Ecological and evolutionary patterns of freshwater maturation in Pacific and Atlantic salmonines

Abstract

Reproductive tactics and migratory strat- egies in Pacific and Atlantic salmonines are inextri- cably linked through the effects of migration (or lack thereof) on age and size at maturity. In this review, we focus on the ecological and evolutionary patterns of freshwater maturation in salmonines, a key process resulting in the diversification of their life histories. We demonstrate that the energetics of maturation and reproduction provides a unifying theme for under- standing both the proximate and ultimate causes of variation in reproductive schedules among species, populations, and the sexes. We use probabilistic maturation reaction norms to illustrate how variation in individual condition, in terms of body size, growth rate, and lipid storage, influences the timing of maturation. This useful framework integrates both genetic and environmental contributions to condi- tional strategies for maturation and, in doing so, demonstrates how flexible life histories can be both heritable and subject to strong environmental influ- ences. We review evidence that the propensity for freshwater maturation in partially anadromous species is predictable across environmental gradients at geo- graphic and local spatial scales. We note that growth is commonly associated with the propensity for fresh- water maturation, but that life-history responses to changes in growth caused by temperature may be strikingly different than changes caused by differences in food availability. We conclude by exploring how contemporary management actions can constrain or