Bioenergetic trajectories underlying partial migration in Patuxent River (Chesapeake Bay) white perch (Morone americana)

Partial migration, the coexistence of resident and migratory individuals within the same population, may be common in fish populations. A proposed mechanism underlying partial migration is differing dispersive responses to early growth conditions, but few studies have explicitly tested this. During their first year of life, white perch (Morone americana) in the Patuxent River (Maryland, USA) exhibit either residency in freshwater natal habitats (resident contingent) or disperse down-estuary into brackish habitats (dispersive contingent). We tested whether white perch juveniles exhibited differing growth and metabolic trajectories based on contingent membership or in response to salinity. A randomized factorial experiment with two contingent types and two salinity treatments (1 and 8) was conducted over a 30-day period. The experiments supported a contingent effect, with the dispersive contingent exhibiting higher consumption rates and a higher scope for growth. In addition, we identified a weak salinity effect with evidence of increased consumption and routine metabolism in mesohaline conditions. Juvenile growth rates calculated from individuals in the field supported laboratory results, with dispersive contingent members exhibiting higher growth rates. We conclude that contingent membership and the related phenomenon of partial migration in this population is associated with varying energetic tactics that significantly influence the scope for growth.