An Otolith Stable Isotope Approach to Evaluate Northwest Atlantic Mackerel Contingent Structure

Atlantic mackerel (Scomber scombrus) support an important commercial fishery and act as a forage fish for many larger species.   Despite this ecological and economic importance, there is large uncertainty regarding the stock structure and migration patterns of this species on both sides of the North Atlantic.  Stock assessments for Northwest Atlantic mackerel currently assume two unit stocks, comprising northern and southern contingents.  We are evaluating whether otolith δ¹⁸O and δ¹³C values can be used to separate contingent structure by testing for a relationship between these isotopes and latitude.  Over 400 archived otoliths from age-1 juveniles (NMFS Northeast Fisheries Center) collected across three years and broad regions were carefully milled to extract material corresponding to the first year of life.  Otolith carbonates are currently being analyzed by isotope ratio mass spectrometry and data will be presented from across the US Southern Mid-Atlantic Bight, New York Bight, and Georges Bank to determine whether the approach is practical.