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The frequency of reproduction and homing success are crucial in determining the lifetime reproductive fitness of iteroparous salmonids. We examined consecutive spawning and homing by Sakhalin taimen (Parahucho perryi), one of the least-studied iteroparous salmonids. The average consecutive return rate was 69.5%, the highest ever reported for an iteroparous salmonid. The consecutive return rates were highest for large spawners, females, and individuals with active inter-tributary movements in the preceding year. We estimated tributary-scale repeat homing rates to be 50–87%, which was significantly higher than expected by chance alone. The repeat homing rates were higher for spawners that entered tributaries with more abundant spawners in the preceding year, which is consistent with the pheromone hypothesis because the abundance of spawners in a tributary is expected to correlate to the concentration of chemical homing cues emanating from offspring in the following year. These extremely high consecutive return and homing rates likely facilitated local adaptation over evolutionary time scales and have contributed to population stability in this river system. These same traits, however, may impede species recovery following habitat restoration efforts within the species historic range (e.g. dam barrier removal or improved passage) due to their limited rate of dispersal.

The conservation of land-locked populations of migratory fishes is increasingly important in the era of dam removal and habitat reconnection. We used an otolith strontium (Sr) tracer (87Sr/86Sr) to test the hypothesis that a land-locked population of an endangered salmonine species, Sakhalin taimen (Parahucho perryi), retains the capacity for anadromy, and that some individuals out-migrate from the reservoir to the sea. Years later, these individuals return but are blocked by the migration barrier of the reservoir dam and are denied reproduction in their natal streams. Juvenile taimen collected from the reservoir and two nearby regions were classified based on their otolith Sr isotopic signatures to their regions of origin with an overall accuracy of 88%. When the same classifier was applied to ocean-caught adult taimen, we predicted some individuals had originated from the reservoir with high posterior probabilities (> 0.9). Whether the land-locked Sakhalin taimen can help sustain the metapopulation dynamics of the species at the watershed scale may depend on whether, and how soon, the disrupted migration pathway is restored.

Sakhalin taimen (Parahucho perryi) range from the Russian Far East mainland along the Sea of Japan coast, and Sakhalin, Kuril, and Hokkaido Islands and are considered to primarily be an anadromous species. We used otolith strontium-to-calcium ratios (Sr/Ca) to determine the chronology of migration between freshwater and saltwater and identify migratory contingents of taimen collected from the Koppi River, Russia. In addition, we examined taimen from the Sarufutsu River, Japan and Tumnin River, Russia that were captured in marine waters. Transects of otolith Sr/Ca for the Sarufutsu River fish were consistent with patterns observed in anadromous salmonids. Two fish from the Tumnin River appeared to be recent migrants to saltwater and one fish was characterized by an otolith Sr/Ca transect consistent with marine migration. Using these transects as benchmarks, all Koppi River taimen were classified as freshwater residents. These findings suggest more work is needed to assess life history variability among locations and the role of freshwater productivity in controlling migratory behavior in taimen.

Rainbow trout introduced into Hokkaido in 1920 have become widely distributed due to extensive release into many reservoirs and lakes for sport-fishing; their presence often results in reductions of native fish populations. We analyzed and predicted the relationship between the probability of occurrence of rainbow trout and the proximity of dams (or attributed reservoirs), using a database of the presence or absence of rainbow trout collected during 1960–2004 in Hokkaido to clarify the spread patterns of exotic species (e.g., rainbow trout) due to large-scale damming over a long period. Rainbow trout were abundant in streams within approximately 10 km of dams in recent years, regardless of whether the stream was up- or down-stream from the dam and after accounting for the effects of other environmental variables (e.g. elevation, population density, and survey year). A delayed increase in trout occurrence below dams as compared with above dams suggests that the occurrence below dams may be largely due to escapement of stocked populations and a continuously increasing abundance since 1970. The management of dams and reservoirs is necessary to prevent further spread of rainbow trout because they can threaten habitats of native Japanese salmonids through various mechanisms.

We analyzed the patterns of total, endangered and exotic fish species richness in 80 reservoirs throughout Japan using Generalized Linear Models (GLMs) with variables of dam characteristics (e.g. reservoir size, isolated period, altitude, DO, pH, rainfall, and air temperature) and watershed characteristics (e.g. watershed area, human population, and land-use patterns). Exotic species richness was positively correlated with total species richness but negatively correlated with endangered species richness. Largemouth bass, rainbow trout, and bluegill were the three most widely distributed species, occupying 47.5, 37.5, and 33.8% of the reservoirs, respectively. The patterns of total and endangered species richness were largely explained by watershed area and annual air temperature. Exotic species richness was determined primarily by the proportion of developed areas within each watershed. Therefore, reservoirs in more developed areas tend to support more exotic fish species that in turn threaten endangered fish species more severely.

Quasi-continuous DO and pH measurements (total 47 days) were conducted during enclosure experiments (6 enclosures; 5 × 5 × 2.5 m), in which a biomass gradient of silver carp was created. After subtracting the air–water exchanges of O2 and CO2, the chemical and biochemical changes in DO (dissolved oxygen) and DIC (dissolved inorganic carbon) were estimated in order to evaluate MQ (metabolic quotient: DO change divided by DIC change) at intervals of 1 hour. By removing small absolute changes below the threshold value (0.01 mM h−1), the averaged values of the 24 MQ means for the respective 1-hour periods ranged from 0.96 to 1.20 in the six enclosures. Because the MQs in the daytime inversely correlated well with the ratio of NH+ 4–N to (NH+ 4–N + NO− 3–N), not the ecosystems, i.e., density of fish, community structure of zooplankton and phytoplankton, but the form of nitrogen uptaken for primary production principally determined the MQs. The higher MQs observed in the daytime compared with the nighttime (from 14% to 21% except 3% for one enclosure) could not be explained by the denitrification and/or dissolution of CaCO3 in the sediments, therefore suggesting the selectively faster decomposition of part of the organic matter provided through primary production, in other words, an accumulation of another part of the organic matter in the diurnal and/or daily time scale.