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It is known that environmental parameters, such as water depth, size of substrate material, and current velocity, influence the organization of stream fish communities. However, few studies have investigated the effects of these parameters on the sympatric freshwater eels. Here, the habitat use of two anguillid eels, the Indo-Pacific eel (Anguilla marmorata Quoy and Gaimard, 1824) and the Japanese eel (Anguilla japonica Temminck and Schlegel, 1846), coexisting in the same river systems in Japan, was investigated. It was found that the tropical species A. marmorata exclusively used habitats with larger substrate materials, fewer fallen leaves and less leaf detritus, higher current velocity, and lower turbidity than the temperate species A. japonica, and the distributions of the two anguillid eels were clearly segregated. Thus, when both species co-occur in the same river systems, clear flowing mainstems and tributaries are preferred habitats for A. marmorata, whereas stagnant muddy estuaries, backwater areas, irrigation channels, and reservoirs are preferred habitats for A. japonica. These habitat segregations were consistent as body size increased and life-stage developments from elver to yellow eel and did not show ontogenetic shifts. The findings indicated that both species had strong habitat preferences, and these environmental parameters must be considered in the conservation of anguillid eels in sympatric zones.

Few studies have examined the movement of adult Pacific salmon Oncorhynchus spp. between the sea and rivers after they have entered a river. In this study, archival tags that recorded temperature and salinity were attached to pink salmon (Oncorhynchus gorbuscha) that had ascended one of four rivers in northern Japan, to examine their individual movement patterns among fresh, brackish, and salt water. Of 44 pink salmon tagged and released in the rivers, nine were recaptured in the same river where they had been tagged and released, four were recaptured in marine nearshore trap fisheries, and one was recaptured in a neighboring river from where it was tagged and released. Among the 14 recaptured tagged fish, abrupt changes in salinity records were found in seven tag records, indicating that tagged fish had returned to brackish or seawater. Six of the seven individuals were males, and the sex ratio was biased toward males when compared with all tag-released individuals. The results suggest that a substantial proportion of homing adult pink salmon may make short-term movements between fresh and saltwater during their spawning migration. Analysis of the tag-recorded water temperatures showed significantly cooler riverine temperatures compared with inferred temperatures of the coastal water (up to 20.5 °C). While many factors, including orienting behavior and male-biased dispersal, likely influenced the observed ‘probing’ behavior, I speculate that the temporal use of freshwaters rivers may be a thermoregulatory behavior to avoid high, stressful temperatures. Future research relating the short-term movements between fresh water and salt water to local environmental conditions is needed to investigate this hypothesis.

Genetic characteristics of the southern Asian Dolly Varden, Salvelinus curilus, and northern Dolly Varden, S. malma malma, in northeastern Asia and the Aleutian Islands were examined using mitochondrial (mt) and microsatellite (ms) DNA markers. The maximum-likelihood tree based on mtDNA control region haplotypes showed two well-supported monophyletic lineages for these species, but the haplotypes found in the Sakhalin Island populations were an admixture of the two mtDNA lineages. Bayesian clustering using msDNA indicated that all individuals from Sakhalin Island grouped with the S. curilus populations, regardless of their mtDNA haplotype. Incongruence between mtDNA and msDNA markers provided strong evidence of historical mtDNA introgression from S. m. malma to S. curilus. Secondary contact by postglacial colonization from different refugia is a plausible explanation for the introgressive hybridization detected in Sakhalin Island.

Alternative life‐history tactics of masu salmon Oncorhynchus masou are well documented. Subsequent to the freshwater parr stage (age ≥ 1 + years), an anadromous form migrates to the sea after smolting, while a resident form matures without seaward migration. In addition to this typical migratory dimorphism, anecdotal reports based on field observations have indicated that some underyearling masu salmon use estuarine waters. However, no empirical evidence indicates saltwater utilisation and subsequent survival in the early parr stage. Here, we used otolith microchemistry to examine whether a portion of masu salmon parr in northern Japan enters coastal habitats. The otolith Sr:Ca ratios of most juveniles collected from six rivers had consistently low values, indicating that masu salmon parr inhabiting these rivers stay only in freshwater. In contrast, in individuals from a steep‐gradient river the Sr:Ca ratios increased at about a 200‐µm distance from the otolith core. These results suggest that some masu salmon parr might use brackish water or sea water temporarily. In addition, three masu salmon parr were found in another steep river where a culvert located only ten metres from the river mouth completely blocked upstream migration for spawning. The Sr:Ca ratios in these fish increased at about >200 µm from the otolith core, indicating the parr had immigrated to the non‐natal river from the sea. Such flexible behaviour at an early life stage may contribute to the spatial expansion of masu salmon, and the movement could moreover help to stabilise its population dynamics.

Climate oscillations during the Pleistocene had profound effects on the evolutionary history of freshwater fishes now distributed across northern temperate regions. The extent of continental glaciation on the western side of the North Pacific, including areas of East Asia, was more limited as compared with regions of North America, Europe and high‐latitude areas of the North Pacific. Therefore, the effects of climate oscillations might have influenced species in dissimilar ways depending on the species' distribution. We used mitochondrial DNA (mtDNA) and microsatellite DNA (msDNA) markers to clarify the evolutionary history of masu salmon Oncorhynchus masou subspecies‐complex (family Salmonidae) distributed in historically non‐glaciated regions in the western North Pacific. No marked regional or subspecies‐specific mtDNA haplotype associations were recognised, except for O. masou subsp., a lacustrine form endemic to Lake Biwa, an ancient lake in central Honshu. The landlocked subspecies O. masou formosanus, with a disjunct distribution on Taiwan Island, exhibited no diagnostic population features differing from the other subspecies, in either mtDNA or msDNA markers. Mismatch distribution and Bayesian skyline plot analyses indicated relatively recent range expansion and rapid population growth for masu salmon during the last glacial period (c. 0.1–0.15 Ma). Contrary to the mtDNA genealogy, Bayesian clustering using msDNA showed two main genetic clusters, mainly northern populations of the subspecies O. m. masou and populations of the subspecies O. m. ishikawae in southern areas of the Japanese Archipelago. Notably, O. m. formosanus on Taiwan Island was included in the O. m. masou group, and O. masou subsp. was included in the O. m. ishikawae group. Our results suggest that the masu salmon subspecies‐complex in historically non‐glaciated regions of the Temperate Northern Pacific is characterised by weak population structuring and shallow genetic differentiation among the subspecies, except for O. masou subsp. owing to its long isolation in Lake Biwa. Incomplete lineage sorting and historical inter‐subspecies hybridisation, possibly due to secondary contact, seem to be plausible explanations for discrepancies in the mitochondrial DNA genealogy and nuclear DNA genetic structure.

