Life-history plasticity in amphidromous and catadromous fishes: a continuum of strategies

Plastic life-history strategies in diadromous fishes have long been acknowledged but have often been viewed as anomalies. Until recently, techniques were lacking to investigate the prevalence and variety of life-history strategies. However, recent technical advances, such as otolith trace element and stable isotope analyses, have provided insights into the life-histories of migratory fish, often revealing considerable plasticity. Reviews of anadromy and catadromy examined the extent of plasticity in these life-histories; however amphidromy has not been reviewed. Amphidromy, the most widespread diadromous life-history (273 + sp.), consists of two types: freshwater amphidromy, where fish rear in the ocean as larvae and return to freshwater as juveniles for growth and reproduction, and marine amphidromy, where fish utilize the marine environment for larval growth, enter freshwater for a short time, and return to the marine environment for further growth and spawning. In this review, a detailed taxonomic examination of plasticity in amphidromous fishes is utilized to determine its prevalence and ecological role. Our results indicate plasticity, as evidenced by variable use of fresh or marine environments for key life-history stages, is present on both evolutionary and ecological scales in most families of amphidromous fishes. Such variability indicates amphidromy is not necessarily a diadromous migration, but is better viewed as a spatially extensive benthic-pelagic migration. Further, adult downstream migration by amphidromous fishes parallels catadromy, suggesting a life-history continuum linking fluvial, amphidromous, catadromous, and oceanadromous life-histories. The role of egg-size/fecundity tradeoffs, migration, salinity, and landscape are discussed in the context of benthic-pelagic centered life-histories.