Summary Pacific salmon (Oncorhynchus spp.) transport marine‐derived nutrients (MDN) and organic matter to freshwater ecosystems, which enhances the productivity of North Pacific ecosystems. Relatively few studies, however, have evaluated the MDN subsidy to both the aquatic system and the terrestrial catchment simultaneously. Using stable isotope analysis, we tested how the dynamics of MDN differed between the river and adjacent riparian forest in rivers of the Shiretoko World Natural Heritage Site in eastern Hokkaido (Japan). In addition, we accounted for temporal and spatial variations in the stable isotope signatures of freshwater organisms due to the presence or absence of spawning salmon. We analysed carbon and nitrogen stable isotopes (δ13C and δ15N) of biofilm, invertebrates, fish, riparian plants and brown bear (Ursus arctos) in the Rusha River during the pre‐spawning and spawning periods and in the Akai River (where there are no salmon). Willow leaves were collected along the 50‐m transects to evaluate how far MDN are incorporated within the riparian area. We counted the number of pink salmon (O. gorbuscha) carcasses in riparian areas and categorised their mode of transport. In addition, we examined the stomach contents of Dolly Varden (Salvelinus malma). The δ13C and δ15N of aquatic organisms increased by 1–4‰ and 1–6‰, respectively, with the arrival of salmon spawners. Aquatic organisms incorporated 23% of their nitrogen from salmon (range: 7–46%). The diet of Dolly Varden switched from aquatic invertebrates to salmon eggs during the salmon spawning run. More salmon carcasses were transported from the stream to riparian areas by flooding than by brown bears. The δ13C and δ15N of blowflies (Calliphora spp.) and brown bears increased significantly during the spawning run. Riparian vegetation, with the exception of Manchurian alder (Alnus hirsuta), incorporated 25% of its nitrogen from salmon. The δ15N values of riparian willow (Salix spp.) were correlated negatively with distance from the stream. The proportion of MDN incorporated in the freshwater biota was lower than that reported for North American rivers, potentially due to the influence of dams and modification of the river environment in this Japanese example. The riparian forest incorporated a relatively high fraction of MDN, however, mainly due to the transport of salmon carcasses from the channel by brown bears and, particularly, flooding. The dynamics of salmon‐derived nutrients thus differed between river and adjacent riparian zones. These results suggested the importance of linkages between freshwater and riparian ecosystems for the extent of the marine nutrient subsidy.

Mature male Pacific salmon (Genus Oncorhynchus) develop a dorsal hump, as a secondary male sexual characteristic, during the spawning period. Previous gross anatomical studies have indicated that the dorsal humps of salmon are mainly composed of cartilaginous tissue (Davidson, 1935). However, the histological and biochemical characteristics of such humps are poorly understood. In this study, the detailed microstructures and components of the dorsal humps of pink salmon (O. gorbuscha) were analyzed using histochemical techniques and electrophoresis. In mature males, free interneural spines and neural spines were located in a line near to the median septum of the dorsal hump. No cartilaginous tissue was detected within the dorsal hump. Fibrous and mucous connective tissues were mainly found in three regions of the dorsal hump: i) the median septum, ii) the distal region, and iii) the crescent-shaped region.Both the median septum and distal region consisted of connective tissue with a high water content, which contained elastic fibers and hyaluronic acid. It was also demonstrated that the lipid content of the dorsal hump connective tissue was markedly decreased in the mature males compared with the immature and maturing males.Although, the crescent-shaped region of the hump consisted of connective tissue, it did not contain elastic fibers, hyaluronic acid, or lipids. In an ultrastructural examination, it was found that all of the connective tissues in the dorsal hump were composed of collagen fibers. Gel electrophoresis of collagen extracts from these tissues found that the collagen in the dorsal hump is composed of type I collagen, as is the case in salmon skin. These results indicate that in male pink salmon the dorsal hump is formed as a result of an increase in the amount of connective tissue, rather than cartilage, and the growth of free interneural spines and neural spines.

The expression of synaptic vesicle exocytosis-regulator SNARE complex component genes (snap25, stx1 and vamp2) was examined in the olfactory nervous system during seaward and homeward migration by pink salmon (Oncorhynchus gorbuscha). The expression levels of snares in the olfactory organ were higher in seaward fry than in feeding and homeward adults, reflecting the development of the olfactory nervous system. The expression of snap25a, b and stx1a was upregulated or stable in the adult olfactory bulb and telencephalon. This upregulated expression suggested alterations in olfactory neuronal plasticity that may be related to the discrimination of natal rivers. The expression of stx1b was downregulated in the adult olfactory bulb, but remained stable in the adult telencephalon. The expression of vamp2 was initially strong in seaward fry, but was downregulated in adults in both the olfactory bulb and telencephalon. Pink salmon has the lowest diversity of maturation age, the largest population, and the most evolutional position in Pacific salmon (genus Oncorhynchus). The expression of snares in the olfactory center of pink salmon reflected the timing of sexual maturation and homeward migration. The present results and our previous studies indicate that snares show distinct expression patterns between two salmon species that depend on physiological and ecological features of migration.

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