山本 俊昭

Thelazia callipaeda (Nematoda: Spirurida: Thelaziidae) parasitizes the eyes of dogs, cats, humans, and various wild mammals, and is transmitted by drosophilid flies. In Japan, T. callipaeda is considered an emerging parasite that has expanded its endemic region northward. However, reports of its detection in mammals other than domestic animals and humans are scarce. This study reports the detection of T. callipaeda in Japanese red fox (Vulpes vulpes japonica), masked palm civet (Paguma larvata), Japanese badger (Meles anakuma), Japanese black bear (Ursus thibetanus japonicus), raccoon (Procyon lotor), Japanese raccoon dog (Nyctereutes viverrinus), domestic dog (Canis lupus familiaris), domestic cat (Felis silvestris catus), and human. Of these, the Japanese red fox, masked palm civet, Japanese badger, and Japanese black bear have been reported as novel host records. Sequence analysis of the cytochrome c oxidase subunit I gene of T. callipaeda revealed two unique lineages specific to Japan, with no regional or host species differences. These results suggest a wide host range for T. callipaeda, highlighting the significant role of wildlife as a reservoir for this parasite in Japan.

Hair hormone analysis has received increasing attention in the fields of wildlife management and conservation over the last decade. However, few studies focused on sex steroid hormones, even though hair sex steroid hormone analysis has the potential to provide information on an individual's reproductive health, leading to a better understanding of animal reproductive biology and the assessment of individual and population health. Here, we tested the assay validation and examined whether hair progesterone concentration (HPC) differs between different sexes and age classes in the Asian black bear, Ursus thibetanus. We also investigated the effects of reproductive condition, age, and body condition index (BCI) on HPC to determine the utility of hair progesterone analysis in bears. The assay validation was reported in our study. We found a significantly higher HPC in adult females than in juvenile females and juvenile and adult males. In addition, we found no effects of age and BCI on HPC but a significant positive effect of reproductive condition, as the HPC in breeding adult females was significantly higher than in nonbreeding adult females. These results suggest that hair progesterone analysis has the potential to evaluate the reproductive condition of Asian black bears.

To clarify the effect of paternal life-history form and egg size on offspring life-history traits (e.g., growth and maturation) in masu salmon (Oncorhynchus masou), we studied an artificially fertilized population for 3 years using mark-recapture methods. The body size of offspring in early life was associated with egg size, but not paternal life-history form. By contrast, the growth rate of individuals during summer and winter seasons was not influenced by egg size or paternal life-history form. Movement within the tributary differed between males and females, with more females moving downstream during early life stages compared to males. However, the effects of egg size and paternal life-history form on the movement of offspring were not detected. Out of 192 males that remained in the tributary, 77 had relatively large body sizes and reached sexual maturity at one year old. The probability of maturation was associated with the body size in September, but not with paternal life-history form and egg size. We concluded that offspring life-history traits are profoundly influenced by environmental conditions, whereas they are slightly influenced by parental genetic effects.

Offspring growth is greatly influenced by both the maternal and paternal genotypes. However, although the effects of maternal genotype on offspring growth are recognised widely, ecological studies of the paternal genotype have not focused on the links with physiological traits, especially at the embryo stage. In this study, we conducted three experiments to determine the effects of two contrasting paternal life history traits (anadromous males, which return to their natal rivers for spawning after migrating to the sea, and resident males, which mature in rivers without migrating to the sea) on offspring growth during early life based on the metabolic rate in masu salmon Oncorhynchus masou. In the first experiment, metabolic rates of fertilised eggs were compared between 24 half-sibships. Results suggested that the metabolic rate of embryos was higher for the offspring of resident males than of anadromous males. In the second experiment, the body sizes of 133 half-sibships at the yolk-sac absorption stage were compared. The offspring from the resident males were larger, which could be related to metabolic rate differences at the embryo stage associated with paternal life history. In the third experiment, the differences in body size observed at the yolk-sac absorption stage were positively correlated with growth after 60 days of fry rearing. Our findings, taken together with previous demonstrations that faster-growing males tend to become younger resident males, are thus consistent with early growth and associated traits, such as metabolic rate, being heritable from the male parent to the offspring.

