Using delta super(13)C and delta super(15)N to determine the migratory history of offshore Louisiana blue crab spawning stocks

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 super(13)C and increasing delta super(15)N 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. 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.