Table 1. Details of the salmon scale samples analyzed, δ13C (corrected for Suess effect) and δ15N ranges and correlation coefficients between these two SI ratios. P-values significance is: *** p<0.001, **p<0.01, *p<0.05.
1Satterfield and Finney, 2002,2Espinasse et al. 2018, 3Johnson and Schindler 2012
Kamchatka Peninsula salmon were collected at the river mouths, while salmon from Columbia River stocks were collected at Bonneville Dam about 230 km upstream from the sea. The Okanagan and Wenatchee time series were mixed until 2005 and identified separately afterwards based on tag data. For four of the stocks processed (see Table 1), only the last annulus, which grows over the last year at sea, was processed. The last annulus was excised from the rest of the scale following the description given by Satterfield and Finney (2002). Most of the scales were initially glued on gum cards. All the scales were immerged in water and rubbed thoroughly until the scales were transparent and free of residual glue. Details on SI analysis of scales from SE Alaska and Kodiak Islands can be found in Satterfield and Finney (2002).
The scales processed during this study were dried in an oven for 24 hours at 60°C and sent for SI analysis at UC Davis SIF (https://stableisotopefacility.ucdavis.edu/13cand15n.html). The samples were analyzed for 13C and 15N isotopes using a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). The system was calibrated using different NIST Standard Reference Materials. Measurement precision was assessed by running replicates of these standards and result in standard deviation consistently below 0.1‰ both for δ13C and δ15N. Isotopic ratios are expressed in the following standard notation:
δX (‰) = (Rsample / Rstandard – 1) x 1000
where X is 13C or 15N and Rsample is the 13C/12C or15N/14N respectively. δ13C and δ15N were determined in parts per thousand (‰) relative to external standards of Vienna Pee Dee Belemnite and atmospheric nitrogen, respectively.
The large amount of anthropogenic carbon dioxide released into the atmosphere has led to a long term decrease in both atmospheric and oceanic δ13C values, known as the Suess effect (Gruber et al., 1999). The extent of this decrease is directly linked to the rate of change of CO2 concentration and therefore has accelerated in recent decades. Analysis of δ13C time series should be corrected by adjusting values to a year of reference. We applied a correction factor of -0.02 ‰ yr-1, in agreement with recent studies (Espinasse et al., 2018; Williams, Risk, Stone, Sinclair, & Ghaleb, 2007) and standardized the time series using 2015 as the year of reference.
C/N ratios are often used to correct δ13C values for the presence of lipids in the materials analyzed (Post et al., 2007). The scales of adult salmon are mainly made out of collagen and as such show constant C/N values varying between 2.5 to 2.9. However, for some of the published data (Bristol Bay and Rivers inlet stocks), C/N were found out of this range. We suggest that the scales which are not rinsed directly after collection on fish might contain mucus residuals that will stick to the scale even when washed carefully before analysis. We applied a correction for the eight Bristol Bay stocks based on the difference between δ13C of scale with C/N > 3.5 and yearly average of δ13C scales having a C/N < 3.5. This resulted in correcting values for 65 scales out of 543 with a maximum correction factor of 1.2 ‰. The correction factor used for Rivers Inlet stock can be found in Boris Espinasse et al. (2018) and is also based on the differences in δ13C values between scales with expected C/N and scales with relatively high C/N. It has been questioned if the scales should be acidified prior SI analysis as the external layer of the scale is comprised of mineral apatite that could potentially skew analyses of δ13C (Tzadik et al., 2017). However, when the scales grow through the fish life cycle, new layers of collagen are added and the contribution of the external mineral layer to the total weight of the scale decreases (Hutchinson & Trueman, 2006). Furthermore, Sinnatamby, Bowman, Dempson, and Power (2007) found no significant differences between δ13C values of Atlantic salmon scales that were acidified or not. Therefore, none of the scales processed during this study were acidified.
SST data were extracted from the COBE SST2 dataset, which can be downloaded freely at https://www.esrl.noaa.gov/psd/. This dataset provided SST interpolated on a 1 by 1 degree grid, and used a new analysis scheme to reduce uncertainties in analyzed SST (Hirahara, Ishii, & Fukuda, 2013). SST were averaged for each grid cell from January to June, which generally provides highest correlation coefficients for both whole and excised scales.