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Feature: Regional Environmental Factors Affecting Bomb-Derived Radiocarbon Age Validation Studies

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Otoliths from adult Greenland halibut are prepared in the laboratory for age estimation studies and analysis of regional differences of bomb-derived radiocarbon content. Photo by Karna McKinney.

THE PRIMARY RESPONSIBILITY OF THE ALASKA FISHERIES SCIENCE CENTER’S AGE AND GROWTH PROGRAM is to provide accurate fish ages for annual stock assessments. Accurate age data are important because the age composition of commercially exploited fish populations is critical in setting harvest limits for the sustainability of the resource. Ages of most commercially important fish caught in the Gulf of Alaska or eastern Bering Sea are estimated by counting (reading) the annual growth rings in the fish’s otoliths (ear bones). More than 20,000 otoliths are read by AFSC scientists each year.

Accurate age estimation is often a difficult task as the interpretation of growth rings is not always clear and can require subjective decisions on what constitutes a year’s growth. To confirm the accuracy of age estimates, the Age and Growth Program uses a number of age validation techniques including the novel use of radiocarbon ( 14 C) derived from above-ground atomic bomb testing. Serving as a time-stamp in fish otoliths from the Cold War era, bomb-derived 14 C is becoming a widely used tool for fish age validation and is considered one of the best methods for this type of research.

The Age and Growth Program expands the use of this age validation method through investigation into the timing and strength of the bomb- derived 14 C marine signal and its relationship to oceanographic gradients in the North Pacific Ocean. Oceanographic processes such as currents, wind mixing, or upwelling may change the level of bomb-derived 14 C in fish otoliths, requiring further exploration to understand such relationships.

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