In Japan, numerous artificial dams constructed for erosion control or hydroelectric power generation have affected almost all rivers and resulted in isolation and fragmentation of many freshwater fish populations. We examined genetic structure and diversity of two temporally isolated samples (spanning 15 years) from 23 white-spotted charr (Salvelinus leucomaenis) populations, including above- and below-dam populations, on Hokkaido Island. We used microsatellite DNA markers to elucidate how habitat fragmentation by damming can spatiotemporally alter population genetic structure. An isolation-by-distance genetic structure was found for anadromous populations on Hokkaido Island, which indicates substantial gene flow among populations. After habitat fragmentation by damming, isolated populations were indiscriminately subjected to genetic differentiation from anadromous (below-dam) populations within river systems, and to the subsequent loss of genetic diversity. Effective population sizes of isolated populations were less than 50. We found that substantial decreases in heterozygosity and allelic richness occurred in some isolated white-spotted charr populations over 15 years. This study shows that habitat fragmentation by damming induces significant spatial changes in population genetic structure and genetic diversity within river systems. Our findings also provide an important implication that most isolated white-spotted charr populations will continuously lose genetic diversity in the future.

Habitat fragmentation caused by damming can greatly reduce the population viability of aquatic organisms, with smaller fragmented populations at higher risk of extinction due to increased demographic, genetic, and environmental stochasticity. However, empirical evidence demonstrating that smaller natural populations are more vulnerable to extinction is limited. We studied the vulnerability to extinction of white-spotted charr (Salvelinus leucomaenis) populations in 30 dammed-off streams in Oshima Peninsula, southwestern Hokkaido Island, Japan, by comparing the incidence of charr populations in streams between 1999 and 2014. Using electrofishing and environmental DNA surveys, we identified three localized extinctions, with the probability of extinction increasing with decreasing watershed area (our surrogate for habitat size). We also found a new population in one dammed-off stream in which white-spotted charr were previously unknown, after installation of a fish ladder, indicating the capacity of white-spotted charr to recolonize reconnected habitat in a short period. Our results suggest that localized extinction of white-spotted charr in small dammed-off streams is ongoing, but that appropriate fish migration corridors can reduce localized extinction risk and increase the probability of species persistence.

The degree of iteroparity in stream-resident forms of masu salmon Oncorhynchus masou was examined using mark-recapture studies in natural streams. In a partially migratory population, at least 10% of resident males survived after maturation and repeatedly matured for up to 5 years. In the landlocked amago salmon subspecies, the post-maturation survival rate was at least 7% and repeat maturation was observed for up to 3 years for both males and females.

Human social activities are realized by a synergy of neuronal activity over various regions of the brain, which is supported by their connectivity. In the present study, we examined associations between social activities, represented by work hours, and brain connectivity as quantified using diffusion tensor imaging (DTI). In 483 healthy participants, DTI analysis was performed using 3 T magnetic resonance imaging, and work hours were calculated, considering hours of paid employment (the “Work for Pay” category), hours of housework (the “Work at Home” category), and hours of school-related study (the “Student” category). The correlations between each class of work time and DTI indices were analyzed. The mean diffusivity (MD) values of the anterior limb of the internal capsule (ALIC) and the superior fronto-occipital fasciculus (SFO) were negatively correlated with total work hours (ALIC: r = -0.192, p = 2.3 × 10−5; SFO: r = -0.161, p = 3.8 × 10-4). We also found that the MD values of the ALIC and the SFO were correlated with work hours in the Work for Pay category (ALIC: r = -0.211, p = 3.2 × 10-6; SFO: r = -0.163, p = 3.4 × 10-4) but not with those in the Work at Home category or the Student category. These results suggest that social activity is associated with the white matter microstructure of the ALIC and the SFO. The main difference between “Work for Pay” and the other two social activities appears to be the type of motivation—for example, external versus internal. Therefore, the white matter microstructure of the ALIC and SFO may be related to externally motivated social activities.

Trout and char (hereafter, trout) represent some of the more cul-turally, economically and ecologically important taxa of freshwater fishes worldwide (Kershner, Williams, Gresswell, & Lobón-Cerviá, 2019a). Native to all continents in the Northern Hemisphere (as well as western Mediterranean Africa), trout belong to seven genera (Oncorhynchus, Salvelinus, Salmo, Hucho, Parahucho, Brachymystax and Salvethymus), which are distributed across more than 60 countries (Muhlfeld et al., 2019). Despite their broad im-portance as indicators of biodiversity in cold-water ecosystems (Haak & Williams, 2013), as well as cultural icons for food and rec-reation, nearly half of the world's recognised trout species (IUCN, 2018) are imperilled or at risk of global extinction (Muhlfeld et al., 2018, 2019). The root causes of their vulnerability include broad-scale alteration of landscapes and watersheds, dams, overhar-vest, pollution, interactions with hatchery-bred conspecifics and non-native species. However, emerging threats such as climate change and related problems such as the spread of diseases and parasites pose significant challenges and uncertainties to native trout and their habitats (Kovach et al., 2016; Muhlfeld et al., 2018). Ultimately, conservation of native trout depends on understand-ing their diversity, a willingness to address threats at their root causes and implementing progressive conservation solutions that promote persistence of these iconic species in the face of growing human pressures.

