Heritable variation in swimming performance in Nile tilapia (Oreochromis niloticus) and negative genetic correlations with growth and harvest weight.

Nile tilapia is predominantly produced in smallholder ponds without aeration. We hypothesize that Nile tilapia with high oxygen uptake efficiency (OUE) may perform better under these conditions than Nile tilapia with low OUE. Critical swimming speed (U, in cm s) is a potential indicator for OUE. Our objectives were to estimate variance components for U and fish size at swim testing early in life, and genetic correlations (r) between U with harvest weight (HW) and daily growth coefficient (DGC) later after grow-out in a non-aerated pond. Substantial heritability was found for absolute U (0.48). The estimated r between absolute U and fish size at testing were all strong and positive (range 0.72-0.83). The estimated r between absolute U and HW, and absolute U and DGC were - 0.21 and - 0.63 respectively, indicating that fish with higher absolute U had lower growth in the non-aerated pond as compared to fish with lower absolute U. These results suggest a juvenile trade-off between swimming and growth performance where fish with high U early in life show slower growth later under conditions of limited oxygen availability. We conclude that U in Nile tilapia is heritable and can be used to predict growth performance.