Bomb Radiocarbon - Gindai
A case study of early overcounting in otoliths
Published in Fisheries and Aquatic Sciences (KOSFAS)
Early overcounting in otoliths – a case study of age and growth for gindai (Pristipomoides zonatus) using bomb 14C dating
Allen H. Andrews and Taylor R. Scofield
Gindai (Pristipomoides zonatus) is one of six snappers in a management complex called the Deep 7 of the Hawaiian Islands. Little is known about its life history and a preliminary analysis of otolith thin sections indicated the species may exhibit moderate growth with a lifespan approaching 40 years. Preliminary age estimates from the previous study were reinvestigated using the same otolith sections in an attempt to validate those ages with bomb radiocarbon (14C) dating. From the misalignment of birth years for the otolith 14C measurements with regional references — the post-peak bomb 14C decline period — it was concluded that previous ages were inflated from overcounting of the earliest growth zone structure in otolith sections. The oldest gindai was re-aged to 26 years once the age reading was adjusted for early overcounting, 13 years younger than the original estimate of 39 years for this fish. In general, the earliest otolith growth of gindai was massive and complicated by numerous subannual checks. The approach of lumping the early growth structures was supported by the alignment of 14C measurements from otolith core material (first year of growth). The result was greater consistency of calculated birthdates with the 14C decline reference, along with minor offsets that may indicate age estimation was imprecise by a few years for some individuals. The revised von Bertalanffy growth function applied to the validated age-at-length estimates revealed more rapid growth (k = 0.378 cf. 0.113) and a lifespan of approximately 30 years. The findings presented here are a case study of how the 14C decline period can be used as a tool in the refinement of age reading protocols.
Otolith section images from gindai (P. zonatus) specimen PZ 90 showing the core extraction (grey shaded area) from the micromill (A) and the revised age-reading scenario (B). The milled extraction is a trough cut into the original aged otolith section at the core region (2.5 mm long x 0.5 mm wide x 0.4 mm deep) within the first year of growth. The marked section image is the obverse of the same otolith section where lumping of fine growth structure was used for the first 8 years and splitting was used for the remaining growth zones out to 26 years (aged to 25–27 years depending on interpretation). Note that there are numerous ways to count this otolith and that it is possible to attain the original age of 39 years by splitting the earliest growth zones (see Figure 5 of Scofield (2013); Appendix 1). In addition, some growth zone structure that was counted to 26 years is difficult to see because it requires a change in the angle of transmitted light, and may also require panning the eye across the concentric zone structure to either unify or split the area of interest. This age reading protocol was used to age all otoliths that were reinterpreted with support from bomb 14C dating in this study.
Bomb 14C plot of the selected series of gindai (P. zonatus) otoliths from the early study by Scofield (2013) with revised dates from a reinvestigation section age. The original estimates of age (X symbol) were used to calculate birth years that are typically well-removed from the expected birth year determined by the regional 14C reference records. Revised age estimates from a combination of 14C alignment and reference-image otolith age-reading led to a stronger alignment of the calculated birth dates with the 14C reference records. Reference records plotted here are from two coral and otolith 14C data sets (Kure Atoll coral and otoliths of the Northwestern Hawaiian Islands (NWHI reference) and Kona of Hawaii Island with otoliths of the main Hawaiian Islands (MHI reference); see Andrews et al. (2016) for details).
Original student study DOWNLOAD:
