Paul W. Webb

Functional Locomotor Morphology of Early Life History Stages of Fishes

Abstract Routine activities of early life history stages of fishes occur in an intermediate hydrodynamic environment (as identified by Reynolds numbers), between a zone where drag is linearly dependent on velocity and resistive forces make large contributions to thrust, and a zone where inertial forces dominate except in the boundary layer immediately adjacent to the body surface. Sprint performance carries larvae into this latter zone; thus, locomotor activities important for survival of both larvae and adults occur in the same hydrodynamic environment and similar selective pressures would be expected to influence locomotor morphology of larvae and adults. The simplest framework for evaluating and interpreting development of larvae recognizes the parental form as the developmental terminus and uses adult forms as references to identify similarities and discrepancies in larva structure. Three measures of locomotor structure are used to examine changes during development: (a) the ratio of caudal peduncle d...

Power Requirements of Swimming: Do New Methods Resolve Old Questions?

Abstract A recurring question in the study of fish biomechanics and energetics is the mechanical power required for tail-swimming at the high speeds seen among aquatic vertebrates. The quest for answers has been driven by conceptual advances in fluid dynamics, starting with ideas on the boundary layer and drag initiated by Prandtl, and in measurement techniques starting with force balances focussing on drag and thrust. Drag (=thrust) from measurements on physical models, carcasses, kinematics as inputs to hydromechanical models, and physiological power sources vary from less than that expected for an equivalent rigid reference to over an order of magnitude greater. Estimates of drag and thrust using recent advances largely made possible by increased computing power have not resolved the discrepancy. Sources of drag and thrust are not separable in axial undulatory self propulsion, are open to interpretation and Froude efficiency is zero. Wakes are not easily interpreted, especially for thrust evaluation. We suggest the best measures of swimming performance are velocity and power consumption for which 2D inviscid simulations can give realistic predictions. Steady swimming power is several times that required for towing an equivalent flat plate at the same speed.

Ontogeny of routine swimming activity and performance in zebra danios (Teleostei: Cyprinidae)

Zebra danios, Danio rerio, 3–39 mm long, were studied to quantify ontogenetic changes in routine (spontaneous) swimming. Swimming speeds and mean acceleration rate increased during the larval period with the most rapid changes occurring when fish were between 5 and 15 mm long. At larger sizes, the rate of increase in performance was small. This pattern presumably resulted from morphological changes which also proceeded rapidly in larvae and levelled off towards adulthood. Swimming bouts began with either a large (105°) or a small (3°) turn. Turning angle changed with size apparently due to hydrodynamic conditions, as indexed by Reynolds numbers (Re). Larvae spent 98% of the time in the viscous and intermediate hydrodynamic regimes, of which 90% occurred at an Re of less than 110, and 23% was in the viscous regime (Re<30). Larvae made exclusively small turns when the bout was in the viscous regime (Re<23) and exclusively large ones in the intermediate regime (75<Re<270). Other species show similar rates of travel at lengths between 3 and 7 mm. At larger sizes, the zebra danio, Pacific mackerel, Scomber japonicus, and red sea bream, Pagrus major travel much greater distances per unit time. When their different sizes and rates of development were taken into account, the majority of species eamined showed similar performance levels throughout the larval period. Two species that cover small distances per unit time at a given length (northern anchovy, Engraulis mordax, and Atlantic herring, Clupea harengus) showed performance comparable to that of faster species when their prolonged larval period was taken into account. The implications of these ontogenetic changes in swimming performance for the development of foraging and anti-predator behaviour are discussed.

Optimal avoidance and evasion tactics in predator-prey interactions☆

Abstract A kinematic analysis of optimal avoidance and evasion techniques for prey is presented. The analysis is mainly directed towards piscivorous interactions but can include other aquatic and terrestrial cases. Avoidance is defined as maneuvering for position by prey, before the predator starts a chase, while evasion is an escape response to an attack. Two separate optimal avoidance methods are found and analyzed—minimizing time within sighting range; and maximizing instantaneous distance. The second method leads to the well-known “fountain effect” of fish school break-up when predators are in the vicinity. The optimal evasion technique involves escape at a small angle (up to 20°) from the heading directly away from the predator. This is in agreement with observations of escaping minnows.

