but further validation is needed across a wider range of conditions [6].

Welfare and Safety Context

Handling Fish for Swimming Studies

When conducting swimming performance assessments, fish must be handled carefully to minimize stress. Standardized protocols include acclimation periods and gradual increases in water velocity. Researchers should monitor fish for signs of fatigue or distress and terminate trials if fish are unable to maintain position. The welfare of experimental animals is a primary consideration in all swimming studies [18].

Environmental Conditions

Water temperature, oxygen levels, and water quality affect swimming performance and must be controlled during experiments. Fish swimming in altered flows may experience increased physiological costs, and these costs must be considered when interpreting results. The ability to maintain stability in three-dimensional flows plays a large role in whether fish seek out or avoid turbulence [4].

Professional Escalation Criteria

Researchers and practitioners should seek expert consultation when:

• Swimming performance data show unexpected patterns that cannot be explained by known morphological or environmental factors
• Studies involve protected or endangered species that require special permits
• Experimental protocols involve invasive procedures or prolonged exercise that may compromise fish welfare
• Results are to be used for regulatory decisions or management actions affecting wild fish populations
• Models produce predictions that conflict with empirical observations across multiple studies

Frequently Asked Questions

What is the difference between anguilliform and carangiform swimming?

Anguilliform swimming involves undulation of the entire body, as seen in eels and lampreys. Carangiform swimming confines undulation primarily to the posterior third to half of the body, with the anterior body remaining relatively stiff. Carangiform swimmers like bluegill sunfish and brook trout generate thrust on their anterior bodies using leading-edge suction mechanics, much like an airfoil, while anguilliform swimmers produce thrust through undulatory mechanics along the whole body [9].

How do fish generate thrust with their anterior bodies?

The anterior body of many fishes is shaped like an airfoil turned on its side. With an oscillating angle to the swimming direction, such an airfoil experiences negative pressure due to both its shape and pitching movements. This negative pressure acts as thrust forces on the anterior body. In carangiform swimmers, this mechanism contributes approximately 28% of the total thrust produced over the body and caudal fin [9].

What is the leading-edge vortex and why is it important?

The leading-edge vortex is a flow structure that forms on the leading edge of the caudal fin during swimming. It dominates the overall thrust production for carangiform swimmers. In schooling fish, a trailing fish can position itself such that the wake-induced flow of the leading fish enhances the leading-edge vortex on its own fin, improving thrust and efficiency by up to 12% [7].

Why do some fish swim under sharks?

Pilot fish and remoras position themselves under or alongside sharks to exploit the boundary layer and wake flows created by the larger animal. This behavior is consistent with the mechanisms of flow refuging, exploiting regions of reduced flow relative to the earth frame of reference, and vortex capture, harnessing the energy of environmental vortices [4].

How does turbulence affect fish swimming?

Flows that display chaotic and wide fluctuations in velocity can repel fishes, while flows that have a component of predictability can attract fishes. The ability to maintain stability in three-dimensional flows, either actively with powered movements or passively using the posture and intrinsic compliance of the body and fins, plays a large role in whether fish seek out or avoid turbulence [4].

What are the key dimensionless parameters in fish swimming?

The most relevant dimensionless parameters in fish swimming are the Reynolds number, which characterizes the flow regime, the Strouhal number, which characterizes the frequency of vortex shedding relative to swimming speed, and the slip number. These parameters determine wake configurations, thrust production, and propulsive efficiency [13].

How do fish fins change shape without muscles?

Fins from ray-finned fishes do not contain muscles, yet fish can change the shape of their fins with high precision and speed. The mineralized layers in the rays, known as hemitrichs, have a flexural stiffness that is 5 to 6 times lower than their axial stiffness, allowing for stiff morphing. The collagenous core region prevents buckling and collapse of the structure at large deformations [10].

What is the significance of the wave of muscle activation traveling faster than the wave of bending?

Calculations confirm that the wave of muscle activation travels down the fish much more rapidly than the wave of bending. This means that muscles are activated before the body reaches its maximum bend at that location, allowing the muscle to generate force while the muscle fibers are still being stretched or at the start of shortening, which is mechanically advantageous for power production [3].

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References and Further Reading

• NCBI Literature Resources. National Center for Biotechnology Information.
• PubMed. National Library of Medicine.
• Large-amplitude undulatory fish swimming: fluid mechanics coupled to internal mechanics.. The Journal of experimental biology, 1999.
• A review of fish swimming mechanics and behaviour in altered flows.. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 2007.
• Fish swimming: patterns in muscle function.. The Journal of experimental biology, 1999.
• Dipole- and vortex sheet-based models of fish swimming.. Journal of theoretical biology, 2023.
• Improved swimming performance in schooling fish via leading-edge vortex enhancement.. Bioinspiration & biomimetics, 2022.
• Unravelling hagfish slime.. Journal of the Royal Society, Interface, 2019.
• Airfoil-like mechanics generate thrust on the anterior body of swimming fishes.. Proceedings of the National Academy of Sciences of the United States of America, 2020.
• Mechanics and properties of fish fin rays in nonlinear regimes of large deformations.. Acta biomaterialia, 2023.
• Swimming with robots: investigating fish locomotion, sensing, and schooling behavior with robotic swimmers.. 2026.
• Numerical Study on the Hydrodynamics of Fish Swimming with Different Morphologies in Oblique Flow. Journal of Marine Science and Engineering, 2024.
• Numerical Investigation of Dimensionless Parameters in Carangiform Fish Swimming Hydrodynamics. Biomimetics, 2024.
• Hydrodynamics of a swimming batoid fish at Reynolds numbers up to 148 000. Journal of Fluid Mechanics, 2023.
• Hydrodynamics of unsteady fish swimming and the effects of body size: comparing the flow fields of fish larvae and adults.. Journal of Experimental Biology, 2000.
• Contour segmentation of fish body with neural network model and characteristic parameter analysis of zebrafish swimming. Journal of Hydrodynamics, 2025.
• The roles of fish median fins on the hydrodynamics and muscle actuation in carangiform swimming. Journal of Fluids and Structures, 2023.
• Swimming performance assessment in fishes. Journal of Visualized Experiments, 2011.
• Prediction of mechanical properties of fish muscle in vivo during steady swimming. Lixue Xuebao Chinese Journal of Theoretical and Applied Mechanics, 2014.
• Where is the rudder of a fish?: The mechanism of swimming and control of self-propelled fish school. Acta Mechanica Sinica Lixue Xuebao, 2010.

This article is educational and does not replace institutional policy, professional advice, or applicable safety and regulatory requirements.