Abstract
The effects of salmon carcasses on forest and stream ecosystems were determined by nitrogen stable isotope analysis in natural streams in eastern Hokkaido, Northern Japan, where numerous chum salmon (Oncorhynchus keta) were migrated upstream from ocean to spawn in autumn. The leaves and soils surrounding riparian forest and aquatic animals were collected at salmon present and absent stream before and after migration. The nitrogen stable isotope ratio (δ15N) of riparian trees (Salix spp. and Ulmus davidiana var. japonica) were not significantly different between the two sites. The grass leaves of Petasites japonicus, collected within 5 m at salmon present stream showed higher δ15N value (6 ‰) than another site (0.5 ∼ 3 ‰), however, this was not significantly different. The δ15N of A0 soil layer at the salmon present site was 3 ‰ higher than absent site, but this was not significantly different. The C-N ratio was the lowest within 5 m of salmon sites, which was also not significantly different. The movement of carcasses was traced by telemetry, and three out of the five carcasses were removed within 10 m. However, one carcass was transported to 500 m distant hillslope ridge. The δ15N in four taxa of stream dwelling invertebrates and juvenile masu salmon (Oncorhynchus masou) were significantly different between the salmon present and absent stream. The temporal change of δ15N values of aquatic invertebrates were compared before and after migration, 0.5 to 3.5 ‰ increases were observed in grazer, collector-gatherer and predator, but not shredder. These results suggest that the salmon carcasses did not significantly effect on riparian vegetation, however, they strongly influenced on stream ecosystems by enhancing trophic levels of aquatic consumers.
