Age Validation of Pacific Cod Using Stable Oxygen Isotopes (δ 18O)

Measurements of stable oxygen and carbon isotopes (δ¹⁸O and δ¹³C) were obtained from Pacific cod (Gadus macrocephalus) otoliths using a high resolution micro-sampling coupled with mass spectrometry. Up to 9-10 discrete measurements were obtained from any one annual growth zone of 9 otoliths from Pacific cod that were tagged with electronic data storage tags and at liberty between one and two years in the Eastern Bering Sea and Gulf of Alaska (Kodiak Island). The sequential δ¹⁸O measurements, representing the full lifespan of the fish, were examined for seasonal variations which can be associated with changes in water temperature. Taking advantage of the well established relationship between δ¹⁸O of biogenic marine aragonite and water temperature, the goals of this study were to: 1) validate Pacific cod ageing criteria of typical growth-zone counts with seasonal signatures of otolith δ¹⁸O, and 2) verify the relationship between otolith δ¹⁸O and temperature using archival tag temperatures. Pacific cod otolith δ¹⁸O signals showed seasonal variation which was assumed to be driven by variations in seasonal bottom temperatures. In more than half of the samples, Pacific cod otolith δ¹⁸O showed the expected cyclical pattern consistent with seasonal variation in temperatures. In some of these the number of δ¹⁸O maxima showed a close correspondence to the expected growth-zone age estimates, validating standard age interpretation methods, but overall the results were not completely definitive. However, there was a statistically significant relationship between δ¹⁸O and archival tag bottom temperatures (r=0.74, p<0.01) which was consistent with other published studies from the North Atlantic and may suggest that Pacific cod otolith aragonite is deposited in oxygen isotopic equilibrium with ambient sea water.