Stable isotopic and trace elemental compositions of otoliths and the stock structure of Pacific cod, Gadus macrocephalus

We examined the stock composition and life history of Pacific cod, Gadus macrocephalus, in the northeast Pacific Ocean from sagittal otoliths collected from three marine fishing areas in Washington State, U.S.A. We analyzed both stable isotope ratios (δ18O and δ13C) and trace elemental concentrations (Sr, Mg, Na, Fe, Mn) for these otoliths. The combination of δ18O and δ13C, and correlation of δ18O vs. 1000Sr/Ca and 1000Mg/Ca showed clear separations between North Puget Sound and coastal cod, suggesting there might be two different spawning stocks in the region. The North Puget Sound cod might represent an ‘Estuary-type’ from the Strait of Georgia, whereas coastal cod might represent an ‘Ocean-type’ from the Pacific west coast. Isotopic variations from five representative otoliths also showed a two-stage life history for Pacific cod, and a critical transition period for cod in migration to the ocean or in age of sexual maturity. These chemical interpretations and conclusions appear in agreement with biological observations of Pacific cod, and are consistent with results of previous studies for other marine fish species in nearby areas.