Basin-Specific Rates of Incorporation of Bomb-Produced 14C in Fish Otoliths: an Example from the North Pacific Ocean

Pacific halibut (Hippoglossus stenolepis) represent one of the northeastern Pacific Ocean’s most valuable fisheries, on average generating ~60 million pounds of annual productivity.  The majority of the fishery is concentrated on the continental shelf, and is monitored and managed by the International Pacific Halibut Commission (IPHC), a joint treaty organization of the United States and Canada. The IPHC has conducted various age-validation studies to ensure the accuracy of the age determination technique employed; the most recent study, and most conclusive, used “bomb-produced” radiocarbon (¹⁴C).  A Pacific halibut reference chronology curve for the Gulf of Alaska (GOA) was developed in 2004, and concordance between ¹⁴C in the reference chronology and a GOA age validation test sample indicated that the existing age-determination technique was valid.  However, unique oceanographic mixing processes exist in the GOA and eastern Bering Sea (EBS), suggesting that regional differences in ¹⁴C dynamics may also exist.  Thus, we could not discount the hypothesis that the established reference curve might only accurately represent halibut reared within the GOA.  Given that recruitment levels and other biological aspects of stock structure are somewhat discontinuous between basins, it is critical that cohorts are properly identified and assigned, on a regional basis, within the assessment model in order to track productivity on an area-by-area basis.  To that end, we sought in the present study to test whether the ¹⁴C reference curve established for the GOA is equally valid for application to halibut resident in the eastern Bering Sea.  To our knowledge, this is the first study to provide a ¹⁴C reference chronology for an EBS teleost, and to examine differences in ¹⁴C reference chronologies across oceanic basins within a single fish species.  The Pacific halibut reference chronology curves developed for the EBS and GOA are markedly different.  The EBS curve displays an earlier and more rapid increase in ¹⁴C, a substantially higher ¹⁴C peak, and an exponential post-peak decay that is much less pronounced in the GOA.  The importance of developing basin-specific metrics in studies that rely upon bomb-produced radiocarbon will be discussed, including examples of other fish species inhabiting the GOA and EBS.