Differential migration in Chesapeake Bay striped bass

Differential migration - greater migration level with increasing individual size - is common in migratory species. Like other forms of partial migration, it provides spatial buffering against regional differences in habitat quality and sources of mortality. We investigated differential migration in striped bass, a species with well-known plasticity in migration behaviors.  A size-stratified sample of Potomac River (Chesapeake Bay) striped bass was implanted with acoustic transmitters and their subsequent coastal migrations recorded over a 4-yr period by telemetry receivers throughout the Mid-Atlantic Bight and Southern New England. A logistic regression predicted that ≥ 50% of both males and females depart the Chesapeake Bay at large sizes > 80 cm total length. Egressing striped bass exited through both the Chesapeake Bay and Delaware Bay (C&D Canal), favoring the former. All large fish migrated to Massachusetts coastal waters and in subsequent years repeatedly returned to regions within Massachusetts and Cape Cod Bays. Within this dominant behavior, minority behaviors included straying, skipped spawning, and residency by large individuals. Analysis of last day of transmission indicated that small resident striped bass experienced nearly 2-fold higher loss rates (70% yr-1) than coastal emigrants (37% yr-1). The study confirmed expectations for a threshold size at emigration and different mortality levels between Chesapeake Bay (resident) and ocean (migratory) population contingents; and supported the central premise of the current assessment and management framework of a two-contingent population: Chesapeake Bay and ocean contingents discriminated by size. An improved understanding of differential migration thus affords an opportunity to specify stock assessments according to different population sub-components, and tailor reference points and control rules between regions and fishing stakeholder groups.