Chum salmon migration routes
Our estimates of chum salmon migration routes successfully reproduced a
known migration pattern from coastal Japan to the Bering Sea (Seebet al . 2004; Urawa et al . 2000; 2001). More importantly,
it is likely that chum salmon move onto the Bering Sea Shelf at the last
stage of their somatic growth. Although the number of chum salmon used
for the δ 15NAAs analysis is
limited (n = 2), we observed similar isotopic profiles ofδ 15NBulk in the vertebral
sections of all the salmon we sampled (n = 8). This strongly
suggests that most chum salmon populations in Japan migrate onto the
Bering Sea Shelf at the later stages of their somatic growth. Note,
however, that the migration routes estimated by stable isotope analysis
did not reflect homing migrations to the Japanese coast from the Bering
Sea, possibly because the vertebrae of the sampled individuals did not
exhibit substantial growth during this homing migration.
Given that the fish somatic growth generally stops after sexual
maturation because they devote surplus energy to reproduction (Lesteret al . 2004), salmon vertebrae are not likely to grow
substantially after maturation; in other words, the timing of maturation
corresponds to that of the formation of the last vertebral section
(Section 10). Ishida et al . (1998) investigated the relationship
between age and fork length in chum salmon and showed that they exhibit
substantial somatic growth during sexual maturation (Fig. S6). Other
physiological studies have also shown higher levels of growth hormone in
maturing salmon than those in immature salmon (Björnsson 1994; Steadet al . 1999), although growth rates rapidly decrease in the late
phase of maturation (Stead et al . 1999). Thus, our data indicate
that salmon migration to the Bering Sea Shelf is strongly related to
sexual maturation. The Bering Sea Shelf supports high phytoplankton
productivity (Lester et al . 2004), which feeds one of the highest
benthic faunal biomass densities in the world’s oceans (Grebmeieret al . 1995; Highsmith 1990). Specifically, large euphausiid
populations in the region should be an important food resource for
maturing chum salmon (Moss et al . 2009). For this reason, this
region is a suitable habitat to fulfil the nutritional demands of sexual
maturation in salmon, and its productivity and environmental conditions
may be key determinants of the age and size at maturation of the
Japanese chum salmon, which have a large influence on individual fitness
(Mangel 1996; Roff 1984).