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).