Rainbow trout provide the first experimental evidence for adherence to a distinct Strouhal number during animal oscillatory propulsion.

The relationship between tail (or wing) beat frequency (f(tail)), amplitude (A) and forward velocity (U) in animals using oscillatory propulsion, when moving at a constant cruising speed, converges upon an optimum range of the Strouhal number (St = f(tail) · A/U). Previous work, based on observational data and supported by theory, shows St falling within the broad optimum range (0.2<St<0.4) and considers this adequate to indicate its importance in governing wing or tail kinematics. This study presents the first evidence using an experimental manipulation that supports the importance of maintaining kinematics at a single optimum (or preferred) St. The tail beat kinematics of rainbow trout, Oncorhynchus mykiss, were disturbed by increasing water temperature (T(water)) from 11 ± 1 to 20 ± 1 °C. Elevated T(water) increased f(tail) and decreased A, whilst St at any given U was conserved. St increased with U, driven by concomitant increases in A, whilst f(tail) was unaffected by U. An increase in T(water) also increased basal metabolic costs, but did not affect the incremental increase in metabolic cost with increasing U. Predicted future changes to T(water) of lakes and rivers (5-10 °C over the next 100 years) may not present major locomotory problems to salmonids.