Figure 8. Gulf of Alaska Dover sole and eastern Bering Sea yellowfin sole test specimens are compared to a Gulf of Alaska Pacific halibut reference chronology.
Figure 9. Stable oxygen isotopes (O-18) as a function of distance from otolith core of a 6-year-old Pacific cod illustrates seasonal variation likely coincident with water temperature. Dotted vertical line represents estimated otolith location corresponding to visible growth bands.
Validating the true age of fish is critical for stock assessments. The main method used by the AFSC’s Age and Growth Program to determine the age of fish is to count growth bands in otoliths.
One method used to validate the age of long-lived fish uses radiocarbon levels in otoliths as a cross-reference. Atmospheric radiocarbon (C-14) from atomic bomb testing during the 1950s and 1960s led to a substantial increase in marine C-14 levels during those years. Radiocarbon incorporated into the otoliths of known-age fish alive during that time can be measured for different species of fish, producing a reference chronology that can be compared to radiocarbon levels in fish aged by reading growth bands.
Ages are considered accurate and valid if the C-14 increase in the test specimens matches the timing and magnitude of the C-14 increase in the reference chronology, as illustrated for Gulf of Alaska Dover sole test specimens compared to a halibut reference chronology in Figure 8.
In comparison, Bering Sea yellowfin sole test specimens are asynchronous with the Gulf of Alaska Pacific halibut reference chronology and are therefore either aged incorrectly, or more likely, require a reference chronology specific to the Bering Sea.
Other validation methods are required for fish that hatched after the era of bomb testing. A promising new method for short-lived species relates the stable oxygen isotope (O-18) signature from otoliths to seasonal variation in water temperature, a well understood principle in calcified marine structures.
Preliminary results with Pacific cod otoliths (Fig. 9) show that the O-18 signature varies seasonally with water temperature and supports the age obtained by counting growth bands in otoliths. This and other new approaches are being explored to assess ageing bias and to validate ages used in stock assessments for the more than 25,000 otoliths aged each year by the AFSC Age and Growth Program.
