Development of Bomb Radiocarbon Reference Chronologies Aids Studies of Age, Growth and Longevity in Bottom Fishes and Reef Fishes

With the goal of improving stock assessments, scientists in the PIFSC Life History Program are continuing studies to refine state-of-the-art bomb radiocarbon (14C) dating methods and apply them to bottom and reef fishes in the Insular Tropical Pacific (ITP). The work is being done in collaboration with the University of Ryukyus in Okinawa, Japan. Because age-validated life history data have been lacking, important aspects of population dynamics have not been fully understood and taken into account in ITP fish resource assessment or management. The bomb-radiocarbon dating research aims to address these shortcomings.

The first application of bomb 14C dating by PIFSC researchers was successful and indicated that prevailing age assumptions for opakapaka (Pristipomoides filamentosus) were incorrect; previous longevity estimates were 5 to 18 years, but bomb 14C dating revealed that the lifespan of opakapaka exceeds 40 years and that growth may be different in the Mariana Islands.

Bomb 14C dating of marine organisms requires accurate regional documentation of how the bomb 14C signal (from testing of nuclear devices in the 1950s and 1960s) was sequestered and transported by the marine environment. For the Hawaiian Archipelago, the low-resolution bomb 14C record from French Frigate Shoals was initially used to age fish collected in the Northwestern Hawaiian Islands (NWHI). Subsequently, PIFSC scientists further developed a continuous record from Kure Atoll in the NWHI. At present, this coral core has provided a validated bomb 14C time series from ~1940 to 2002 (Figure 1). Additional samples (4 per year) have been extracted and are slated for 14C analysis later in 2013 to provide the higher resolution chronology that is needed.

Figure 1.  Preliminary plot of two bomb 14C records from the NWHI.  The pre-existing record from French Frigate Shoals is 
        partial (1958-1978) and low-resolution.  The new record from Kure Atoll is annual and covers the pre-bomb to post-peak decline 
        periods (1940-2002).  Sub-annual 14C data for Kure Atoll will be added later this year to the Kure Atoll record to 
        account for seasonal for Kure Atoll will be added later this year to the Kure variation (necessary for properly understanding the 
        bomb 14C age uncertainty for fishes).
Figure 1. Preliminary plot of two bomb 14C records from the NWHI. The pre-existing record from French Frigate Shoals is partial (1958-1978) and low-resolution. The new record from Kure Atoll is annual and covers the pre-bomb to post-peak decline periods (1940-2002). Sub-annual 14C data for Kure Atoll will be added later this year to the Kure Atoll record to account for seasonal for Kure Atoll will be added later this year to the Kure variation (necessary for properly understanding the bomb 14C age uncertainty for fishes).

A regional reference record for bomb 14C dating does not exist for Guam and CNMI. To apply the technique in that region, three 14C references (Okinawa, Japan, Nauru, and French Frigate Shoals) were comingled, covering a huge region of vastly different oceanographic conditions. A comparison of these records revealed strong temporal differences in the bomb 14C signal (Figure 2). Hence, it is necessary to develop and apply a more geographically relevant bomb 14C reference chronology from within the U.S. Territories. A coral core has been located that can be used to establish a bomb 14C record for the Guam-CNMI region. It was taken from Double Reef, Guam and has been used for studies on regional climate change.

Figure 2.  Plot of regional bomb 14C records surrounding Guam-CNMI. The great differences in bomb 14C signal 
        timing makes it necessary to develop a new record for the Guam-CNMI region.
Figure 2. Plot of regional bomb 14C records surrounding Guam-CNMI. The great differences in bomb 14C signal timing makes it necessary to develop a new record for the Guam-CNMI region.

Collaborators at the University of the Ryukyus, Okinawa, Japan are working with PIFSC scientists to develop a bomb 14C record from the Guam coral core. A sample series from this coral core will cover a time period similar to that for the Kure Atoll coral record (~1940-2000). The new coral core record will provide a strong basis for successful age validation of bottom and reef fishes of the Guam-CNMI region. Studies of onaga (Etelis coruscans) and humphead wrasse (Cheilinus undulatus) from this area are currently underway. Otoliths from Guam humphead wrasse have been analyzed for 14C and validation of age for these specimens awaits the bomb 14C record that will come from the Double Reef coral core.