Costs of being extreme — Do body size deviations from population or sire means decrease vitality in rainbow trout?

In animals, large or small body size may carry health costs. Large deviations from physiologically benign growth range can divert resources from vital body functions and cause developmental defects, thereby constraining the welfare and productivity of animals. When analyzing the phenotypic costs associated with growth variation, it is unclear whether slow or fast growth should be determined relative to an individual's own family background or relative to population's overall genetic level. That is, should the body size of an individual be related to its parental (genotypic) average, instead of a general population mean? Using the data from 60,518 individual rainbow trout, Oncorhynchus mykiss, we tested whether the negative and positive deviations of tagging body weight (one-summer-old fish, average weight 51g) relative to either a common population mean or sire-family means differently predict the probability of mortality and vertebrae defects during the subsequent grow-out period. Based on the logistic regression analysis, neither types of body weight deviation predicted the probability of vertebrae defect. In contrast, the probability of mortality showed a significant negative relationship with deviations of weight. The smallest individuals with large negative deviation from their sire-family means possessed the highest risk of death. However, there was a high positive correlation between the deviation types calculated from sire-family and population means (r=0.93), and thus their ability to predict mortality was very similar. This finding does not support our physiological capacity hypothesis stating that the phenotypic vitality costs would be more strongly associated with an individual's body size relative to its family's genetic mean level, rather than relative to the overall population mean. Rapid fingerling growth was not phenotypically related to either of the vitality costs studied. The largest fish, both with respect to population and sire-family means, had the lowest risk of death and were no more or less liable to developmental vertebral defects, compared to their smaller counterparts. These results emphasize the importance of narrowing growth variation, especially from the lower end of the growth profile, using mating and selection designs to reduce fish mortality during the communal grow-out period.