Aims Many parasitic helminths are known to alter host immune responses and consequently affect the progression of autoimmune and allergic diseases. The parasitic nematode Trichinella sp has been reported to suppress several experimental diseases in rodents, including experimental autoimmune encephalomyelitis, type 1 diabetes, colitis, airway inflammation and autoimmune arthritis. We tried to clarify requirement of Th2 cytokines in the anti-arthritic effects of Trichinella spiralis (Ts) against collagen-induced arthritis (CIA). Methods and results We infected Ts and then induced CIA in STAT6KO DBA/1 mice, comparing the disease progression with that in wild-type (WT) DBA/1 mice, Ts significantly mitigated arthritis in WT mice, in addition to the impairment of anti-type II collagen (IIC) IgG production in a subclass-independent manner. The genetic absence of STAT6 in the mice did not abrogate the anti-arthritic effects of Ts. Alteration of splenic cytokines was not related to the anti-arthritic effects of the parasite. Moreover, lack of IL-10 did not abrogate the anti-arthritic effects of Ts. Conclusion Our results suggest that the anti-arthritic effects of Ts do not require host Th2 signals.

In the Orumappu River, northeastern Hokkaido, 667 wild masu salmon smolts were tagged and released. During annual monitoring within the river, 10 individuals were recaptured, and an additional individual was reported from a coastal fishery. All adult masu salmon were recaptured the year after their release, and all had spent one winter at sea. When examining scales of marked fish, four individuals lacked winter annuli and one individual had 2 or 3 resting zones (closely spaced circuli) in the oceanic zone. Accordingly, we believe it is uncertain to assess age of adult masu salmon based solely on scale annuli.

Many studies have indicated that invasive species can drive declines in native species through interspecific competition. In freshwater ecosystems, brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) have been implicated in reducing native species, especially congeneric salmonid fishes. However, demonstrating causality in the wild is problematic because non-natives might replace natives in areas where the former were extirpated by other environmental changes. Using the recently developed technique of multispatial convergent cross mapping (CCM) and a long-term monitoring data set (2002–2017) with spatial replication on both introduced and native salmonids in a Japanese stream, I tested whether an increase in non-native trout caused a decrease in native charr (Salvelinus leucomaenis). Native charr decreased their population density over time in contrast to the non-native brown trout and rainbow trout. The dominant species changed from native charr (64%) in 2002 to non-native trout (97%) by 2017. Multispatial CCM identified significant causal forcing of the native charr by both rainbow trout and brown trout, lending support for the hypothesis of displacement, rather than replacement, of a native species by non-native species. These results therefore suggest that eradicating the invasive trout species may aid the recovery of native charr in the region.

Partial migration, in which a portion of the population migrates while the rest of the population remains as residents, is a common form of migration. Alternative migratory tactics (AMTs) of partial migration are often determined by polygenic threshold traits. However, the ultimate mechanisms that drive inter-population variations in threshold traits are not well understood. We present a simple schematic model to explain how the threshold trait changes with fitness consequences under opposing natural and artificial selection forces. We conducted a field test to evaluate the effects of migration difficulty (as a natural selective force) and selective captive breeding (as an artificial selective force) on threshold traits of a partially migratory fish. Male masu salmon Oncorhynchus masou in the Shari River system have AMTs divided into three population categories of hatchery, wild/above the waterfall, and wild/below the waterfall (control). The wild/above the waterfall salmon live in a high-migration-cost situation, and the threshold trait changed in a direction that promoted residency. In hatchery salmon, which are produced by migrant-selective captive breeding, the threshold trait changed in a direction that promoted migration. In contrast, Dolly Varden charr Salvelinus malma displayed only resident tactics, and the threshold trait did not differ between the populations above and below the waterfall, indicating that environment did not explain the variation in the threshold trait. Our results support the model and suggest that opposing natural and artificial selection forces drive variations in the threshold traits and migratory patterns in the studied species. Our conceptual framework for the ultimate mechanism may help to better understand adoption of AMTs and production of diverse intraspecific traits in migratory animals.

Many animal species exhibit size dimorphism between sexes. Sexual selection, whereby male–male competition favors larger body sizes, has been considered a likely cause of sexual size dimorphism. Habitat features in breeding areas could affect the outcome of male–male competition, yet few attempts have been made to relate breeding habitat features with interpopulation variation in sexual size dimorphism. In this study, we examined interpopulation variation in sexual size dimorphism by studying the landlocked amago salmon (Oncorhynchus masou ishikawae) at a microgeographic scale. We found that female body size was independent of stream size but that male body size decreased with smaller stream sizes. A likely explanation is that the relationship between reproductive success and the size of males is influenced by the availability of refuges that are only available to small-bodied males. Sexual differences in body size increased with decreasing stream sizes, supporting the hypothesis that the reproductive success of larger males is reduced in smaller streams. In contrast, the maturation-length threshold increased with stream size for both sexes. The stream-size-based interpopulation variation in sexual size dimorphism and size at maturity in landlocked amago salmon may therefore have arisen through a combination of sexual and natural selection.

