1 INTRODUCTION
Migratory species often exhibit coordinated movements underpinned by
individual-specific timing of migration, whereby individuals arrive
consistently early or late, relative to other individuals in a
population, at specific habitats or life-history stages (Bell et al.,
2009; Biro & Adriaenssens, 2013; Tamario et al., 2019). Timing of
migration has proven to be crucial in defining fitness consequences
related to reproductive success and offspring survival (Brodersen et
al., 2012; Tibblin et al., 2016). Intra-individual consistency in animal
movements and resultant repeatability in migration timing have received
increased scientific attention as empirical evidence, based on advanced
tracking technologies, accumulates for many migratory species
(Herbert-Read, 2016; Westley et al., 2018), particularly
birds (Gilsenan et al., 2020;
Kentie et al., 2017) and fishes (Brodersen et al., 2012; Forsythe et
al., 2012; van Wijk et al., 2016).
Migration timing has been shown to be partially determined by an
endogenous circannual rhythm that
synchronizes behaviors with photoperiod in birds and fishes (Eriksson &
Lundqvist, 1982; Pulido et al., 2001; Styrsky et al., 2004). This rhythm
arises from genetic evolution in animals, and it is therefore important
to recognize that migratory behaviors are heritable and best understood
in the context of evolutionary adaptation (Brönmark et al., 2013; Quinn
et al., 2000; Thompson et al., 2019). Although these rhythms are driven
by endogenous hormone cycles cued by photoperiod, migration timing in
individuals can be affected by environmental factors (Franklin et al.,
2022; Harrison et al., 2017). Because of consistent individual
differences in animal behavior (Biro & Adriaenssens, 2013; Nilsson et
al., 2014), the behavioral traits of individuals responding to
environmental controls with pronounced seasonality tend to be repeatable
across years (Dahl et al., 2004; Gilsenan et al., 2020; Quinn et al.,
1997). However, identifying proximate environmental drivers and
quantifying their effects on migratory behaviors have been technically
difficult, especially in a field setting, because researchers generally
can only recognize migration at its end point (e.g. a breeding site),
and not at the time and location at which the migration was initiated
(Porlier et al., 2012; Winkler et al., 2014). An additional complication
is that animals typically migrate when environmental variables
monotonically increase (or decrease), inevitably leading to spurious
correlations with day-of-year, even though no causal mechanisms exist
(Dahl et al., 2004; Sinnatamby et al., 2018).
Alternatively, repeatability in migration timing could be manifested
when animals form cohesive groups (e.g. bird flocks and fish schools) of
socially interacting individuals with strong, long-lasting group
fidelity (Fraser et al., 2005; Hay & McKinnell, 2002; Klimley &
Holloway, 1999). Group migration, or collective navigation, is known to
facilitate rapid transfer of beneficial behavioral traits (Gil et al.,
2018; Thorsteinsson et al., 2012), help avoid predation, cancel errors
made by less experienced members (Berdahl et al., 2016), and improve
homing precision to natal locations (Bett & Hinch, 2015). This
mechanism, however, may not be entirely independent of the environmental
control mechanism described above because at least some group members
would still rely on external cues for decision making (Couzin, 2009;
Herbert-Read, 2016, Herbert-Read et al., 2017). Even though the social
aspect of animal migration has been investigated in laboratory and
theoretical studies, it remains largely unexplored in field-based
ecological studies because of logistical and methodological constraints
(Brönmark et al., 2013; Gil et al., 2018).
Sakhalin taimen Parahucho perryi is a critically endangered,
long-lived (>25
years), iteroparous salmonid
inhabiting far eastern Russia and
Hokkaido, Japan (Rand, 2006). During the spring spawning season, mature
Sakhalin taimen migrate upstream to headwater streams in small groups
from their non-breeding habitats in the estuary (Rand & Fukushima,
2014). A recent tagging study revealed that up to 87% of Sakhalin
taimen spawners return to the same tributaries across consecutive years,
one of the highest rates of site fidelity ever documented for
iteroparous salmonids (Fukushima & Rand, 2021). The group migration and
strong site fidelity of the species are consistent with the hypothesis
that social interactions influence their movement patterns during the
spawning migration. However, the rate of upstream migration of this
species is significantly influenced by water temperature and stream
discharge (Rand & Fukushima, 2014), suggesting the importance of
environmental controls on migration timing as well.
In this study, we investigated the migratory behavior of adult Sakhalin
taimen by tracking spawners and post-spawners in a river system in
Hokkaido, Japan, with multiple spawning streams. We first characterized
migration timing at specific waypoints and examined the degree to which
the migration timing was repeatable among individuals across years. We
then explored whether the repeatability in migration timing resulted
from individual-specific responsiveness to seasonality in the
environmental controls, or from social interactions among comigrating
individuals. We discuss our findings in relation to the concepts of
migratory connectivity (Webster et al., 2002) and differential migration
(Briedis & Bauer, 2018; Brodersen et al., 2012).