Migratory costs and the evolution of egg size and number in introduced andindigenous salmon populations

The trade-off between reproductive investment and migration should be an im portant factor shaping the evolution of life-history traits among populatio ns following their radiation into habitats with different migratory costs a nd benefits. An experimentally induced difference in migratory rigor for fa milies of chinook salmon (Oncorhynchus tshawytscha). of approximately 86 km and 413 in elevation, exacted a cost to somatic energy reserves (similar t o 17% reduction in metabolizable mass) and ovarian investment (13.7% reduct ion in ovarian mass). This cost was associated with a reduction in egg size and paralleled the phenotypic pattern of divergence between two introduced New Zealand populations of common origin, presently breeding at sites with different migration distances, The genetic pattern of divergence of these same populations, detected under common rearing, was consistent with compen sation for migratory costs (the population that migrates farther invested m ore in ovarian mass), but egg number more than egg size was associated with this evolution. These evolutionary patterns are consistent with what is kn own of the inheritance of these traits and with trade-offs and constraints favoring initial evolution in offspring number over offspring size. Analysi s of egg number-size patterns of other Pacific salmon populations in their native range supported the hypothesis that migration strongly influences pa tterns of reproductive allocation, favoring a higher ratio of egg number to egg size with greater migration distance.