Bomb Radiocarbon Dating Improves Hapu‘upu‘u Age Estimation

Researchers in the PIFSC Fish Life History Program are improving estimates of age in Hawaiian bottomfish species using bomb radiocarbon dating. This method of age determination uses a signal of radioactivity in a fish's hard parts (such as otoliths) originating from the testing of nuclear bombs in the 1950s and 1960s. Its application to valuable Hawaiian bottomfish has led to key advances in determining the age and lifespan of these fishes. The technique was recently used with great success on opakapaka (Pristipomoides filamentosus) in a study by PIFSC scientists Allen Andrews, Ed DeMartini, Jon Brodziak, Ryan Nichols, and Robert Humphreys. (article In Press)


Work now underway using bomb radiocarbon dating is providing validated age estimates of the Hawaiian grouper, locally known as hapu‘upu‘u (Hyporthodus quernus). Results to date indicate hapu‘upu‘u are long-lived, information vital for accurate, age-structured stock assessment of this species in the Hawaiian Archipelago.

Prior estimates of age and growth for hapu‘upu‘u, based solely on visual counts of growth zones in otoliths, were published in 2008 by PIFSC scientists Ryan Nichols and Ed DeMartini. Their preliminary study demonstrated that otolith growth zones could be quantified and provided estimates of length-at-age that seemed reasonable for small to medium sized fish. However, for fish that had reached sexual maturity, the difficulty of growth zone counting increased with increased fish length, and age estimates were dubious for the largest fish. A maximum age of 34 years was estimated from growth zone counts, but this estimate was derived from a fish considerably smaller than the largest observed hapu‘upu‘u. Because annual growth zones were difficult to interpret and no age validation studies had been performed to date, PIFSC researchers decided to use bomb radiocarbon dating to age hapu‘upu‘u.

Otoliths from adult hapu‘upu‘u were selected based on growth-zone age estimates and collection year. In this process, core material from the birth year of the fish, within the otolith center, was sampled with a micromilling machine (a computer controlled drill). The extracted samples were analyzed for radiocarbon using the mass spectrometry facility at the Woods Hole Oceanographic Institution. As they grow, otoliths reflect the ocean chemistry during the time they are formed. Thus by comparing measurements of radiocarbon in the otolith to a marine radiocarbon reference, the fish's age can be determined.

Preliminary results from the first stage of the bomb radiocarbon dating of hapu‘upu‘u have been very promising. The graph above shows the radiocarbon reference record for waters off Hawaii from the mid-1940s through the mid-1990s. Also plotted are recent radiocarbon measurements in otoliths of 5 hapu‘upu‘u. Plotted circles indicate when each fish was captured. To determine a fish's year of birth, the measured radiocarbon level from the fish's otolith must be projected back in time to the radiocarbon reference series; projections are indicated by arrows in the plot. An age for each fish was calculated by subtracting the estimated birth year from the capture year. In this first set of adult hapu‘upu‘u studied, validated age estimates ranged from 14 to 26 years for fish whose total lengths ranged from 48 to 75 cm.

Based on these length-at-age data, additional adult hapu‘upu‘u specimens have been selected for bomb radiocarbon analysis to provide a more comprehensive range of validated age estimates. Of particular interest is validating age estimates for the largest fish (up to 110 cm total length) to determine the potential lifespan of hapu‘upu‘u. In a recently published bomb radiocarbon dating study on a member of the same genus from the Indo-Pacific, the eightbar grouper (Hyporthodus octofasciatus), PIFSC researcher Andrews and his colleagues validated the lifespan as no less than 43 years. This age determination was for several fish specimens much smaller than the largest fish observed, so the actual lifespan for the species may be considerably greater than 43 years. Hence, it is also likely that the lifespan of hapu‘upu‘u exceeds the 34-year estimate from growth zone counting.