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The implementation of management strategies for marine fishes relies on the identification of subpopulations as well as the characterization of their spatial and temporal distribution relative to environmental conditions. In the Pacific off North America, there are currently three spawning subpopulations of Pacific sardines (Sardinops sagax caeruleus). The spawning center of the northern (or cold) subpopulation is located off southern California and occurs in the spring at sea surface temperatures (SST) of 12-16°C. Spawning can also extend northward of Washington and Oregon. Spawning of the southern (or temperate) subpopulation can take place between spring and fall off central Baja California at SST of 17-22°C. Peak spawning also occurs in the Bahia Magdalena region in the winter and there is less intense spawning during the summer at SSTs > 23 °C. The oxygen isotopic composition (?¹⁸O) of the otolith carbonate of marine fishes has been successfully used as a natural tracer of stock structure and natal origin, as well as to reconstruct the thermal history of individuals and infer migration patterns. Otolith ?¹⁸O values permanently record the temperature and salinity under which the carbonate precipitated. The otoliths of individuals that have grown in waters with different temperature and/or salinity regimes can thus exhibit differing isotopic values. Further, if the isotopic composition of the water (?_w) in a given area is known or can be estimated based on salinity, ?¹⁸O values can be used to estimate the temperature of carbonate precipitation. Our objective is to reconstruct the temperature to which individual sardines were exposed to during the larval and early juvenile period by subsampling carbonate extracted from otolith cores. We hypothesize that back-calculated temper

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