Researchers have long assessed the ecological impacts of metals in running waters, but few such studies investigated multiple biological groups. Our goals in this study were to assess the ecological impacts of metal contamination on macroinvertebrates and fishes in a northern Japanese river receiving treated mine discharge and to evaluate whether there was any difference between the metrics based on macroinvertebrates and those based on fishes in assessing these impacts. Macroinvertebrate communities and fish populations were little affected at the downstream contaminated sites where concentrations of Cu, Zn, Pb, and Cd were 0.1–1.5 times higher than water-quality criteria established by the U.S. Environmental Protection Agency. We detected a significant reduction in a few macroinvertebrate metrics such as mayfly abundance and the abundance of heptageniid mayflies at the two most upstream contaminated sites with metal concentrations 0.8–3.7 times higher than the water-quality criteria. There were, however, no remarkable effects on the abundance or condition factor of the four dominant fishes, including masu salmon (Oncorhynchus masou). These results suggest that the richness and abundance of macroinvertebrates are more sensitive to metal contamination than abundance and condition factor of fishes in the studied river. Because the sensitivity to metal contamination can depend on the biological metrics used, and fish-based metrics in this study were limited, it would be valuable to accumulate empirical evidence for ecological indicators sensitive to metal contamination within and among biological groups to help in choosing which groups to survey for general environmental impact assessments in metal-contaminated rivers.

We obtained the following five results in 10 rivers while elucidating the potential threat of introduced rainbow trout to native salmonids. First, native salmonid density decreased with increasing trout density. Second, four environmental variables, namely altitude, distance from the sea, river width, and river depth, did not significantly affect trout density. Third, the distribution of trout expanded, and trout were the dominant species in an unconfined river. Fourth, the trout invasion has changed salmonid faunas over the last 20 years. Lastly, smolt trout were collected from a river. Therefore, the potential threat to native salmonids due to trout introduction is continuously increasing.

Positively size-selective vulnerability to fishing is well established in recreational fisheries. Size-selective harvesting can either induce an indirect selection response of behavioural traits that are correlated with size or exert direct selection pressures on behaviours that contribute to vulnerability. In addition, learning to avoid future capture may always happen. Behavioural change caused by fishing may in turn affect the size-selective properties of angling. To test this prediction, field experiments with amago salmon (Oncorhynchus masou ishikawae) were conducted. We demonstrated that, as expected, large fish were more vulnerable than smaller individuals in a low fishing pressure (LP) stream and that positively size-selective angling was not (or no longer) present in a high fishing pressure (HP) stream. Moreover, fish in the HP stream were by far less vulnerable to angling with natural bait than fishes in the LP stream. Laboratory studies showed that offspring from HP streams were intrinsically shyer than offspring collected from LP streams. We propose that the increased timidity of individuals inhibiting HP streams disrupted the generally positive relationship among salmon body size and its vulnerability to angling. Fisheries-induced timidity in response to high exploitation rates reduces catchability, affects the size-selective properties of angling, reduces the value of fishery-dependent stock assessments, and potentially affects trophic interactions.

White-spotted charr (Salvelinus leucomaenis) typically occupy the upstream reaches of Japanese streams, whereas masu salmon (Oncorhynchus masou) are generally found downstream. Temperature varies predictably with altitude; thus, it is thought to be an important determinant of the altitudinal niche partitioning. We examined (i) the species composition and habitat availability (e.g., water temperature and velocity) in reaches along an altitudinal gradient (elevation: 0–270 m, gradient: 0.6–11%), (ii) microhabitat use at the individual level, and (iii) swimming stamina against a fixed water flow velocity using stamina tunnel tests in the Ohkamaya River, Hokkaido, Japan. The proportion of white-spotted charr increased in an upstream direction from 5 to 95%, whereas summer water temperature and average velocity increased downstream (temperature: 15–18°C, velocity: 17–40 cm s−1). Underwater observations revealed that white-spotted charr used slow velocity microhabitat more than masu salmon under sympatric and allopatric conditions (charr: 7–13 cm s−1, salmon: 15–23 cm s−1). Masu salmon swam twice as long as white-spotted charr against a fixed-velocity (66 cm s−1). Our results suggest that velocity was an important determinant of the observed altitudinal distribution patterns of masu salmon and white-spotted charr.

Among years, fry-to-adult survival of hatchery-reared chum salmon Oncorhynchus keta was positively correlated with the length (in days) of the fry out-migration period with temperatures suitable for migration. Furthermore, survival decreased with increasing difference in mean temperature between May and June. Thus, prolonged out-migration periods increased the probability of survival from fry to adult, lending support to the hypothesis that long migration periods decrease the risk of mortality (bet-hedging), and increase the probability of migration when environmental conditions in fresh water and the ocean are suitable (match-mismatch).

Evaluating the contribution of wild and hatchery fish to a fishery is essential to understand economic feasibility as well as the impact of hatchery fish on the ecosystem. However, a precise estimate of this contribution is often difficult to obtain, particularly when hatchery and wild fish are mixed in the catch. In this study, we quantified the contribution of hatchery and wild Pink Salmon Oncorhynchus gorbuscha to the mixed-stock commercial fishery in Japan by identifying the ratio of otolith-marked hatchery fish to unmarked and presumably wild fish. The contribution of hatchery fish to the total coastal catch of Pink Salmon in Japan was estimated to be 16.6% and 26.4% in 2011 and 2012, respectively. Thus, the majority of the commercial salmon catch originated from naturally spawned wild fish. Economic yield per release by Japanese hatcheries was 2.2 yen (¥2.2) (≈US$0.022) and ¥1.5 in 2011 and 2012.Received October 5, 2014; accepted February 7, 2015

Annual and spatial variations in the growth condition of the arabesque greenling (Pleurogrammus azonus) were examined in relation to temperature and food availability. We found age-dependent effects of temperature and sex-dependent effects of zooplankton biomass on the growth condition. Growth condition increased with bottom temperature in the 1+ group, but this effect was reversed in the older groups. This age-dependent effect of temperature on growth condition was consistent with regional differences in growth patterns. Arabesque greenling living in cool sea areas reached a larger final adult size (i.e. larger L ∞), but the juveniles grew slower (i.e. smaller k) than those living in warm sea areas. Based on our results, we suggest that the negative effect of climate warming on growth condition might be more severe for older individuals and that a climate change-driven decrease in the somatic growth of adult fish could bring a population growth rate decline through maternal effects.

