To Sea or Not to Sea? Anadromy Versus Non-Anadromy in Salmon ids

Abstract

Abstract “Migration” has been defined in many ways that have a variety of connotations (Endler 1977; Dingle 1996). Here we follow Endler (1977) in considering migration to be “relatively long-distance movements made by large numbers of individuals in approximately the same direction at approximately the same time ...usually followed by a regular return migration.” This definition is useful because it corresponds to salmonid migrations, and because it excludes dispersal, which is the subject of another chapter (Hendry et al. 2003a-this volume). Breeding often takes place at one terminus of the migration but not the other, and migrations may sometimes be undertaken multiple times over the course of an individual’s life. Moreover migratory tendency varies at multiple levels: among species, populations, sexes, individuals, and years. In this chapter, we use salmonids (primarily salmon, trout, and charr) as a model system for examining evolutionary factors that may generate and maintain this variation. Variation in migratory tendency is present at many levels in salmonids, making them a good system for examining how, when, and why migration will evolve, as well as how other traits will evolve in parallel. Salmonid migrations take place between a variety of contrasting environments (small streams vs. large rivers, streams vs. lakes, fresh water vs. the ocean) but we focus our analysis on migrations between fresh water and the ocean (“anadromy”; McDowall 1988). In salmonids, anadromy takes the form of individuals that hatched in fresh water migrating to the ocean, and later returning to breed in fresh water. In some instances, individuals may move back and forth between fresh water and the ocean several times in their life, and may enter and leave fresh water without breeding.