Craig R. Kastelle

Using 210Pb/226Ra disequilibrium to validate conventional ages in Scorpaenids (genera Sebastes and Sebastolobus)

Age determination of rockfish (genera Sebastes) and thornyheads (genera Sebastolobus) is typically accomplished by counting growth zones in burnt cross-sections of the otoliths. Interpretation of patterns seen in the otolith’s growth zones is often difficult and subjective so age validation is desirable. Unfortunately, age validation has been nonexistent for many of these species. Their biology and life history often make age validation methods used on other species impossible. Therefore, we used radiometric ageing which employs the disequilibrium of 210 Pb and 226 Ra in the otolith. Two methods were used to obtain material from the otoliths: core samples and whole otolith samples. The 210 Pb/ 226 Ra ratios generally confirmed ageing criteria used for rockfish, but in some cases there was a bias between growth zone counts and radiometric ages. We investigated two potential sources of bias. First, the initial ratio of 210 Pb/ 226 Ra as incorporated into the otolith may have been overestimated. Second, the otolith core removal process may have inadvertently left excessive amounts of younger aged material not belonging to the core. Both would cause radiometric ages to be biased low. Published by Elsevier Science B.V.

Testing for loss of 222Rn from Pacific halibut (Hippoglossus stenolepis) otoliths

Abstract Radiometric ageing of fish by measuring the disequilibrium of 210 Pb/ 226 Ra found in otoliths has been used by numerous researchers in over 15 different studies. The ultimate goal of the previous research has been to validate conventional ageing methods. If 222 Rn, an intermediary between 226 Ra and 210 Pb, escapes from the otoliths, the radiometric age would be biased young. The goal of this study was to test for this bias using Pacific halibut ( Hippoglossus stenolepis ) otoliths collected in 1960 and analyzed in 1997. If 222 Rn loss occurred, the estimated radiometric age would be less than the 37 year span between collection and analysis. In comparison to other fish species, Pacific halibut otoliths were found to have an unusually low activity of 210 Pb and 226 Ra. Low activity measurements lead to radiometric age estimates with very large estimated errors. All estimated radiometric ages for the Pacific halibut otoliths were much greater than 37 years. This suggests that 222 Rn is not escaping from the otoliths and that radiometric ages are not biased low. However, considering the estimated error, the results are not definitive. This study should be repeated using a different species that is known to have a higher activity of 210 Pb and 226 Ra.

Evaluation of micromilling/conventional isotope ratio mass spectrometry and secondary ion mass spectrometry of δ18O values in fish otoliths for sclerochronology

RATIONALE Stable oxygen isotope ratios (δ18 O values) measured in fish otoliths can provide valuable detailed information on fish life history, fish age determination, and ocean thermography. Traditionally, otoliths are sampled by micromilling followed by isotope ratio mass spectrometry (IRMS), but direct analysis by secondary ion mass spectrometry (SIMS) is becoming more common. However, these two methods have not been compared to determine which, if either, is better for fish age validation studies. Hence, the goals were to: (1) determine if the δ18 O signatures from the two different methods are similar, (2) determine which method is better for fish age validation studies, and (3) examine biogeographic and migration history. METHODS Both analytical techniques, micromilling/IRMS and SIMS, were used to measure δ18 O values in six Pacific cod (Gadus macrocephalus) otoliths. A series of measurements was made from the center of each otolith to its edge to develop a life-history δ18 O signature for each fish. RESULTS The sampling resolution of SIMS analyses was 2-3 times greater than that obtained by micromilling/IRMS. We found an offset between SIMS and micromilling/IRMS δ18 O values, about 0.5‰ on average, with SIMS yielding lower values. However, the δ18 O patterns from both methods (i.e., the number of δ18 O maxima) correspond to the estimated age determined by otolith growth-zone counts, validating fish age determination methods. CONCLUSIONS Both techniques resolved δ18 O life-history signatures and showed patterns consistent with seasonal variation in temperatures and changes due to fish migration. When otoliths are large, micromilling/IRMS can provide adequate resolution for fish age validation. However, SIMS is the better option if greater sampling resolution is required, such as when otoliths are small or specimens are longer lived and have compact growth zones.