Hovering in still water (0 BL s^-1) at 0 (A,B) and 50 ms (C,D). This behavior involves maintenance of both horizontal and vertical body position, and is characterized by moderate erection of the dorsal fin and low-speed sculling of the pectoral fins beneath the body. The left and right pectoral fins move out of phase with each other such that when one fin is protracted the contralateral fin is retracted. Abbreviations as in Fig. 1.

Low-speed turning (non-fast-start escape response). While swimming steadily at 0.5 BL s^-1, trout are exposed to a visual and auditory stimulus (at 0 ms; A,B), which elicits rapid abduction of the strong-side pectoral fin (i.e. the fin closer to the source of the stimulus). The weak-side pectoral fin shows slower and delayed abduction. These propulsor motions are accompanied by slight dorsal fin erection and abduction of the strong-side pelvic fin (at 110 ms; C,D). During the turning maneuver, the body of the fish yaws and translates toward the weak side (at 130 ms; E,F). Abbreviations as in Fig. 1.

Braking maneuver. Trout swimming steadily at 0.5 BL s^-1 (at 0 ms; A,B) react to an upstream stimulus by abducting the left and right pectoral fins simultaneously and erecting the dorsal fin (at 200 ms; C,D). The pectoral fins' trailing edges are elevated and protracted resulting in a characteristic `cupping' of the fins along their longitudinal axes. These fin motions decelerate the body and cause the snout to pitch ventrally (at 375 ms; E,F). Abbreviations as in Fig. 1.

Velocity of body excursions in three dimensions during turning and braking maneuvers. Text labels on the X, Y and Z reference axes (cf. Fig. 1) signify the direction of body movement. Each plotted point reflects the distance traveled by a reference point on the pectoral fin base over the entire fin stroke duration. Turning is characterized by anterior body movement and significantly faster translation toward the weak side than braking. Both maneuvers involve sinking in the water column (i.e. Y velocity<0).

Visualization of pectoral-fin wake flow during hovering. (A) Schematic illustration of the left pectoral fin in a protracted position (cf. Fig. 2A) intersecting a horizontal laser plane (broken line). High-speed video images of this plane (XZ) recorded from below were used to calculate velocity vector fields, an example of which is shown in (B). The fish maintains its position in still water using asymmetrical left-right pectoral fin motions (direction indicated by red arrows). As the fin at left protracts, fluid behind the fin is entrained and drawn anteriorly. At the same time, the fin at right retracts and sheds attached flow posteriorly. These momentum flows are balanced on the following half-stroke as each fin assumes the other's position. The center of mass of the body (CM) of trout used in this study was located at a longitudinal position 39±2% BL (mean ± S.D.) posterior to the snout.

Representative wake flow patterns during pectoral-fin turning and braking maneuvers. (A,C) Line drawings (not to scale) depict trout in ventral view during turning with the strong-side pectoral fin abducted (A) (cf. Fig. 3D) and in lateral view during braking with the fin `cupped' (C) (cf. Fig. 4C). Boxed regions indicate areas within the laser light sheet for which velocity vector fields were calculated. (B) During slow turning, pectoral fin abduction generates a single vortex within the horizontal plane with an anterolateral-facing fluid jet. Fin adduction on the following half-stroke contributes no additional vorticity within this plane of analysis. (D) During braking, elevation and abduction of both pectoral fins at once generates a strong vortex on each side of the body visible in the vertical plane (clockwise flow at right side of panel); subsequent depression and adduction of the fins produces weaker counterrotating vortices (counterclockwise flow centered above base of fin). The central fluid jet between paired rotational centers is oriented anterodorsally. In B and D, the mean free-stream flow velocity (0.5 BLs^-1 in the X direction) has been subtracted from each velocity vector. CM, center of mass of the body.

Kinematic repertoire of the pectoral fin of rainbow trout. (A) During steady swimming, the fin remains adducted against the body (cf. Fig. 1A). The enlarged image of the fin below the body illustrates the angle of inclination of the fin base (dotted line) and the first fin ray (thick line), whose proximal end is indicated by an asterisk. During the maneuvering behaviors examined in this study, pronounced rotation and flexion of the pectoral fin was observed. In B—D, white and red areas indicate fin surfaces that face laterally and medially, respectively, when the fin is at rest in an adducted position (as in A). (B) While hovering, trout twist the fin along its spanwise axis (cf. Fig. 2A) to enable fore-and-aft sculling beneath the body. (C) Turning is characterized by rotation of the fin in the opposite direction above the ventral body margin (cf. Fig. 3C). (D) Braking involves fin rotation in the same direction as during turning, but to a greater degree such that the fin surface which faces medially at rest becomes dorsolaterally oriented (cf. Fig. 4C). Note that the pectoral fin base rotates to a nearly horizontal orientation during maneuvering locomotion. The considerable kinematic versatility of the trout pectoral fin permits a range of locomotor functions comparable to that of more derived teleost fishes.

Experimental evaluation of the braking hypothesis of Breder (1926). (A) Fishes such as trout possessing pectoral fins located ventrally on the body are predicted to exert an anteriorly directed braking force (black vector). The reaction to this horizontal momentum flow (gray vector) decelerates the body. We tested Breder's hypothesis that the line of action of the braking force lies below the center of mass of the body (CM) using anatomical and hydrodynamic measurements from Oncorhynchus mykiss. (B) An arbitrarily oriented braking reaction force (stroke-averaged) is shown to illustrate two angles within the parasagittal plane: (i) the angle α between the longitudinal axis of the fish and the line of action of the braking force acting on the fin; (ii) the angle β between the longitudinal axis of the fish and the line connecting the center of mass of the body with the centroid of the pectoral fin in its fully extended position during braking. Breder's hypothesis is supported if α is significantly less than β.