Pelvic fin locomotor function in fishes: three-dimensional kinematics in rainbow trout (Oncorhynchus mykiss)

The paired pelvic fins in fishes have been the subject of few studies. Early work that amputated pelvic fins concluded that these fins had very limited, and mainly passive, stabilizing function during locomotion. This paper is the first to use three-dimensional kinematic analysis of paired pelvic fins to formulate hypotheses of pelvic fin function. Rainbow trout (Oncorhynchus mykiss) were filmed swimming steadily at slow speeds (0.13-1.36 BL s(-1)) and during manoeuvres (0.21-0.84 BL s(-1)) in a variable speed flow tank. Two high-speed cameras filmed ventral and lateral views simultaneously, enabling three-dimensional analysis of fin motion. During steady swimming, pelvic fins oscillate in a regular contralateral cycle. This cyclic oscillation appears to have active and passive components, and may function to dampen body oscillation and stabilize body position. During manoeuvres, pelvic fins move variably but appear to act as trimming foils, helping to stabilize and return the body to a steady swimming posture after a manoeuvre has been initiated. Fins on the inside of the turn move differently from those on the outside of the turn, creating an asymmetric motion. This paper challenges the understanding that pelvic fins have a limited and passive function by proposing three new hypotheses. First, pelvic fins in rainbow trout have complex three-dimensional kinematics during slow-speed steady swimming and manoeuvres. Second, pelvic fins are moved actively against imposed hydrodynamic loads. Third, pelvic fins appear to produce powered correction forces during steady swimming and trim correction forces during manoeuvres.