| Summary: | Hosts can defend themselves against parasites by either preventing or limiting infections (resistance), or limiting parasite-induced damage (tolerance). However, it remains underexplored how these defense types vary over host development with shifting patterns of resource allocation priorities. Here, we studied the role of developmental stage on resistance and tolerance in Atlantic salmon ( Salmo salar ). This anadromous fish has distinct life stages related to living in fresh and sea water. We experimentally exposed one-year old salmon, either at the freshwater stage or at the stage transitioning to the marine phase, to the trematode Diplostomum pseudospathaceum . Using 56 pedigreed families and multivariate animal models, we show that developmental transition is associated with reduced resistance, but does not affect tolerance. Furthermore, by comparing tolerance slopes (host fitness against parasite load) based on additive genetic effects among infected and unexposed control relatives, we observed that the slopes can be largely independent of the infection, that is they may not reflect tolerance. Together, our results suggest that the relative importance of different defense types may vary with host development and emphasize the importance of including control treatments for more confident interpretations of tolerance estimates. Funding provided by: Academy of Finland Crossref Funder Registry ID: Award Number: 314254 Funding provided by: Academy of Finland Crossref Funder Registry ID: Award Number: 314255 Funding provided by: Academy of Finland Crossref Funder Registry ID: Award Number: 327255 Funding provided by: Academy of Finland Crossref Funder Registry ID: Award Number: 310632 Funding provided by: H2020 European Research Council Crossref Funder Registry ID: Award Number: 742312 Funding provided by: European Maritime and Fisheries Fund Crossref Funder Registry ID: Award Number: 43521
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