Stock assessments for Northwest Atlantic mackerel (Scomber scombrus) assume a single stock, comprised of northern and southern contingents, each with distinct natal regions in the US and Canada. I hypothesized that otolith δ18O and δ13C values would provide discrimination between these regions based upon hydrographic differences, and such information could illuminate seasonal migrations and contingent structure. Otoliths from regions throughout the North Atlantic Ocean were carefully milled to

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... xtract carbonates corresponding to the first year of life. Significant differences occurred in otolith isotope composition across the Atlantic basin, and within the Northwest Atlantic stock, despite annual variability. In the Northwest Atlantic, two separate natal habitats and associated contingents were supported for juvenile mackerel, but incursions by the northern contingent into US waters occurred in older fish (age>2). These findings indicate that stock structure assumptions should be revisited in the assessment and management of Northwest Atlantic mackerel.