Temperature-driven life-history modifications by adaptation occur in ectotherms, and therefore, life-history modifications by adaptation need to be taken into consideration when predicting population responses to the climate change. Partial migration is a common form of life-history diversity in which a population contains both migratory and resident behaviours. Salmonid fish exhibit a wide range of life-history diversity and, in particular, partial migration. We evaluated the effect of temperature-driven life-history modifications on population dynamics in partially migratory masu salmon (Oncorhynchus masou) by field observations and theoretical models. Field observations revealed that spatial patterns of alternative migratory tactics were associated with temperature gradients. The occurrence of resident males increased, whereas the proportion of migrant males and the proportion of delayed migrants including both sexes decreased with increasing temperature and, thereby, with improved early growth conditions. The expected fitness for each migratory tactic was computed in a life-history model with early growth conditions as a function. Individual fitness was maximized by adopting resident tactics under favourable early growth conditions, early migrant tactics under intermediate early growth conditions and delayed migrant tactics under unfavourable early growth conditions. The results suggest that individuals exhibited a status-dependent conditional strategy, that is, the adoption of alternative migratory tactics is influenced by the status of individuals to make the best of a situation. A simulation model suggests that increased residency by males to increased temperature leads to a substantial decrease in the number of migrants. Moreover, the decrease in the number of delayed (older) migrants with increasing temperature magnified fluctuations in abundance. Our findings indicate the importance of temperature-driven life-history modifications for predicting dynamics of natural populations under climate warming.

The environmental conditions faced by stream-dwelling fish in many regions are highly variable during fall and winter, and may include very low water temperatures or ice formation (Brown et al. 2011). Such extremes can result in increased overwinter mortality (Hurst 2007). To minimize this, stream-dwelling fish have developed phenology responses to cope with the variable conditions seen during winter (Huusko et al. 2007; Shuter et al. 2012). In the absence of direct behavioral observation, seasonal changes in population density provide indirect evidence of behavioral repertoires in fish response. Moreover, the seasonal changes in population density may influence population processes via a density-dependent effect (Bailey et al. 2010). Numerous studies have evaluated the seasonal change in population densities in stream-dwelling salmonids between spring and fall (e.g., Niemela et al. 2001). However, few studies have investigated seasonal changes in population density during fall and winter, especially at the community level. Morita et al. (2011) examined the annual seasonal change in the densities of three salmonid species (white-spotted charr Salvelinus leucomaenis leucomaenis, masu salmon Oncorhynchus masou, and rainbow trout Oncorhynchus mykiss) in the Shoro river system. The authors noted that the density of white-spotted charr decreased in both tributaries between fall and winter, whereas the densities of masu salmon and rainbow trout increased in the warmer of the two streams during this period. Unfortunately, however, the study lacked spatial replication as the authors were only able to compare seasonal changes in population densities between two tributaries. Thus, the relationship between salmonid densities and water temperature in winter remains poorly understood. To address this, we evaluated the seasonal changes in the population density and ratio of two salmonid species, white-spotted charr and masu salmon, between fall and winter in four tributaries of the Kushiro river system, eastern Hokkaido, Japan. White-spotted charr (hereafter charr) and masu salmon (hereafter salmon) are the dominant species within the Kushiro river system and generally occur sympatrically (Hariu 2010). The field study was conducted in four tributaries: Hitominai Stream, Oriyomappu Stream, Ooshima Stream, and Minamitawa Stream (see Sahashi and Morita 2013: fig. 1). The sampling reaches included the typical biotopes of the two species in each tributary (see abiotic characteristics of the four tributaries in the electronic supplementary material of Sahashi and Morita 2013). The length of the study reaches was 80 m in Hitominai Stream, Ooshima Stream, and Minamitawa Stream. The length of the reaches at Oriyomappu Stream was 55 m in fall and 27.5 m in winter. In each tributary study reach, we deployed fixed data loggers (Stow-Away TidbiT; Onset Computer Corp., Bourne, Massachusetts, USA) on the stream bed to measure water temperature at hourly intervals between 1 June and 4 September 2010, and 1 November and 1 February 2011. G. Sahashi Division of Marine Bioresource and Environmental Science, Graduate School of Fisheries Sciences, Hokkaido University, Hakodate 041-8611, Japan

Although dorsal and caudal fin deformities (curvature), abnormal body color patterns, and fluctuating asymmetry of meristic characters have been previously reported for white-spotted charr (Salvelinus leucomaenis), the incidence of skeletally deformed individuals in the wild are extremely rare. Here, we report on skeletally deformed individuals of white-spotted charr with extremely truncated upper jaw, in a headwater reach of the Yoneshiro River, northern Honshu, Japan. In this study, we examined the genetic diversity of deformed and non-deformed populations of white-spotted charr using polymorphic microsatellite DNA markers. We also compared individual growth histories of deformed individuals with normal individuals using a back-calculation of fish lengths at previous ages from otolith annuli. We collected 41 and 100 individuals from the below- and above-waterfall sites, respectively. Of these, two individuals from the above-waterfall site were skeletally deformed, whereas the other individuals had a normal morphology. Microsatellite DNA genetic diversities of the above-waterfall population were notably lower than those of the just below-waterfall population. Individuals of the above-waterfall population had significantly higher internal relatedness and lower standardized heterozygosity values compared with those of the below-waterfall population, indicating higher inbreeding situation of the above-waterfall individuals compared with individuals in the below-waterfall site. Back-calculation of individual growth using otoliths revealed that deformed individuals grew significantly slower than sympatric normal individuals. Although incidence of deformed individuals in the wild might be related with accumulated inbreeding as suggested for other fishes, mechanisms for their long-term persistence in the wild remain unclear.

