Energy metabolism fuels swimming and other biological processes.We compared the swimming performance and energy metabolism within and across eight freshwater fish species.Using swim tunnel respirometers,we measured the standard metabolic rate(SMR)and maximum metabolic rate(MMR)and calculated the critical swimming speed(Ucrit).We accounted for body size,meta-bolic traits,and some morphometric ratios in an effort to understand the extent and underlying causes of variation.Body mass was largely the best predictor of swimming capacity and metabolic traits within species.Moreover,we found that predictive models using total length or SMR,in add-ition to body mass,significantly increased the explained variation of Ucrit and MMR in certain fish species.These predictive models also underlined that,once body mass has been accounted for,Ucrit can be independently affected by total length or MMR.This study exemplifies the utility of multiple regression models to assess within-species variability.At interspecific level,our results showed that variation in Ucrit can partly be explained by the variation in the interrelated traits of MMR,fineness,and muscle ratios.Among the species studied,bleak Alburnus alburnus performed best in terms of swimming performance and efficiency.By contrast,pumpkinseed Lepomis gibbo-sus showed very poor swimming performance,but attained lower mass-specific cost of transport(MCOT)than some rheophilic species,possibly reflecting a cost reduction strategy to compensate for hydrodynamic disadvantages.In conclusion,this study provides insight into the key factors influencing the swimming performance offish at both intra-and interspecific levels.