Introduction
Sockeye salmon (Oncorhynchus nerka ) migrate from freshwater to the coastal ocean as juveniles and rapidly migrate from the coast to the open ocean where they typically reside for 2 to 3 years before returning to their natal streams to spawn (Burgner, 1991). Although it is considered that the early marine life phase is the period most critical to survival, there are indications that much of the cumulative juvenile-to-adult mortality of salmon occurs on the high seas (McKinnell, Curchitser, Groot, Kaeriyama, & Trudel, 2014; Welch et al., 2011). It is also during this period, and especially during maturation, that the fish accumulate most of their mass (Ishida, Soto-o, Ueno, & Sakai, 1998) and establish the body condition necessary to undertake successful spawning migration and reproduction. However, our understanding of the high seas life phase and its role on determining stock specific recruitment dynamics remains limited. This knowledge gap reflects the challenges posed by the massive spatial scale of the North Pacific, the wide dispersal of fish, and the associated expense and logistical challenges of sampling.
Many sockeye salmon spawning grounds are located at considerable distance from the sea (up to 100’s km), requiring substantial reserves for upstream migration. The energy requirements depend on water temperature, that will affect salmon metabolic rate, distance, and river flow strength, both of which determine the relative migration duration (Crossin et al., 2008; J. Stevenson Macdonald, Patterson, Hague, & Guthrie, 2010; Rand et al., 2006). It has previously been demonstrated that high adult returns and high productivity (adult recruits produced per spawner) are not necessarily linked. In 1997, Fraser River sockeye returned to the river with poor body condition (smaller size, low lipid content) and concurrently experienced extremely high river discharge (60 to 100% greater than mean levels), resulting in the largest en-route mortality recorded for the Fraser system (Macdonald, 2000). The poor fish condition when entering the freshwater system that year has been suggested to be have been due to warmer sea surface temperature (SST), associated with a record setting El Niño event, and density-dependent effects (S. M. McKinnell, 2000). Warmer than usual temperatures experienced by the fish in the high seas were suggested to have resulted in altered salmon phenology (early maturating) and reduced nutritional health (low lipid content).
Another direct effect of a warming North Pacific Ocean on salmon is a reduction in the area of suitable thermal habitat (Abdul-Aziz, Mantua, & Myers, 2011; Healey, 2011; Welch, Ishida, & Nagasawa, 1998). This is expected to result in an intensification of competition for resources. Recent studies indicate that competition for food among salmon species in the open ocean can play a role in regulating populations (Ruggerone & Connors, 2015; Springer & van Vliet, 2014). Increasing pink salmon abundance (Oncorhynchus gorbuscha ), in part due to hatchery enhancement, combined with the limited carrying capacity of the North Pacific Ocean could result in unfavorable conditions for sockeye development. However, the influence of food composition and abundance on salmon development and condition remains largely unknown, although Pacific salmon seem to be able to adapt their diet when food conditions change (Kaeriyama et al., 2004).
A prerequisite to determining the extent to which these different processes affect sockeye salmon stocks is knowledge of salmon distributions in the high seas (Chittenden, Beamish, & McKinley, 2009). However, this is an extremely challenging task due to the enormous technical and logistical difficulty associated with tracking and monitoring salmon populations at the scale of ocean basins. As a consequence, salmon distributions in the ocean have been only coarsely defined to date and are almost completely unknown when it comes to specific stocks, with the exception of intensive tagging studies carried out by fishery vessels in the 50-60’s (French, Bilton, Osako, & Hartt, 1976). In the future, at-sea genomic analysis may help to resolve this knowledge gap, however, biogeochemical approaches may provide a means to retrospective resolve estimate ocean distributions. The Carbon (δ13C) and Nitrogen (δ13N) stable isotope ratios of salmon scales provide a useful tool to estimate the environmental conditions experienced by fish and their distribution (MacKenzie et al., 2012; Torniainen et al., 2014; Trueman, MacKenzie, & Palmer, 2012). In particular, covariance of SST and δ13C provide a reliable indicator to track animal location (MacKenzie et al., 2011; McMahon, Hamady, & Thorrold, 2013). This relationship is based on the water temperature control of aqueous [CO2], which in turns controls carbon isotope fractionation by autotrophs. Because autotrophs preferentially take up the lighter carbon isotope (12C), high [CO2] concentration (low SST) result in low δ13C, and conversely a decrease in concentrations (high SST) lead to higher δ13C as autotrophs have reduced access to the light isotope (Goericke & Fry, 1994; Rau, Takahashi, & Marais, 1989). Although other mechanisms linked to phytoplankton physiology and community composition do affect phytoplankton δ13C values (Burkhardt, Riebesell, & Zondervan, 1999; Riebesell, Burkhardt, Dauelsberg, & Kroon, 2000), water temperature is considered to be the main driver of δ13C variation at high latitude (Magozzi, Yool, Zanden, Wunder, & Trueman, 2017). Recently, the δ13C / SST correlation has been used to identify salmon feeding grounds in the North Atlantic (MacKenzie et al., 2011) and for one sockeye stock in the North Pacific (Espinasse, Hunt, Doson Coll, & Pakhomov, 2018).
In this study, we build on the preliminary work of Espinasse et al. (2018) to assess stock specific sockeye salmon distributions in the North Pacific through analysis of archived scales for 17 North Pacific sockeye salmon stocks. These scales were all collected in or near the freshwater system and differed in the time range covered, sampling resolution (yearly, every 3rd year, irregular), and material type (whole scale or last annulus). Our primary objectives were to determine whether salmon feeding grounds can accurately be described based on the δ13C / SST relationship and the robustness of this method. We discuss the implications of our results for our understanding of salmon high seas migration history.