COMPOSITION AND INTERANNUAL VARIABILITY IN TRACE ELEMENT PROFILES OF PACIFIC SARDINE OTOLITHS

Temperature is a primary factor separating Pacific sardine Sardinops sagax into cold and temperate stocks in the California Current Ecosystem. We collected otoliths of age-1 sardine captured in the spring off central California (1996–97) and in the Southern California Bight (SCB; 1991–92 and 1995–2004) where the cold and temperate stocks presumably overlap during seasonal migrations. To assess whether the sardine had distinguishing chemical characteristics, we compared trace element profiles of otoliths with seawater temperatures to evaluate composition and interannual variability. Ca, Mg, P, Sr, Mn, and Ba in dissolved whole otoliths were analyzed by inductively coupled plasma mass spectrometry. Cohorts from central and southern California differed by collection site in Mg/Ca and P/ Ca ratios. Mg/Ca and P/Ca ratios covaried and correlated with sea water temperature in the SCB in most years. High inter annual variability in some trace element ratios within sites made it difficult to characterize site-specific otolith profiles. Otolith composition was likely influenced by temperature and local conditions. Stock mixing in the SCB may have contributed to interannual variability as well. INTRODUCTION Cooperation between Canada, the United States (US), and Mexico in management of Pacific sardine Sardinops sagax (Jenyns 1842) fisheries along the North American coast would be facilitated by knowledge of the distribution of regional stocks of mature and immature sardine along with their spawning habits and migration patterns. The range of Pacific sardine encompasses much of the California Current Ecosystem (CCE, fig. 1). Three stocks are believed to exist with seasonally synchronous, north-south migrations: a cold, northern stock along the California coast that migrates to the Pacific Northwest in the summer as adults to feed; a temperate, southern stock along the Pacific coast of Baja California, Mexico, that migrates to southern California; and a warm, gulf stock within the Gulf of California that migrates to the Pacific coast as far as Bahía Magdalena, Mexico (FélixUraga et al. 2004, 2005; Smith 2005). The biological and environmental factors that divide Pacific sardine into regional stocks are not definitive. Unique genetic profiles within stocks have not been identified (Hedgecock et al. 1989; Grant and Bowen 1998; Pereyra et al. 2004; García-Rodríguez et al. 2011). Other methods that have offered clues to describing regional stocks and migration patterns include: egg, larval, and adult surveys (Lo et al. 2005, 2010, 2011); vertebral counts and tags (Smith 2005); temperature at catch (Félix-Uraga et al. 2004, 2005; Demer et al. 2012; Demer and Zwolinski 2014); analysis of spawning habitats (Reiss et al. 2008; Zwolinski and Demer 2013); fish and otolith morphometric analysis and aging (Javor et al. 2011; Javor 2013; Vergara-Solana et al. 2013); and otolith stable isotope measurements (Valle and Herzka 2008; Dorval et al. 2011; Javor and Dorval 2014). Trace element composition in immature sardine otoliths could reflect chemical and temperature differences in spawning regions. In US waters, sardine spawn primarily in the spring off central and southern California (Lo et al. 2005, 2010). They spawn in the late fall and winter in the Gulf of California, and in the summer in Bahía Magdalena off southern Baja California (Smith 2005). Seawater temperature of sardine habitats ranges from less than 10 ̊C in the Pacific Northwest (Emmett et al. 2005) to over 25 ̊C in their southern distribution in Mexico (Mitchell et al. 2002; Félix-Uraga et al. 2004, 2005). Temperature is believed to be the primary physical driver of sardine population and individual growth rates, abundance, and large-scale migrations along the North American Pacific coast (Demer et al. 2012; Hill et al. 2014; Dorval et al. 2015). Fisheries biologists often use otolith trace element composition to identify fish stocks. Factors that influence elemental partitioning into otoliths include temperature, salinity (Hoff and Fuiman 1993; Fowler et al. 1995; Elsdon and Gillanders 2002; Bath Martin and Wuenschel 2006), age, and ontogeny (Begg et al. 1998; Rooker et al. 2001; Brophy et al. 2003; Ruttenberg et al. 2005). Using several experimental approaches to influence trace element chemistry of otoliths in a companion study of juvenile Pacific sardine, Javor and Dorval (2016) demonstrated Mg and P were metastable in the JAVOR AND DORVAL: COMPOSITION AND INTER-ANNUAL VARIABILITY IN TRACE ELEMENT PROFILES OF PACIFIC SARDINE OTOLITHS CalCOFI Rep., Vol. 58, 2017 96 otoliths of juveniles over time as they grew, and these two elements covaried in cultured sardine. Mg/Ca and P/Ca ratios decreased with growth in age-0 sardine at 16 ̊–19 ̊C, and Mn/Ca ratios increased. The overall goal of this study was to use otolith trace element composition to identify and compare cohorts of age-1 Pacific sardine fished off central California (Monterey Bay) and in the Southern California Bight (SCB). The central California sardine belong to the cold stock, while the SCB sardine consist of seasonally influenced, migrating populations from cold and temperate stocks (Félix-Uraga et al. 2005). We approached the broad problem by addressing three specific issues: 1) How similar are trace element profiles of sardine otoliths within and between collection sites in the SCB? 2) How do otoliths of sardine cohorts in the SCB compare to each other and with seawater temperature over multiple years? 3) How do trace element profiles in otoliths of cohorts compare in age-1 sardine captured off central and southern California? This investigation is a companion report to comparative surveys of oxygen stable isotopes in sardine otoliths (Dorval et al. 2011; Javor and Dorval 2014) and an experimental study of trace element behavior in otoliths of laboratory-reared, juvenile sardine (Javor and Dorval Javor CalCOFI 2016 TE2