HR: 0800h
AN: PP31C-0536    [Abstracts]
TI: High Natural Variability in Delta18O Values of the Otoliths of Young Pacific Sardines Captured in Mexican Waters
AU: Herzka, S Z
EM: sherzka@cicese.mx
AF: Center for Scientific Research and Higher Education of Ensenada, Km 107 Carretera Tijuana-Ensenada, Ensenada, BC 22860, Mexico
AU: * Valle, S
EM: svalle@cicese.mx
AF: Center for Scientific Research and Higher Education of Ensenada, Km 107 Carretera Tijuana-Ensenada, Ensenada, BC 22860, Mexico
AB: Measurements of the oxygen isotope ratios of marine fish otoliths have been used to infer patterns of habitat utilization, mixing among subpopulations and geographical population structure. Otoliths, or "ear stones", are small, metabolically inert three-dimensional structures comprised primarily of carbonate in the form of aragonite. They grow continuously throughout life by deposition of periodic rings, which has made them invaluable for estimating age and reconstructing growth histories. Oxygen isotopes incorporated into otolith carbonate are deposited at or near equilibrium with their relative abundance in seawater, and do not appear to be influenced by kinetic effects or metabolic processes. The greater discrimination against ¹⁸O during CaCO₃ formation at higher temperatures results in a predictable negative relationship between temperature and otolith δ ¹⁸O values. However, biological effects, such as natural variations in otolith growth rates, can indirectly influence the isotopic composition measured in whole otoliths or in subsamples corresponding to different stages of the life cycle. Temperature estimates derived from δ ¹⁸O values thus represent an integrated measure of the conditions experienced by an individual. We measured δ ¹⁸O values in whole otoliths of young (ca. 1 yr old) Pacific sardines (Sardinops sagax caeruleus) collected throughout the Mexican Pacific to quantify natural variability, reconstruct integrated temperature histories and evaluate the suitability of isotopic measurements as natural tracers of mixing processes. Isotopic values and back-calculated temperature estimates of young sardines of similar sizes captured simultaneously were highly variable (up to 2.0 ‰ and 10 ° C). The limited variations in salinity within each location (< 0.8 psu) imply differences in thermal history among individuals and hence mixing during the first year of life. Based on the schooling behavior and vertical habitat utilization patterns of Pacific sardines, it is unlikely that differences in vertical distribution are the cause of the large variation in isotopic values. We tested the null hypothesis of limited horizontal movement of local populations by comparing (1) local δ ¹⁸O values with predicted carbonate isotopic values and (2) ‘life-to-date' sea surface temperatures (SSTs) with back-calculated temperature estimates. Some individuals exhibited values outside the range of locally predicted carbonate isotopic ratios and ‘life-to-date' SSTs, suggesting they were not permanent residents. An empirical otolith growth model was developed to examine the magnitude of the effect of growth rate variations and seasonal temperature fluctuations on δ ¹⁸O values. For a 1 yr old sardine, the percent monthly increase in otolith radius (used as a proxy for accretion rate) varied between 4-18 %\. We used the model in conjunction with average monthly SST series to calculate δ ¹⁸O values with and without weighing for monthly growth rate variations. Failing to account for monthly variations in otolith growth rates led to differences of 0.15 to 0.69 ‰ relative to unweighed estimates. Differences in otolith growth rates among individuals probably cannot fully account for the observed high variability, although they certainly influence δ¹⁸O values and hence back-calculated temperature estimates. Further research must be conducted to evaluate the potential effect of indirect biological effects such as variations in carbonate accretion rates on otolith isotopic values.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting