Critical Swimming Speeds of Coho Salmon (Oncorhynchus kisutch) Fry to Smolt Stages in Relation to Salinity and Temperature

Critical swimming speeds (fork lengths per second L/s) of early fry, advanced fry, and pre-smolt coho salmon, Oncorhynchus kisutch, surpassed those of smolts. Performances were inversely related to size, varying from a peak of 7.3 L/s in fry to 5.5 L/s in smolts. In early fry to pre-smolt stages, salinity had little effect whereas in smolts a distinct maximum performance occurred near 13‰. Swimming performances of all stages varied directly with temperature: maxima occurred between 20 and 23 °C. As temperature decreased, critical speeds of early fry declined at a faster rate than that of later stages, the values at 23 and 3 °C being 7.2 and 3.5 L/s in early fry and 5.5–3.5 in smolts. This study shows that on a size-related basis underyearling coho are capable swimmers in estuarine conditions. Thus, their apparent failure to survive premature seaward migrations cannot be explained by their inability to perform important locomotor-dependent behaviors.