DOI: 10.5061/dryad.hqbzkh1dw
title: Stock specific isotopic composition of maturing sockeye salmon in the North Pacific

description: 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.
description: <i>Data collection and
 scale processing</i> 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 <sup>13</sup>C and <sup>15</sup>N 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 δ<sup>13</sup>C and δ<sup>15</sup>N. Isotopic
 ratios are expressed in the following standard notation:   δX
 (‰) = (Rsample / Rstandard – 1) x 1000   where X is
 <sup>13</sup>C or <sup>15</sup>N and Rsample is
 the <sup>13</sup>C/<sup>12</sup>C or
 <sup>15</sup>N/<sup>14</sup>N respectively.
 δ<sup>13</sup>C and δ<sup>15</sup>N 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 δ<sup>13</sup>C
 values, known as the Suess effect (Gruber et al., 1999). The extent of
 this decrease is directly linked to the rate of change of
 CO<sub>2</sub> concentration and therefore has accelerated in
 recent decades (Swart et al. 2010). Analysis of
 δ<sup>13</sup>C time series should be corrected by adjusting
 values to a year of reference. We applied a correction factor of -0.02 ‰
 yr<sup>-1</sup>, 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 δ<sup>13</sup>C 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
 δ<sup>13</sup>C of scale with C/N &gt; 3.5 and yearly
 average of δ<sup>13</sup>C scales having a C/N &lt; 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 δ<sup>13</sup>C
 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 δ<sup>13</sup>C (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 δ<sup>13</sup>C values of
 Atlantic salmon scales that were acidified or not. Therefore, none of the
 scales processed during this study were acidified.