Genetically determined resistance and tolerance to Diplostomum sp. parasite in farmed rainbow trout.

Parasite infectivity, virulence and host resistance have been in the centre of the scientific interest when it comes to host–parasite relationships. In addition to resistance, hosts may also vary in their tolerance against parasites. This is important to notice because resistance and tolerance have different consequences in host–parasite co‐evolution. Here, we show that families of farmed rainbow trout (Oncorhynchus mykiss) show both host defence strategies, resistance, and tolerance, against infectivity and virulence of Diplostomum sp. (Trematoda) parasites. Both strategies have moderate genetic variation and are genetically independent of each other. It is also shown that the families having the highest performance measured as higher weight, better condition factor and lower mortality in absence of the parasites suffer the most when parasitism increases. For practical breeding programmes, this means that both resistance and tolerance can be improved by selection without compromising one of the strategies. These results give new insight into defence strategies against parasites in fish and into processes of fish–parasite co‐evolution. [ABSTRACT FROM AUTHOR]