Domestication is associated with reduced burst swimming performance and increased body size in clonal rainbow trout lines

• We assess sprint speed as a possible trade-off for increased body size.
• We use clonal rainbow trout lines, varying in generations under size selection.
• The size-selected clonal lines had increased body size but slower sprint speed.

Domestication selection combined with inadvertent selection for high growth rate and large body size are common in hatchery salmonids. However, evolutionary trade-offs of various fitness components with body size and domestication are likely. This study explores potential trade-offs between growth and sprint swim performance among five clonal lines of rainbow trout (Oncorhynchus mykiss), varying in the number of generations under domestication. We hypothesized that clonal lines of trout derived from highly domesticated populations would have higher growth rates but slower sprint swimming performance relative to clonal lines derived from less-domesticated populations. Individuals from each of the five lines were repeatedly measured for body size and sprint swimming performance over 10–15 weeks. Our findings indicate significant differences in body size and sprint swimming performances between the less-domesticated and the more-domesticated lines, with more-domesticated lines being larger but poorer swimmers. Implications for the survival and reproduction of hatchery salmonids released into the wild are explored.