High resolution bomb dating for testing the accuracy of age interpretations for a short-lived pelagic fish, the Atlantic herring

Although stock assessments of Atlantic herring (Clupea harengus harengus) in the northwest Atlantic have relied on tens of thousands of annual age determinations each year for more than 20 years, recent analyses have suggested that there may have been systemic ageing error. Tracking of dominant year-classes and otolith exchanges confirmed the presence of substantial ageing bias among some readers, although these approaches could not be used to identify an accurate set of ages (if any). We applied bomb radiocarbon in a high resolution dating approach targeted at the 1962 year-class to assess ageing accuracy by multiple age readers and laboratories. Although bomb radiocarbon age validation studies are typically restricted to long-lived species, the availability of archived otoliths through the 1960s and 1970s made herring an ideal candidate for an approach targeted at a single year-class, and allowed the extent of any ageing error to be quantified. The results clearly demonstrated that current age reading practices under-aged fish >6 yr of age by up to 45%, although younger fish were aged accurately. Age underestimation was due to incorrect annulus interpretation rather than non-annual otolith growth. By focusing on the period of most rapid radiocarbon increase (1962), the margin of uncertainty around the targeted bomb radiocarbon ages was reduced to 0.66 yr. This study represents the first time the bomb dating method has reached sub-annual accuracy, which makes it well suited to the age validation of short lived fish species. The use of the targeted approach has considerable promise for improving the accuracy of other bomb radiocarbon studies without the problematic assumptions associated with curve estimation and environmental effects.