Bomb Radiocarbon Dating Used to Validate Age, Growth and Longevity of Bluespine Unicornfish

Scientists in the PIFSC Life History Program (LHP) have collaborated with colleagues at the University of Hawaii-Manoa (UH) to validate age estimates up to approximately 50 years for the bluespine unicornfish (Naso unicornis), commonly known as 'kala', collected off Kaneohe Bay, Oahu. Jeff Eble, a recent graduate of the University of Hawaii, made the age estimates by counting growth zones visible in the pair of fish ear bones known as 'otoliths'. To test the validity of these estimates, Eble provided LHP researchers with the otoliths for bomb radiocarbon dating.

Kala, or bluespine unicornfish (Naso unicornis). Photo courtesy of Richard Field, FishBase.
Kala, or bluespine unicornfish (Naso unicornis). Photo courtesy of Richard Field, FishBase.

Bomb radiocarbon dating is a method of age validation for fishes that uses a radiocarbon signal created by atmospheric nuclear testing in the 1950s and 1960s. As a result of the nuclear testing, reef-building corals have stored this signal in their skeletons, which can be used to trace radiocarbon levels back to days before the nuclear testing. Fish otoliths can also store the radiocarbon signal, if the fish was alive during that time. By extracting the core (earliest growth) of the otolith and measuring radiocarbon, researchers can validate age estimates of the fish by correlating the otolith core value to coral reference values from Hawaii.

LHP researchers at PIFSC have been successful using this method on bottomfishes of Hawaii, but the bluespine unicornfish is the first reef fish tackled and it has provided some challenges. Otoliths of this species are very small and as a consequence the earliest growth has a very low mass. Based on the size and weight of otoliths from juvenile bluespine unicornfish, the center of adult otoliths could be targeted properly using a micromilling machine. In the left image below, the earliest growth can be seen in the otolith as a more opaque region near the otolith center. The image on the right shows how the micromilling machine was used to remove the core or earliest growth.

Otolith from a bluespine unicornfish before (left) and after (right) the core was removed with a micromilling machine.  The length 
        and width of the extracted area on the right image was 2.3 mm by 1.8 mm, resulting in a mass of 0.9 mg.
Otolith from a bluespine unicornfish before (left) and after (right) the core was removed with a micromilling machine. The length and width of the extracted area on the right image was 2.3 mm by 1.8 mm, resulting in a mass of 0.9 mg.

Otoliths from the smallest to the largest bluespine unicornfish were analyzed for bomb radiocarbon. Based on a comparison of the measured radiocarbon values from these specimens to Hawaiian coral records, age was validated for 15 fish that ranged from 12 to 55 cm in fork length (FL). Validated age for these specimens ranged from 1 to 53 years and exemplified rapid growth in the first 10 years of life, and then maintenance of an asymptotic length near 50 cm FL for the remaining 40 years. These data will be combined with length-at-age results for other bluespine unicornfish, also from counting otolith growth zones, to determine growth parameters for this species.