Dynamics of white perch Morone americana population contingents in the Patuxent River estuary, Maryland, USA

Alternative migratory pathways in the life histories of fishes can be difficult to assess but may have great importance to the dynamics of spatially structured populations. We used Sr/Ca in otoliths as a tracer of time spent in freshwater and brackish habitats to study the ontogenetic movments of white perch Morone americana in the Patuxent River estuary. We observed that, soon after the larvae metamorphose, juveniles either move to brackish habitats (brackish contingent) or take up residency in tidal fresh water (freshwater contingent) for the first year of life. In one intensively studied cohort of juveniles, the mean age at which individuals moved into brackish environments was 45 d (post-hatch), corresponding to the metamorphosis of lavae to juveniles and settlement in littoral habitats. Back-calculated growth rates of the freshwater contingent at this same age (median = 0.6 mm d–1) were significantly higher than the brackish contingent (median = 0.5 mm d–1). Strong year-class variability (>100-fold) was evident from juvenile surveys and from the age composition of adults sampled during spawning. Adult samples were dominated by the brackish contingent (93% of n = 363), which exhibited a significantly higher growth rate (von Bertalanffy, k = 0.67 yr–1) than the freshwater contingent (k = 0.39 yr–1). Combined with evidence that the relative frequency of the brackish contingent has increased in year-classes with high juvenile recruitment, these results implicate brackish environments as being important for maintaining abundance and productivity of the population. By comparison, disproportionately greater recruitment to the adult population by the freshwater contingent during years of low juvenile abundance suggested that freshwater habitats sustain a small but crucial reproductive segment of the population. Thus, both contingents appeared to have unique and complementary roles in the population dynamics of white perch.