We evaluated the migratory patterns of white-spotted charr in eastern Hokkaido, Japan using: (1) field observation of fish, (2) salinity-recording archival tags, and (3) analysis of otolith Sr and Ca concentrations. Field observations suggest that the majority of anadromous white-spotted charr outmigrate to the sea in spring (April to June) and ascend the river between late summer and autumn (August to November). Salinity records from archival tags revealed a fluctuating pattern consisting of both low and high values within a month, indicating frequent short-term movements between freshwater (including non-natal rivers) and brackish/salt water habitats. Otolith Sr:Ca ratios exhibited a sharp rise at about 40–50% of the otolith radius, which was followed by several spikes consisting of both low and high values, indicating that fish migrated between freshwater and marine environments multiple times during their life history. However, the peaks in Sr:Ca ratios were generally associated with annulus zones on the otolith, indicating that the anadromous migration was generally an annual event.

As a case study of local adaptation, we compared the life history traits of white-spotted charr Salvelinus leucomaenis between populations isolated in an alpine environment (altitude above 2000 m) and a normal environment (ca. 1000 m) in the Fuji River basin, near the southern limit of its distribution (35° N, 138° E). Our results showed that white-spotted charr in alpine streams had lower growth rate, longer lifespan, earlier spawning season, and were older and larger size at maturity, compared with those habiting a normal mountain stream. We concluded that charr in alpine streams were adapted to low water temperatures and long starvation periods by specialized life history traits.

SUMMARY 1. Endangered native populations of stream salmonids in Japan face three major threats: (i) negative interactions with introduced hatchery-reared fish, (ii) fragmentation of habitat by impassable dams and (iii) recreational angling. 2. To prevent imminent extinction of many local populations, we evaluated these threats and possible conservation actions for red-spotted masu salmon (Oncorhynchus masou ishikawae) and white-spotted charr (Salvelinus leucomaenis japonicus) in the Fuji River system in central Japan. 3. Red-spotted masu salmon and white-spotted charr occupied only 0.73 and 2.4% of suitable thermal habitats, respectively, with masu salmon typically occupying habitats closer to human population centres. 4. Population viability analysis resulted in a 100-year probability of extinction of 78.1% for masu salmon and 48.1% for charr. However, extinction risk of both species was predicted to be

Genetic structure and diversity of nine Japanese kokanee (landlocked) Oncorhynchus nerka stocks and anadromous O. nerka from the North Pacific and the Canadian Lake Cultus population were examined using microsatellite and mitochondrial DNA. Sequence analyses of the cytochrome b region of mtDNA for Japanese kokanee O. nerka stocks on Honshu and Hokkaido islands revealed that most Japanese stocks were monomorphic of one major haplotype, which was also dominant in the Lake Cultus population and anadromous O. nerka in the North Pacific. Assignment tests using microsatellite DNA revealed that there was no clear-cut population structure in Japanese kokanee O. nerka stocks.

Relationships between the vertical distribution and thermal habitat, and body size of chum salmon Oncorhynchus keta were studied in the Bering Sea in summer using trawl surveys at various depths. Chum salmon abundance decreased with increasing depth, but the patterns of decrease differed between size groups. The abundance of small salmon fell rapidly with depth, whereas that of large salmon decreased gradually to 40 m depth, and abruptly below that. The average fork length of chum salmon collected from each trawl correlated positively with trawl net depth and negatively with water temperature. Since the optimal temperature for growth decreases with body size in this species, the observed body size-related vertical habitat use by chum salmon may indicate size-dependent thermal preferences.

We examined seasonal changes in population densities of stream salmonids (masu salmon Oncorhynchus masou, white-spotted charr Salvelinus leucomaenis, and rainbow trout O. mykiss) in two tributaries of the Shoro River, eastern Hokkaido, Japan. In one small tributary, water temperature was relatively high during the winter, and populations of salmon and trout increased through immigration at this time of the year, becoming dominant components of the salmonid assemblage; the density of charr in this stream decreased during the winter, but charr was dominant during the summer. In another medium-sized tributary, the water temperature fell to close to 0°C during the winter, and densities of salmon and charr decreased in this season, through emigration; trout were very rare in this stream. Seasonal patterns of stream salmonid densities vary among species and between localities, resulting in seasonal changes in species composition. For a comprehensive understanding of population processes, a whole-river survey across seasons will be necessary.

We examined latitudinal variation in riverine growth and parr maturation of an endemic Asian salmonid, masu salmon ( Oncorhynchus masou ), in 12 rivers located between 36.6°N and 45.4°N. Masu salmon parr showed considerable variation in growth and maturation patterns among populations. Body sizes were generally larger, and parr maturation was common at southern latitudes. Male parr matured at smaller sizes at more southern latitudes. Latitudinal variation in riverine growth and maturation of masu salmon parr was largely attributed to latitudinal changes in temperature and population density. Parr size at age increased with increasing temperature and decreased with population density. Riverine growth conditions were an important environmental factor determining parr maturation for both males and females; however, the occurrence of mature female parr required extremely favorable growth conditions. Water temperature in May, approximately four months before maturation, was the most important environmental factor affecting the maturation of male parr. Our study supports the hypothesis that freshwater residency was promoted by favorable growth conditions at southern latitudes.

