Bomb radiocarbon dating - Unicornfish
Bluespine unicornfish can live half a century
Published with Canadian Journal of Fisheries and Aquatic Sciences 2016
A half-century lifespan for a keystone browser, the bluespine unicornfish (Naso unicornis), with a novel approach to bomb radiocarbon dating in the tropical North Pacific Ocean
ABSTRACT: Bluespine unicornfish (Naso unicornis) from Hawaii were aged to >50 years using cross-sectioned sagittal otoliths. Fish length was a poor indicator of age because of rapid and variable early growth, exemplified by fish aged to be 4 years near-maximum length. Growth was deterministic with adult ages decoupled from body length. Otolith weight and thickness were evaluated as proxies for age and both were encouraging; thickness explained more variance but weight was easier to measure. Length-at-age was validated through ontogeny using bomb radiocarbon (14C) dating. Use of the post-bomb 14C decline period from a regional reference chronology enabled age validation of young fish—a novel approach for the Pacific Ocean. A probabilistic procedure for assigning bomb 14C dates (CALIBomb) was used for the first time to determine fish birth years. The age-reading protocol was generally validated and it was possible to describe length-at-age despite difficulties in counting otolith growth zones beyond 30-40 years. Growth curves differed between sexes and a 4-parameter generalized von Bertalanffy growth function provided the best fit.
Published CJFAS: http://dx.doi.org/10.1139/cjfas-2016-0019
PROJECT BACKGROUND
Bluespine unicornfish (Naso unicornis) is a fish commonly seen by snorkelers in Hawaii and is known locally as Kala. This fish may seem like a small to medium-sized reef fish to most people, but it can be a regular part of fish markets reaching a maximum size of up to 27 inches. It’s range is considerable as a tropical fish ranging from Hawaii throughout the southern tropical Pacific and Indo-Pacific, and into the Indian Ocean to the eastern coast of Africa.
As is usually the case, little is known about the life history of this species. I have been working with Ed DeMartini (PIFSC - Life History Program) to complete an age validation project that includes the age-at-maturity, as well as aspects of reproduction. This work is an extension of work started by Jeff Eble (former University of Hawaii graduate student) and is a collaborative effort amongst the three of us. Age has been estimated from growth zone counting in otoliths and I have extracted a series of otolith cores from fish of differing estimated age that have been tested with bomb radiocarbon dating. These samples were measured for radiocarbon at Woods Hole Oceanographic Institution at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) and the final paper is in press.
One of the challenges, and hence the “state-of-the-art” claim in the title above, is with the size of the otoliths from this fish. They are small and for me to validate the age I need to extract the core (first year of growth or less) from the otolith and analyze it for radiocarbon. This was very difficult and near the limits of both 1) what I can extract with the micromill (with confidence) and 2) what NOSAMS can measure for radiocarbon with a reasonable level of precision. The mircostructure was sampled with a series of extractions made with a New Wave Research micromilling machine (Elemental Scientific Lasers, Bozeman, MT, USA; http://www.nwrlasers.com/milling/micromill/).
The latest from bomb radiocarbon dating confirmed the longevity estimate that Jeff Eble made from counting growth zones in otolith sections. This species can live more than 50 years. These estimates were confirmed with bomb radiocarbon dating.
Main Hawaiian Islands where the bluespine unicornfish were collected for the study.
Resultant von Bertalanffy growth functions fitted to the validated age estimates. Note the strong decoupling of age from fish length — fish with similar lengths near maximum size can be 5 to more than 50 years old.
Young bluespine unicornfish commonly seen at Hanauma Bay.
