The stock-specific distribution of maturing salmon in the North Pacific has been a persistent information gap that has prevented us from determining the ocean conditions experienced by individual stocks. This continues to impede understanding of the role of ocean conditions in stock-specific population dynamics. We assessed scale archives for 17 sockeye salmon (Oncorhynchus nerka) stocks covering the entire North Pacific, from the Columbia River (Washington State and British Columbia) to Kamchatka Peninsula (Russia), to infer salmon locations during their last growing season before returning to their spawning grounds. The approach used, first pioneered in salmon stocks in the Atlantic, relies on the relationship between temporal changes in δ¹³C in salmon scales and sea surface temperature to estimate salmon distribution based on correlation strength. An advantage of this approach is that it does not require fish sampling at sea, but relies on existing fishery agency collections of salmon scales. Significant correlations were found for 7 of the stocks allowing us to propose plausible feeding grounds. Complementary information from δ¹⁵N, historical tagging studies, and connectivity analysis were used to further refine distribution estimates. This study is a first step toward estimating stock-specific distributions of salmon in the North Pacific, and provides a basis for the application of the approach to other salmon scale archives. This information has the potential to improve our ability to relate stock dynamics to ocean conditions, ultimately enabling improved stock management. For example, our estimated distributions of Bristol Bay and NE Pacific stocks demonstrated that they occupy different areas with a number of the former being distributed in the high productivity shelf waters of the Aleutian Islands and Bering Sea. This may explain why these stocks seem to have responded differently to changes in ocean conditions, and the long term trend of increased productivity of Bristol Bay sockeye.

Data collection and scale processing

The literature was reviewed for published stable isotope data of sockeye salmon scales. Johnson and Schindler (2012) reported on stable isotope data for eight stocks distributed in Bristol Bay with time series spanning over four decades of scales collected every three years. Satterfield and Finney (2002) reported on a 30-year time series with yearly resolution for a stock located on Kodiak Island in SW Alaska, and Espinasse et al. (2018) reported on stable isotope data for the Rivers Inlet stock (BC coast) which covered more than 50 years with irregular sampling resolution. In addition, we accessed archived scales for two major stocks in the Kamchatka Peninsula (Ozernaya and Kamchatka) (Bugaev et al., 2008), for two stocks of the Columbia River (Okanagan and Wenatchee), for two stocks in SE Alaska (Chilkoot and Chilkat) and for one additional stock on Kodiak Island. The stock locations can be seen in Figure 1 and the details of the time series resolution are provided in Table 1.

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 region between the last annulus and the periphery of the scale, which grows over the last year at sea, was processed. This part 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 stable isotope 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 stable isotope analysis at UC Davis SIF (https://stableisotopefacility.ucdavis.edu/). 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 resulted in standard deviations 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 or ¹⁵N/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 (Swart et al. 2010). 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 (Williams, Risk, Stone, Sinclair, & Ghaleb, 2007; Espinasse et al., 2018) 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 is also based on the differences in δ¹³C values between scales with expected C/N and scales with relatively high C/N (Espinasse et al. 2018) (Fig. S1.1). It has been questioned if the scales should be acidified prior stable isotope 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.