Log Transformed Variance (Lnvar) from Individual Growth Curves as a Potential Indicator of Resilience in Nile Tilapia (Oreochromis Niloticus)

Abstract

Resilience is defined as the ability of the animal to cope with environmental changes or the consistency in performance over time. Resilience can be measured as variation in the deviations of actual weights from expected weights on longitudinal measurements. Log transformed variance of these deviations (LnVar) is the most promising indicator for resilience based on its moderate heritability. In this study, we calculate LnVar by fitting the expected individual growth curve based on longitudinal observed weight (LnVarind). This growth curve represents the ideal growth trajectory for fish in the absence of disturbances. We use a dataset from Nile tilapia (Oreochromis niloticus) that were grown in either an aerated or a non-aerated freshwater pond and where weight was measured at five time-points during the grow-out period. We hypothesize that LnVarind should be more expressed in non-aerated than aerated ponds. The objectives of this study were 1) to estimate genetic parameters for LnVarind in Nile tilapia, 2) to estimate the genetic correlation between LnVarind and growth to explore the effects of selection for growth rate on LnVarind and 3) to estimate the genetic correlation for LnVarind between aerated and non-aerated ponds. The heritability estimate for LnVarind (0.28) in the non-aerated ponds was higher than LnVarind in aerated ponds (0.06). In the aerated ponds, genetic correlation (rg) of LnVarind was-0.44±0.23 with daily growth coefficient (DGC) and-0.45±0.24 with harvest weight (W5). In the non-aerated ponds, the rg with DGC and W5 were-0.68±0.12 and-0.52±0.17 respectively. This high and negative genetic correlation suggests that selection for fish with high growth rate will also improve LnVarind. However, genetic correlation of LnVarind between aerated and non-aerated ponds was 0.50 suggesting that genetic improvement for growth in the aerated environment will not automatically lead to improved LnVarind in the non-aerated environment. Therefore it is beneficial to incorporate information of records on relatives in the non-aerated ponds if the breeding program is aiming for this environment.