Early overcounting in otoliths: a case study of age and growth for gindai (Pristipomoides zonatus) using bomb 14C dating

Abstract

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 (14 C) dating. From the misalignment of birth years for the otolith 14 C measurements with regional references-the post-peak bomb 14 C 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 14 C measurements from otolith core material (first year of growth). The result was greater consistency of calculated birthdates with the 14 C 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 bomb 14 C decline period can be used as a tool in the refinement of age reading protocols.

Bomb-Produced Radiocarbon Across the South Pacific Gyre—a New Record From American Samoa With Utility for Fisheries Science

“…An apparent misalignment in time for measured otolith 14 C values from the coral 14 C decline can in some cases be explained by oceanography and early life history, as was attributed to a consistent offset of Pacific bluefin tuna (Thunnus orientalis) birth years (Ishihara et al 2017)-these values were within the constraints of the collective coral 14 C records of the North Pacific and provided strong support for the age reading protocol to ∼30 yr. In addition, an age reading protocol was refined based on a wide offset of otolith 14 C values from the regional decline reference and resulted in validated life history characteristics for a deep-water snapper that were drastically different from an initial otolith age reading interpretation (Andrews and Scofield 2021). In each case, the circumstances are specific to the early life history of the species and regional oceanography but the differences in 14 C levels for the tropical-subtropical Pacific across time and space are now reduced and appear to be converging.…”

“…For example, if a species is estimated to live 20 yr and adults of all sizes are available from a collection year of 2020, then use of juveniles (e.g., age-0 fish) collected across the span of calculated adult birth years (2000 to 2020) can provide direct evidence that adult otolith cores (the juvenile portion of the otolith) will align with the 14 C reference if the age reading protocol is accurate. Alternatively, a full study can be facilitated with limited juvenile collections or otolith edge material of young fish that cover a portion of the decline period, with the assumption that the trend of otolith 14 C alignment continues in unison with the coral 14 C reference (i.e., Andrews 2020; Barnett et al 2018;Andrews et al 2020b, Andrews andScofield 2021). Use of the post-peak bomb 14 C decline has also evolved with a novel line of research that uses core material of fish eye lenses to validate age (Patterson et al 2020) and in the use of new technology (laser ablation accelerator mass spectrometry) to trace continuous bomb 14 C signals within an otolith (Andrews et al 2019b).…”