Control of Posture, Depth, and Swimming Trajectories of Fishes

Abstract Perturbations vary in period and amplitude, and responses to unavoidable perturbations depend on response time and scale. Disturbances due to unavoidable perturbations occur in three translational planes and three rotational axes during forwards and backwards swimming. Stability depends on hydrodynamic damping and correcting forces, which may be generated by propulsors (powered) or by control surfaces moving with the body (trimming). Hydrostatic forces affecting body orientation (posture) result in negative metacentric heights amplifying rolling disturbances. The ability to counteract perturbations and correct disturbances is greater for fishes with more slender bodies, which appears to affect habitat choices. Postural control problems are greatest at low speeds, and are avoided by some fishes by sitting on the bottom. In currents, body form and behavior affect lift, drag, weight, and friction and hence speeds to which posture can be controlled. Self-correcting and regulated damping and trimming mechanisms are most important in stabilizing swimming trajectories. Body resistance, fin trajectory, multiple propulsors, and long-based fins damp self-generated locomotor disturbances. Powered control using the tail evolved early in chordates, and is retained by most groups, although fishes, especially acanthopterygians, make greater use of appendages. As with most areas of stability, little is known of control costs. Costs and benefits of low-density inclusions and hydrodynamic mechanisms for depth control vary with habits and habitats. Control may make substantial contributions to energy budgets.

Avoidance Responses of Fathead Minnow to Strikes by Four Teleost Predators

Summary1.Predator avoidance behavior of fathead minnow (Pimephales promelas) attacked by four teleosts (tiger musky,Esox sp; rainbow trout,Salmo gairdneri; smallmouth bass,Micropterus dolomieu; rock bass,Ambloplites rupestris) was analyzed using stop-action video-tape recordings of predator-prey interactions. The predators represented a range of body forms.2.75 to 90% of minnows responded to strikes by trout, bass and rock bass, but only 28% responded to strikes by tiger musky. Responses were 54 to 94% successful in evading a strike.3.Two prey avoidance response patterns were found. Type-1 responses were low intensity, non-sustained turning maneuvers away from more distant and more slowly moving predators. Type-2 responses were high intensity turning maneuvers followed by sustained swimming away from closer and faster-moving predators. All responses to tiger musky were type-2 responses. Prey speeds in avoidance responses were lower for rock bass than for the other three predators.4.Response thresholds for fathead minnow escape maneuvers were calculated at the start of the motor response, and evaluated with respect to predator size. Predator size was calculated from the dimensions of the silhouette viewed by the prey as the mean of predator depth and width. Prey reaction distance decreased and predator visual angle increased with predator size for trout, bass and rock bass. Reaction distance was smaller, and visual angle larger for responses to tiger musky.5.The rates of change of the visual angle at the start of a response, the apparent looming threshold (ALT), were similar for trout, bass and rock bass, but 15–80 times larger for tiger musky.6.The results suggest that configuration differences between the predators are important contributors to the stimulus initiating avoidance responses. It is suggested that the rounded body cross-section of esocids is associated with higher response thresholds than elliptical and lenticular cross-sections of trout, bass and rock bass.

Locomotion in the Biology of Large Aquatic Vertebrates

Abstract As aquatic vertebrates increase in size, hydrofoils, which use lift to generate thrust, are increasingly used as propulsors. One factor affecting the magnitude of the lift force is the area of the propulsor. Resistance to cruising and sprints is mainly due to drag, but inertia is important during maneuvers when animals accelerate or turn. The inertia of the body and entrained water, which is proportional to body volume, resists acceleration. Because a thrust that is proportional to surface area is used to maneuver a resistance that is proportional to volume, acceleration performance and maneuverability are expected to decline with increasing size, This trend is ameliorated to some extent by the high swimming speeds attainable by warm-bodied vertebrates and the reduced resistance to acceleration characteristic of the skeletons of dolphins and ichthyosaurs. Maneuvers are essential for capture of elusive prey and avoidance of predators. As they increase in size, aquatic vertebrates use various means...

Hydrodynamic stability of swimming in ostraciid fishes: role of the carapace in the smooth trunkfish Lactophrys triqueter (Teleostei: Ostraciidae).

SUMMARY The hydrodynamic bases for the stability of locomotory motions in fishes are poorly understood, even for those fishes, such as the rigid-bodied smooth trunkfish Lactophrys triqueter, that exhibit unusually small amplitude recoil movements during rectilinear swimming. We have studied the role played by the bony carapace of the smooth trunkfish in generating trimming forces that self-correct for instabilities. The flow patterns, forces and moments on and around anatomically exact, smooth trunkfish models positioned at both pitching and yawing angles of attack were investigated using three methods: digital particle image velocimetry (DPIV), pressure distribution measurements, and force balance measurements. Models positioned at various pitching angles of attack within a flow tunnel produced well-developed counter-rotating vortices along the ventro-lateral keels. The vortices developed first at the anterior edges of the ventro-lateral keels, grew posteriorly along the carapace, and reached maximum circulation at the posterior edge of the carapace. The vortical flow increased in strength as pitching angles of attack deviated from 0°, and was located above the keels at positive angles of attack and below them at negative angles of attack. Variation of yawing angles of attack resulted in prominent dorsal and ventral vortices developing at far-field locations of the carapace; far-field vortices intensified posteriorly and as angles of attack deviated from 0°. Pressure distribution results were consistent with the DPIV findings, with areas of low pressure correlating well with regions of attached, concentrated vorticity. Lift coefficients of boxfish models were similar to lift coefficients of delta wings, devices that also generate lift through vortex generation. Furthermore, nose-down and nose-up pitching moments about the center of mass were detected at positive and negative pitching angles of attack, respectively. The three complementary experimental approaches all indicate that the carapace of the smooth trunkfish effectively generates self-correcting forces for pitching and yawing motions — a characteristic that is advantageous for the highly variable velocity fields experienced by trunkfish in their complex aquatic environment. All important morphological features of the carapace contribute to producing the hydrodynamic stability of swimming trajectories in this species.

Do Brown Trout Choose Locations with Reduced Turbulence

Abstract The physical habitat requirements of cover, depth, and current speed for brown trout Salmo trutta are associated with high shear zones in stream flows, which in turn result in high turbulence. Observations were made on current speeds and turbulence intensity (TI) in a sand-bed trout stream. Exemplary transects showed that current speeds ranged from 0 to 60 cm/s and that TI ranged from 0 to 0.7. Turbulence intensity was inversely related to current speed. Brown trout were usually found in the lower 5 cm of the stream, where shear forces result in high turbulence. Locations occupied by brown trout had lower TI than similar locations without brown trout but higher TI than is typical of an average stream.

Boxfishes as unusually well-controlled autonomous underwater vehicles

Boxfishes (family Ostraciidae) are tropical reef‐dwelling marine bony fishes that have about three‐fourths of their body length encased in a rigid bony test. As a result, almost all of their swimming movements derive from complex combinations of movements of their median and paired fins (MPF locomotion). In terms of both body design and swimming performance, they are among the most sophisticated examples known of naturally evolved vertebrate autonomous underwater vehicles. Quantitative studies of swimming performance, biomechanics, and energetics in one model species have shown that (i) they are surprisingly strong, fast swimmers with great endurance; (ii) classical descriptions of how they swim were incomplete: they swim at different speeds using three different gaits; (iii) they are unusually dynamically well controlled and stable during sustained and prolonged rectilinear swimming; and (iv) despite unusually high parasite (fuselage) drag, they show energetic costs of transport indistinguishable from those of much better streamlined fishes using body and caudal fin (BCF) swimming modes at similar water temperatures and over comparable ranges of swimming speeds. We summarize an analysis of these properties based on a dynamic model of swimming in these fishes. This model accounts for their control, stability, and efficiency in moving through the water at moderate speeds in terms of gait changes, of water‐flow patterns over body surfaces, and of complex interactions of thrust vectors generated by fin movements.