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Age & Growth Program

Preliminary Age Validation of Pacific Cod Using Stable Oxygen Isotope (δ18O) Signatures in Otoliths

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Figure 9.  Map of collection locations for each Pacific cod otolith specimen in the eastern Bering Sea.

Pacific cod (Gadus macrocephalus) is the second most important groundfish managed by the North Pacific Fishery Management Council, with harvests totaling more than 250,000 metric tons (t) annually and recent ex-vessel values of more than $250 million. Pacific cod are assessed using age-structured models where age data are key to estimating mortality, recruitment, and harvest.  Because the age data for Pacific cod are so controversial, the stock assessments for each of the last several years have included, as an alternative, at least one model with the age data excluded.  Over the five most recent assessments, the model with age data excluded has always resulted in a higher estimate of the acceptable biological catch (ABC), ranging from 2% to 13% (6,000-30,000 t) above the ABC estimated by the preferred model with age data included. 

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Figure 10.  Micromilling system: full Carpenter CM-2 system and close up of micromilling stage.

Figure 11. Pacific cod otolith estimated to be 5 years old by age readers in the Age and Growth Laboratory. The sequence of δ18O values show the “signature” revealing five distinct peaks which represent annual lows in bottom temperature. The plotted data represents a “life history transect” sampling in the otolith from birth to capture.    
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Figure 12. Plot of relationship between temperature and d18O. The d18O in the most recently deposited material at the edges of juvenile otoliths is related to a broad range of bottom temperatures measured at capture.    

Needless to say, Pacific cod age determination is one of the most difficult of the over 30 species aged each year by the Age and Growth program. Historically, uncertainty has existed in interpretation of otolith growth zones during the process of age determination. Because Pacific cod are relatively short lived, proven age validation techniques using bomb radiocarbon are at present not possible.  However, we initiated an alternative strategy to estimate Pacific cod age accuracy with new geochemical techniques such as stable oxygen isotope (δ18O) signatures in otoliths. This approach is based upon the well-established principle that variability in marine carbonate δ18O is inversely related to water temperature. Therefore, the δ18O signature in an otolith, determined with high-resolution sequential microsampling of otoliths and mass spectrometry, inversely mirrors annual seasonal temperature cycles, and can be used as a tool for age validation. This project is utilizing forty specimens collected from the eastern Bering Sea (Fig. 9). To date we have analyzed nearly half of the samples.

Members of the Age and Growth program have also developed the capability to precisely sample otoliths, producing calcium carbonate samples as small as 25 µg with a computer controlled Carpenter CM-2 micromilling system (Fig. 10). We sampled Pacific cod otoliths from specimens estimated by growth zone counts to be 2 to 5 years old.

High-resolution micromilling provided up to ten samples (data points) per posited annual growth zone; which in some cases approached 50 sequential samples per specimen. The sequential sampling represented each specimen’s full life history, birth to capture. Collaborating with staff at Oregon Status University, we then analyzed each sample using flow ratio isotope mass spectrometry for δ18O. The δ18O life history profile revealed annual seasonal information for age validation and life history information such as possible migration (Fig. 11). We also sampled the most recently deposited material at the edges of juvenile otoliths (n=10) and related this to a broad range of bottom temperatures measured at capture (Fig. 12), demonstrating the strength of the relationship between δ18O in otoliths and water temperature (r2 = 0.74). 

Craig R. Kastelle, Thomas E. Helser, Jennifer McKay, Delsa M. Anderl



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