To examine the potential linkage between sex-biased growth and dispersal in demersal fish, we studied the movement distance and growth of tag-recaptured sablefish (Anoplopoma fimbria). Tagging was conducted by teams from Japan and the United States. From 1980 to 1984, 58,918 sablefish were captured by Japanese vessels and tagged with anchor tags, and between 1980 and 2007, 3319 sablefish were recaptured in the Bering Sea and northeastern Pacific. Of these 3319 fish, we analyzed data on 659 individuals for which we have reliable data on recapture location, sex and fork length at the time of release and recapture, and elapsed time longer than 10 days. Annual growth increments (mean±S.D.) were 2.5 ± 6.5 cm (males) and 3.5 ± 8.6 cm (females), and the movement distances (assumed to be straight) were 712 ± 800 km (males) and 877 ± 920 km (females). Females grew larger and moved longer distances than males. We calculated the growth increment deviation that accounted for the effects of elapsed time, curvilinear growth, and initial fork length at release using a von Bertalanffy growth model. The growth increment deviation increased significantly with the distance moved over elapsed time. In addition, the relationship between distance moved over elapsed time and the growth deviation differed significantly between sexes; females grew faster than males at a given movement distance. We suggest that female-biased dispersal is a factor generating female-biased growth in sablefish.

Large fish often inhabit colder waters than small fish. Using a simple bioenergetic model, we found that the optimal temperature for growth should decrease with increasing body size. We predicted that this mechanism would produce an ontogenetic change in thermal preference and then tested our predictions with Pacific salmon, Oncorhynchus spp. In a laboratory experiment, the slope of a regression of growth increment on initial size became steeper with increasing temperature, so that the optimal temperature for growth decreased with increasing body size. In field observations, larger and older salmon inhabited cooler areas, whereas smaller and younger salmon inhabited warmer areas. These patterns were consistent with a size-dependent effect of temperature on condition factor, a parameter shown experimentally to be a measure of the most recent growth performance. Temperatures for maximising condition factor were lower for larger fish. Thus, an ontogenetic change in individual thermal preference toward cooler areas maximises the growth performance of fish, and the negative effects of climate warming on growth are hypothesised to be more severe for larger fish.

Habitat fragmentation by damming can affect the persistence of single species population and also coexistence of two or more species through intensified competition. This study examined the effects of habitat fragmentation by damming on the coexistence of two stream-dwelling salmonids: the southern form of white-spotted charr (Salvelinus leucomaenis japonicus) and the red-spotted masu salmon (Oncorhynchus masou ishikawae). We examined charr, salmon, and dam distributions in 27 streams of the Fuji River basin, central Japan. In the 1970s, there were streams with five sympatric and 22 allopatric populations (n = 13 for charr, n = 9 for salmon). However, from the 1970s to 2004, 356 impassable dams were constructed in the surveyed streams, and four of the five sympatric streams became allopatric. In the extant sympatric stream, more than 20 dams fragmented habitat. Species distributions were separated by dams (with decreasing altitude) in the following order: extirpation area, charr-dominant area, and salmon-dominant area. Within the uppermost sympatric section (i.e., situated between the dams), salmon congregated in the largest uppermost pool just below the dam; despite these conditions, salmon frequency increased in the downstream direction at the stream scale. The results suggest that habitat fragmentation threatens the coexistence of stream-dwelling charr and salmon at both the basin and stream scales. We believe that exclusion of one species by another is likely in extremely fragmented habitats with minimal gradients and little variation in physical conditions (through reduced stream gradient and increased sand sedimentation caused by damming). In addition, multiple sites of damming ensure that there are no salmonid refuges from the collapse of metapopulation structure. In such fragmented habitats, even small tributaries serve important roles, as they are used mainly by salmonid fry and juveniles. We propose that habitats of native salmonids should be maximized by reconnecting fragmented habitats as part of a broader management plan.

Many studies have investigated the ecology of charrs in freshwater, however, little is known about charrs in the ocean. This study examined the distribution, seasonal abundance, and some biological features of Dolly Varden (Salvelinus malma) in the Pacific Ocean. An analysis of by-catch data of Japanese offshore salmon monitoring showed that Dolly Varden were distributed across a wide range in the offshore waters of the Pacific Ocean, including the Japan Sea, Bering Sea, and Okhotsk Sea. The catch per unit effort showed a sharp increase from May to August, followed by a sharp decrease in September. Offshore areas served as an important summer habitat for anadromous Dolly Varden.

Latitudinal variation in egg size and number in anadromous masu salmon Oncorhynchus masou was examined. Relatively greater variation in egg size occurred among rivers than among females within rivers or within females. Egg size was generally greater and egg number generally lower at more northerly latitudes.

Dam construction has serious consequences, and one of the most serious concerns is the fragmentation of riverine ecosystems. We reviewed the influence of habitat fragmentation on white-spotted charr Salvelinus leucomaenis populations. First, habitat fragmentation by damming has serious consequences in terms of alternative life-history strategies. Most fish in dammed-off areas do not migrate to the sea and instead become resident forms. This loss of the anadromous form negatively affects populations through decreased spawning biomass. In addition, the smaller population sizes in dammed-off habitats can negatively affect population dynamics through demographic, environmental, and genetic stochasticity. Therefore, the population viability is reduced in small, dammed-off habitats. White-spotted charr populations also likely experience different selection pressures after damming. Many of these effects of habitat fragmentation due to damming are not immediate but rather occur gradually over several generations. Because most Japanese dams were constructed after 1970, some effects of damming may not yet be obvious.

We review the ecology and conservation of three lesser-known chars (genus Salvelinus): Dolly Varden (S. malma), white-spotted char (S. leucomaenis), and bull trout (S. confluentus). Dolly Varden is distributed across the northern Pacific Rim and co-occurs with bull trout and white-spotted char at the southern extremes of its range. In contrast, bull trout and white-spotted char are naturally isolated, with the former restricted to North America and the latter distributed in northeastern Asia. Though the range of Dolly Varden overlaps with the two other chars, it is most closely related to Arctic char (S. alpinus), whereas bull trout and white-spotted char are sister taxa. Each species exhibits diverse life histories with respect to demographic characteristics, trophic ecology, and movement. This diversity appears to be tied to environmental variability (e.g., temperature, habitat connectivity), resource availability (e.g., food), and species interactions. Increasingly, these interactions involve ...

