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Age & Growth Program

Fish Age Validation with Radiocarbon from Atomic Bombs

figure 2, see caption
Figure 2.  Results of bomb-produced radiocarbon (C-14) age validation studies.  The reference is Pacific halibut (line is a LOESS smooth of measured C-14 representing 1944 to 1981).  The test species displayed in comparison to the reference are northern rockfish, Dover sole, and yellowfin sole.

The Age and Growth Program works to test and improve the accuracy of fish ages generated for stock assessments. Historically the program has used a number of methods for age validation studies, but in recent years bomb-produced radiocarbon (C-14) has been a method of choice.

During the Cold War era in the 1950s to 1960s, above-ground testing of atomic bombs led to a substantial increase in marine C-14. Any marine organism alive at that time incorporated bomb-produced C-14 in their calcified hard parts, providing a time stamp for age estimation.The bomb radiocarbon age validation method relies on fish of known age that were alive during the era of increasing C-14 and on measurements of C-14 in their otolith cores to create a "reference."

The otolith core is material deposited in the first year of life. In age validation studies, C-14 in the otolith cores of "test" specimens is compared to the reference. If the increases in C-14 from the test and reference specimens are synchronous, the test specimens' posited birth years, based on otolith growth zone counts, are considered accurate.

The Age and Growth Program has recently used bomb-produced C-14 in a number of species to validate the accuracy of growth-zone-count ages. Age validation studies on Pacific ocean perch and Dover sole from the Gulf of Alaska (GOA) have been completed and published. Studies on three other species—yellowfin sole, northern rockfish, and Greenland halibut—are in various stages of completion.

A reference from the GOA used extensively for these studies is Pacific halibut (Fig. 2 above). For an example of this method, Figure 2 shows three test species in comparison to the reference: northern rockfish from the GOA, Dover sole from the GOA, and yellowfin sole from the eastern Bering Sea (EBS).

Northern rockfish and Dover sole are examples where the test specimens are in phase with the Pacific halibut reference. For these two species, the appearance of a small bias where the measured test values of C-14 lie to the left of reference is expected due to larger core sizes that represented more than the first year of life. Therefore, northern rockfish and Dover sole age estimates were deemed accurate.

The third test species used as an example in Figure 2 is yellowfin sole from the EBS. Yellowfin sole from the EBS have a much larger bias to the left of the GOA reference and have much higher levels of C-14. These dissimilarities are likely due to geographical differences in marine bomb-produced C-14 between the GOA and EBS and point out the need for area-specific C-14 references.

In two other species, giant grenadier and shortraker rockfish, this age validation method was not successful, probably because their juvenile stages reside in deeper water than the reference Pacific halibut, and thus have a different C-14 uptake rate.

The Age and Growth Program is continuing this area of age validation research in other species and is working to develop new references for areas such as the EBS.

By Craig Kastelle

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