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.