Radioisotope Methods Improve Reliability of Opakapaka Age Determination

Using radioisotope methods, researchers in the Center's Fishery Biology and Stock Assessment Division are making key advances in determining the ages and lifespan of a valuable Hawaiian bottomfish. Reliable estimates of age and longevity for opakapaka (Pristipomoides filamentosus) are needed for stock assessments of this species in the Hawaiian Archipelago.

Opakapaka (Pristipomoides filamentosus).
Opakapaka (Pristipomoides filamentosus).

Previously, information on opakapaka ages was gleaned from analysis of growth zones on the fish's otoliths. Because opakapaka ololiths lack well developed annual growth zones, daily growth increments have been used. In an important early paper, Ralston and Miyamoto (Fishery Bulletin, Vol. 81, No. 3, 1983) used daily growth increments to develop a length-at-age growth curve, and then used this model to estimate ages of larger fish by extrapolation. However, the validity of age projections for the largest and oldest adult fish has remained in question. Of the specimens they studied, Ralston and Miyamoto estimated the oldest to be about 17 yrs. Other age and growth studies across the geographical range of opakapaka confirm the difficulty of age interpretation and report a wide range of maximum ages - from 5 to 30 years. More reliable estimates of age for the largest fish, and for opakapaka longevity, are a research priority.

Age estimates of young opakapaka using otoliths have been validated by captive rearing, tag and recapture, length frequency analysis, and daily growth increments, but application of these methods is limited to the earliest ages and to cases where changes in fish length or otolith growth can be directly observed.

To determine ages of adult opakapaka, PIFSC researchers have recently applied a pair of different radioisotope approaches to the fish otoliths. The first method, lead-radium dating, has been used successfully for nearly 30 species of fish by other investigators and is based on a known radioisotope decay series. It can be used to determine ages for fish ranging from a few years to about 100 years.

The second method, bomb radiocarbon dating, is based on a time-specific event marker. It can be used to age fish born during the 1950s and 1960s, when atmospheric testing of thermonuclear devices was conducted, and therefore is currently effective only for fish up to about 53 years old. However, bomb radiocarbon dating allows ageing of individual fish with high precision (± 1 to 2 years). Lead-radium dating, on the other hand, can be used for fish of any age but detection limits require the pooling of otoliths from many fish to attain measureable lead-radium levels. A group of fish of similar size is used, and the result is an estimate of average age for the group. Where both methods are feasible, using both can provide the most information in an exploratory study.

Recent application of these methods to opakapaka produced the following results:

Method Fish length (FL, cm) Estimated age (yrs)
Lead-radium dating 70-75 >34.4
  ~60 >18.2
  ~66 >31.4
Bomb radiocarbon dating 50.7-76.8 7-43

Together with other reliable age determinations from the Hawaiian Archipelago (graph below), the radiometric dating results will be used to develop an accurate age and growth model that can be applied to strengthen stock assessments.


Recent age determinations for Hawaiian Archipelago opakapaka using lead-radium dating and bomb 
            radiocarbon dating of otoliths significantly augment earlier results based on other methods. Horizontal 
            lines around data points for radiometric age measures represent levels of uncertainty. In addition to 
            individual points for the various methods, plots of von Bertalanffy growth curves are shown based on the 
            early work of Ralston and Miyamoto and an unpublished tag-recapture study by PIFSC scientist Don 
            Kobayashi.
Recent age determinations for Hawaiian Archipelago opakapaka using lead-radium dating and bomb radiocarbon dating of otoliths significantly augment earlier results based on other methods. Horizontal lines around data points for radiometric age measures represent levels of uncertainty. In addition to individual points for the various methods, plots of von Bertalanffy growth curves are shown based on the early work of Ralston and Miyamoto and an unpublished tag-recapture study by PIFSC scientist Don Kobayashi.

The opakapaka ageing research is continuing with the processing of additional lead-radium and bomb radiocarbon samples. A formal report will be prepared and results will be published in a peer-reviewed journal. Following the opakapaka study, both lead-radium and bomb radiocarbon dating will be applied to two other bottomfish species, ehu and hapu'upu'u.