Hutchings, J.A Age- and size-specific costs of reproduction within populations of brook trout, Salvelinus fontinalis. Oikos 70: Abstract Age and size at maturity influence a survival cost of reproduction in brook trout, Salvelinus fontinalis, within three unexploited populations in southeastern Newfoundland, Canada. Post-reproductive, overwinter survival increased significantly with female body size in all three populations but was positively associated with male body size in only one. Reproduction increased the age-specific risk of overwinter mortality by 17-89% and appears to be costly for brook trout because it reduces the lipid reserves upon which individuals depend to survive winter. This survival cost of reproduction (overwinter survival of reproductive individuals relative to that of non-reproductive individuals) increased with age but was negatively associated with body size within each population. The increase in cost with increasing age can be attributable to senescent decline in body condition. Physiological constraints imposed by metabolic allometry and the diversion of lipids from soma to gonads may be responsible for higher reproductive costs in small females; higher costs among small males may result from physical injury during mate competition. Empirically-based interactions among reproductive cost, age and body size provide a theoretical basis for predicting life history responses by salmonid fish to environmental change.