Carryover effects of early growth and river flow on partial migration in striped bass Morone saxatilis

High plasticity in the migrations of marine fishes can result from partial migration, where early life conditional responses to nursery conditions carryover into juvenile and adult migrations. Striped bass Morone saxatilis movement behaviors during the larval and early juvenile period were investigated for evidence of partial migration through otolith Sr/Ca profile ana lysis. Striped bass expressed expected behaviors, with contingents remaining in freshwater habitats through the early juvenile period (through 35 mm TL) or moving across the salt front to brackish water habitats (salinity >= 3); still, the range of sizes at which migration occurred was unexpectedly wide. Using a multivariate clustering analysis, 3 distinct size-based migratory contingents were identified. Two of these migrant groups dispersed following metamorphosis to the juvenile stage, while the third (and smallest) dispersed during the larval period. Both tendency to migrate and the size at which movement commenced were related to early growth, with migrants experiencing lower growth than residents. In one year, a high flow event appeared to influence the timing of migration, coinciding with the majority of dispersal events. Expected dividends for migratory juveniles in the form of improved forage (diet and stable isotope analysis), condition and growth were not detected. Partial migration within natal estuaries during ontogeny appears to be a conditional response in striped bass, possibly resulting from the interaction of liability traits and innate thresholds and influenced by environmental conditions.