Gillnet fisheries are strongly size-selective and seem to produce changes in size at maturity for exploited fishes. After Word War II, large-scale gillnet fisheries targeted Pacific salmon (Oncorhynchus spp.) in the high seas area of the North Pacific and the Bering Sea, but these fisheries were closed in 1993. To assess the effects of this high seas gillnet fishery (and its closing) on size at maturity, we examined long-term trends in size at 50% probability of maturing (L50) for chum salmon (O. keta) from three populations in Hokkaido, Japan. The L50 trends were statistically different among rivers, but showed similar temporal patterns with decreases in the 1970s and early 1980s and increases after the 1985 brood year. While fishery-induced evolution seemed largely responsible for this temporal change in L50 during the fishing period, natural selection and phenotypic plasticity induced by environmental changes could contribute to the increases in L50 after the relaxation of fishing pressure.

Over the last few decades, the size at which Pacific salmon Oncorhynchus spp. attains maturity has decreased in many populations, whereas the age at maturity has increased. Both fish- eries-induced evolution and environmentally-induced phenotypic plasticity could contribute to the changing age and size at maturity of Pacific salmon. We evaluated the potential for genetic changes in the maturation schedule of Japanese chum salmon using the probabilistic maturation reaction norm (PMRN) method. We found that the recent decrease in size at maturity, and increase in age at maturity, of Japanese chum salmon can be largely attributed to a phenotypic response to a reduced growth rate, but that fisheries-induced evolution should not be ruled out. Recent claims concerning fisheries-induced evolution of the maturation schedule are based on the decline in the age-specific body size at which the probability of maturing is 50%, a feature of PMRNs. However, the PMRN could change with changing environmental conditions. Therefore, a genetic change cannot be diagnosed only by the PMRN method.

Using wild white-spotted charr ( Salvelinus leucomaenis ) as a model, we investigated the effects of cutting the line and leaving the hook embedded in deeply hooked fish in a multiple catch-and-release experiment. Ten fishing episodes involving 415 tagged fish (fork length range: 96–311 mm) were conducted at weekly intervals. A total of 735 captures (1.8 times per fish), 88 captures were deep-hooked and released by cutting the line. Eleven of the fish were deeply hooked twice, and released by cutting the line in both instances. After the experiment, 93.5% (72/77) of the deep-hooked fish were recaptured, indicating that the survival rates of fish released by cutting the line are sufficiently high to be encouraging. Of the 82 hooks in the 72 fish sampled that were deep-hooked and released by cutting the line, 27 (32.9%) hooks had been evacuated. The hooks remaining after cutting the line in earlier fishing episodes tended to be corroded or had been evacuated. Logistic regression analyses showed that the probability of hooks being uncorroded and remaining in a charr gut decreased with time. The estimated average time before a remaining hook started to corrode was 22.1 ± 22.7 days after cutting the line and the average time to hook evacuation was 53.3 ± 36.3 days. Our study demonstrated that cutting the line is an effective release method for deep-hooked fish under natural environmental conditions.

The geographic distribution pattern of mitochondrial DNA (control region) sequence polymorphisms from 73 populations of a salmonid fish, Dolly Varden Salvelinus malma, over most of its range in the North Pacific rim, was examined to assess how its spatial population genetic structure has been molded. The observed 68 haplotypes were grouped into three main lineages, which correspond to western, central, and eastern regions in the North Pacific. The two outlier-haplotype groups gave close agreement with DNA types from two congeneric species, white-spotted charr S. leucomaenis and Arctic charr S. alpinus, respectively. These results suggest that the present-day genetic structure of S. malma reflects historical patterns of isolation and re-colonization, and also historical hybridization with co-distributed species. We also placed the haplotypes of S. malma within our study areas into a pre-existing evolutionary relationship of S. alpinus and S. malma throughout the Northern Hemisphere. Phylogenetic analysis revealed that the Western Lineage S. malma was basal to all other lineages of S. malma and S. alpinus. Our data serve as a biogeographic hypothesis for salmonid fishes that the Sea of Japan and/or Sea of Okhotsk regions represents a place of origin for S. malma and S. alpinus groups currently distributed in circumpolar regions.

In the last quarter of the 20th century, the size at maturity of many North Pacific salmon (Oncorhynchus spp.) populations decreased. During this same period, the age at maturity increased, implying that the growth rate of Pacific salmon decreased, probably owing to environmental changes. To elucidate these trends, we identified the rule of age and size at maturity of Japanese chum salmon (Oncorhynchus keta), which was that slow-growing salmon initiated maturation at an older age and smaller size than did fast-growing salmon. We then simulated the potential modification of age and size at maturity in response to changing growth rate using a size-structured model with age- and size-specific maturation rates. This showed that reducing the growth rate without assuming a genetic change was sufficient for realistic modeling of recent changes. In addition, the observed rule of age and size at maturity was consistent with the optimal age and size at maturity in terms of maximizing the fitness. Our results attributed the recent trends in chum salmon's increasing age and decreasing size at maturity to an adaptive phenotypic response to a reduced growth rate.

Anadromy in the Dolly Varden Salvelinus malma at the southern limits of its distribution were examined by an intensive field survey and measurements of otolith Sr and Ca concentrations. Body sizes of anadromous individuals were small, only slightly larger than those of mature parr, and the degree of anadromy was extremely low.

The migratory history of the white-spotted charr Salvelinus leucomaenis was examined using otolith microchemical analysis. The fish migrated between freshwater and marine environments multiple times during their life history. Some white-spotted charr used an estuarine habitat prior to smolting and repeated seaward migration within a year.