RESUMEN
Parasites impose costs on their hosts. The capability to fight against them is of great advantage, but may also be traded off with other traits. Although often observed at the phenotypic level, our knowledge of the extent to which such trade-offs are genetically determined is relatively poor. We tested this possibility with a farmed rainbow trout population suffering from natural Diplostomum spp. infections that cause cataracts in fish. We estimated the heritability of cataract severity and examined phenotypic and genetic correlations between cataract and a set of performance traits measured three times during a 3-year rearing period. A cataract score was used as an indicator of the host's capability to resist and/or tolerate the parasite. Our results showed moderate heritability for the cataract. Nevertheless, we found no evidence for a genetic or phenotypic trade-off between parasite resistance/tolerance and the measured performance traits. Initial body weight was not correlated with the cataract score. Phenotypic and genetic correlations of cataract severity with body mass and condition measured in the second and third year were strongly negative, indicating reduced growth and condition in fish with a high cataract score. The reduced body size and condition in cataract-bearing fish were probably reflected in the phenotypic association between a high cataract score and delayed maturity age in females. Put together, our study did not provide evidence of genetic or phenotypic trade-offs between Diplostomum resistance/tolerance and a number of performance traits. Therefore, selection for lessened Diplostomum-caused cataracts is unlikely to have a negative impact on the studied performance traits.