During offshore sampling conducted between 2005 and 2007, we discovered large concentrations of the blue crab Callinectes sapidus spawning between 4 and 50 km offshore of the Louisiana coast; the greatest concentrations were found on high-relief sandy shoals. We used natural abundance carbon and nitrogen isotopes to evaluate C. sapidus migratory source locations within south-central Louisiana estuarine systems as well as residency on the shoals. There was an east-west trend of decreasing delta C-13 and increasing delta N-15 for offshore crab tissue (muscle and ovary) related to proximity to the Atchafalaya River. This indicates that crabs predominately migrate directly offshore from their home estuary rather than migrating long distances alongshore. Many delta C-13 values for offshore crab muscle and ovary were depleted relative to typical salt marsh values, indicating that some female blue crabs migrate directly offshore from low salinity regions higher in the estuary. The convergence of muscle and ovary isotope values towards a proxy for offshore residence (i.e. Callinectes similis mean isotope values) indicated that migrating C. sapidus utilize offshore prey resources and do not typically re-enter inshore estuaries during the spawning season. A correlation between crab delta C-13 values and the body size of shell-associated epibiotic acorn barnacles Chelonibia patula indicated that crabs found on shoals acquire offshore isotopic values over time. These findings have important research implications for studies involving animal migrations, as well as for management of this ecologically and economically important species. The findings provide evidence of a direct link between the inshore fishery and female blue crabs captured offshore in unique sandy shoal habitats, which likely support at least 20% of the known Louisiana blue crab spawning stock west of the Mississippi River.