Partial migration of an estuarine fish

L.A. Kerr, D.H. Secor, P. Piccoli

University of Maryland Center for Environmental Science 

Chesapeake Biological Laboratory

Abstract

‘Partial migration’ is a term adopted from avian literature to describe life cycle diversity observed in Salmonidae but has not been widely applied outside this family.  It describes a population in which a portion of the population is migratory and another portion is sedentary.  Lifetime patterns of white perch (Morone americana) habitat use in the Patuxent River estuary (Chesapeake Bay, MD) were examined using otolith strontium:calcium (Sr/Ca) measures, which were determined to be positively related to salinity in this system.  Sixty adult otoliths (30 female and 30 male) were analyzed for Sr/Ca at sub-annual periodicity to test for age- and sex-dependent differences in habitat use and to examine the persistence of divergent habitat use initiated during the juvenile stage.  Lifetime patterns in habitat use revealed white perch exhibit partial migration, with a portion of the population residing in freshwater natal habitats (retentive contingent) and another portion migrating down-estuary into brackish water habitats (dispersive contingent).  Adult habitat use was significantly affected by juvenile habitat use (ages 1 and 2), whereas sex and its interaction with year had no significant effect.  Thus, alternative life history tactics, initiated during the juvenile period, persisted over the lifetime of the individual and had population-level consequences.  Members of the dispersive contingent dominate the adult population and exhibit faster growth rates compared to retentive fish.  However, faster growth rates are thought to be balanced by greater mortality rates.  Freshwater residents, exhibit slower growth rates and are expected to have relatively high survival, but anthropogenic pollution and habitat degradation in the freshwater regions of estuaries may compromise this tradeoff.  The interaction of movement patterns, habitat use and growth demonstrate that life cycle diversity (partial migration) is an important factor in population dynamics of white perch.