Massive populations of sardines inhabit both the western and eastern boundaries of the world’s subtropical ocean basins, supporting both commercial fisheries and populations of marine predators. Sardine populations in western and eastern boundary current systems have responded oppositely to decadal scale anomalies in ocean temperature, but the mechanism for differing variability has remained unclear. Here, based on otolith microstructure and high-resolution stable isotope analyses, we show that habitat temperature, early life growth rates, energy expenditure, metabolically optimal temperature and, most importantly, the relationship between growth rate and temperature were remarkably different between the two subpopulations in the western and eastern North Pacific. Varying metabolic response to environmental changes partly explain the contrasting growth responses. Consistent differences in the life-history traits are observed between subpopulations in the western and eastern boundary current systems around South Africa. These growth and survival characteristics can facilitate the contrasting responses of sardine populations to climate change.

Otolith Increment Width data (JP_Otolith_IW_YYYY.xlsx (YYYY: 2006-2010, 2014, 2015), CA_otolith_increments.xlsx)

Extracted sagittal otoliths were cleaned to remove the attached tissue in freshwater and then air-dried. Otoliths of JP sardine were embedded in epoxy resin (Petropoxy 154, Burnham Petrographics LLC) on slide-glass, while those of CA were glued to slide-glass using enamel resin and then ground and polished with sandpaper to expose the core. Using an otolith measurement system (RATOC System Engineering Co. Ltd.), the number and location of daily increments were examined along the axis in the postrostrum from the core. 

Otolith Isotope data (Otolith_isotope_ratios.csv)

The otolith portions deposited during hatch-30, 31-45, 46-60, 61-75, 76-90, 91-105, and 106-120 dph for the JP sardine and hatch-30, 31-60, 61-90, 91-120, and 121-150 dph for CA sardine, were milled out sequentially using a high-precision micromilling system Geomill 326. The milling depth was set to 50 um for the area near the core and 100 um for the rest. After each milling, the otolith was observed under a microscope to check for otolith fractions that had cracked outward from other milling area, and these were removed using a needle if present. The milled powders were then collected using a needle and a stainless-steel cup and poured into response vials. After each collection of powder, the otolith was cleaned with an air duster to avoid cross-contamination between the milling paths. The d18O and d13C values of collected otolith powder were determined using the micro-volume analysis system MICAL 3c at the National Institute of Technology, Ibaraki College, for the area nearest to the core, and the automatic system DELTA V + GAS Bench for the rest at the Atmosphere and Ocean Research Institute, the University of Tokyo. The otolith powders were reacted with phosphoric acid at 25 deg C, and the released CO2 was purified before being introduced into the mass spectrometer for the MICAL3c system. The response with phosphoric acid was performed at 72 deg C for DELTA V. 

Seawater Isotope data (Seawater_d18O_California.csv)

Surface and sub-surface seawater samples for the isotope analysis were collected during the 1708SR CalCOFI cruise survey conducted by the California Cooperative Oceanic  Fisheries Investigations in August 2017. At every two stations on three line transects extending offshore from the Southern California Bight, seawater samples for d18O analysis were taken from 10m and 50m depths using CTD-attached Niskin bottles and preserved in sealed glass vials to prevent evaporation. After membrane-filtration (pore size: 0.45 um, Toyo Roshi Kaisha, Ltd.), d18O values were measured at the National Institute of Technology, Ibaraki College using the Picarro L2130-i system with a precision better than 0.05 permil. The cruise information and temperature and salinity data were obtained from CalCOFI database (https://calcofi.org/data/oceanographic-data/).

All data can be opened by Microsoft Excel.