The migratory history of the brown trout, Salmo trutta, collected from Japanese rivers, was examined in terms of strontium (Sr) and calcium (Ca) uptake in the otolith, by means of wavelength dispersive X-ray spectrometry on an electron microprobe. Sea-run (anadromous) and freshwaterresident (nonanadromous) types of S. trutta were found to occur sympatrically. Otolith Sr concentration or Sr : Ca ratios of anadromous S. trutta fluctuated strongly along the life history transect in accordance with the migration (habitat) pattern from sea to freshwater. In contrast, the Sr concentration or the Sr : Ca ratios of nonanadromous fish remained at consistently low levels throughout the otolith. The higher ratios in anadromous S. trutta, in the otolith region from the core to 1500 µm, corresponded to the initial seagoing period, probably reflecting the ambient salinity or the seawaterfreshwater gradient in Sr concentration. The findings clearly indicated that otolith Sr : Ca ratios reflected individual life histories, enabling the sea-run S. trutta to be distinguished from the freshwater-resident brown trout.

The environmental history of the shirauo, Salangichthys microdon, was examined in terms of strontium (Sr) and calcium (Ca) uptake in the otolith, by means of wavelength dispersive X‐ray spectrometry on an electron microprobe. Anadromous and lacustrine type of the shirauo were found to occur sympatric. Otolith Sr concentration or Sr : Ca ratios of anadromous shirauo fluctuated strongly along the life‐history transect in accordance with the migration (habitat) pattern from sea to freshwater. In contrast, the Sr concentration or the Sr : Ca ratios of lacustrine shirauo remained at consistently low levels throughout the otolith. The higher ratios in anadromous shirauo, in the otolith region from the core to 90–230 μm, corresponded to the initial sea‐going period, probably reflecting the ambient salinity or the seawater–freshwater gradient in Sr concentration. The findings clearly indicated that otolith Sr : Ca ratios reflected individual life histories, enabling these anadromous shirauo to be distinguished from lacustrine shirauo.

Densities of wild masu salmon, Oncorhynchus masou Brevoort, were investigated before (late‐September) and after (mid‐November) autumnal habitat shifts in a small river in Hokkaido, northern Japan. Abundance of instream cover habitat formed by coarse woody debris, submerged vegetation and undercut banks was quantified. Density of 1+ and older masu salmon was significantly correlated with cover abundance in autumn, although cover was not significant for density of 0+ fish, indicating that clear‐cut reaches were utilized by 0+ masu salmon in autumn. In early winter, densities of 0+ and 1+ and older masu salmon were both correlated with cover availability. These results suggest that the amount of cover habitat is an important regulator for densities of juvenile masu salmon during winter.

Smolts of anadromous masu salmon Oncorhynchus masou aged 3+ years were found in a northern Japanese river. This is the first recording of 3+ year smolts in Japan. These fish appeared to originate in the cold upper river where 2+ year parr were found during autumn.

To increase commercial marine catches of masu salmon Oncorhynchus masou in northern Japan, hatchery‐reared juveniles are stocked at several life stages. To minimize mortality from freshwater angling, fish are often stocked in the fall when the fishing season is almost finished. In the fall of 1994–1998, we experimentally stocked hatchery‐reared age‐0 masu salmon (mean weights, 4.1–13.9 g) in the Masuhoro River of northern Japan. Mark–recapture experiments were conducted to estimate the numbers of hatchery‐origin masu salmon smolts in the spring. Numbers of masu salmon parr in the river were estimated in July 1998–1999 when the smolt runs were almost finished. The proportion of fish smolting at age 1 (2.2–15.7%) was positively correlated with mean weight at release. Overwinter survival (9.0–17.0%) was also correlated with fish weight at release. Our study showed that size at release is an important factor when stocking age‐0 masu salmon in the fall to maximize the number of smolts produced and thereby support marine fisheries.