Offspring traits are influenced by complex interactions among parent genotypes and phenotypes. However, ecological studies of these effects have tended to focus on one of either maternal or paternal influences. In this study, we investigated the effects of both parental phenotypes on offspring dispersal, growth and early sexual maturity in masu salmon. We used wild-caught parental fish in a half-sib mating design that allowed comparison of offspring from males of two different life-history types (precocious males that had lived only in fresh water and anadromous males that had migrated to the sea). These males were mated with anadromous females and the eggs planted in natural streams in three different years. Natal dispersal distance depended on offspring sex, body size and paternal life history: female offspring moved further downstream than males, whilst daughters of precocious fathers tended to move further downstream than those of anadromous sires. There was a maternal effect on offspring growth, with larger eggs resulting in larger offspring at least until these became precociously mature at the end of the first summer. However, while faster growing male offspring were more likely to become precociously mature, there was no evidence that the probability of precocious maturation was influenced by parental life history. We conclude that, although body size of young salmon was significantly influenced by egg size and there were sex differences in dispersal, their growth rate and divergence in life history types were mainly environmentally driven.

Despite being well-researched in several other bear species, the act of infanticide has yet to be documented in Asiatic black bears (Ursus thibetanus). We captured video footage of an apparent infanticide event in May 2016 using sensor cameras placed outside the winter den of a known female in Nagano Prefecture, Japan. We observed the presence of a known male outside the den, interacting with the female and her offspring. The female ultimately died and her body did not appear to have been consumed by the male bear. From our observations we considered both sexually selected infanticide and nutritional demand as possible motives behind the male's attack.

We analyzed the genotypes of three pregnant females and their litters to investigate the phenomenon of multiple paternity in wild raccoon dogs (Nyctereutes procyonoides) using 17 microsatellite markers. If a female has mated with only one male during estrus, then the maximum number of paternal alleles will not exceed two among littermates with the same father. The results revealed two out of three litters had three or four paternal alleles at one or five microsatellite loci. Therefore, the female had mated with more than one male during estrus. To the best of our knowledge, the present study is the first to report the possibility of multiple paternity in wild raccoon dogs.

Although kinship (parent-offspring or siblings) contact has been suggested as a driving factor for sarcoptic mange epizootic in raccoon dogs (Nyctereutes procyonoides), no effect has been reported. In contrast, habitat fragmentation caused by urbanization may result in a high occurrence of sarcoptic mange, because habitat fragmentation may promote contact infection by increasing the population density of raccoon dogs. The habitat distribution of raccoon dogs may therefore influence epizootic sarcoptic mange. The genetic relationship between raccoon dogs was analyzed to examine Sarcoptes scabiei transmission between kin. The relationship between S. scabiei infection and the habitat of raccoon dogs was also investigated. Seventy-five raccoon dogs from Takasaki, Gunma prefecture, were examined from 2012 to 2018; 23 were infested with S. scabiei. The genotypes were determined using 17 microsatellite loci, and the relationships were categorized into four patterns by the ML-Relate software. There was no significant difference between infested pairs and other two pairs (Chi-squared test: χ2=0.034, df=1, P=0.85). Although it was difficult to predicate because the mortality rate was unclear in this study, kinship contact does not seem to be an important factor for sarcoptic mange epizootic. S. scabiei infection rates were significantly associated with the location of village sections (OR=1.55, 95% CI=1.11–2.17, P=0.011). It is suggested that direct/indirect contact between individuals living closely together is an important factor for the transmission of S. scabiei.

1. Identifying spatial extent and temporal patterns of population genetic structure is important for understanding evolutionary relationships among local populations and for designing conservation programmes. However, little attention has been devoted to understand whether genetic structure is temporally stable or unstable, and how genetic characteristics may change over time. The goal of this study was to assess the temporal stability of the genetic structure of masu salmon (Oncorhynchus masou). 2. To examine the temporal component of genetic variation in genetic structuring within a river system, we used 810 masu salmon collected from eight tributaries of the Atsuta River, Hokkaido, Japan, over three consecutive years (i.e. 200709). We assessed the temporal stability of the genetic structure in 15 microsatel-lite loci by comparing genetic diversity, effective population size and the relationships between genetic and geographical distances among samples collected from different tributaries and over time. 3. Significant genetic differences were observed among most of the temporal samples collected from the same tributary, and analysis of molecular variance indicated that the temporal component explained about 10 times more of the genetic variation (1.98%) than did the spatial component (0.18%). 4. Isolation by distance (IBD) was detected only in 2009. In addition, a significant correlation in population genetic structure was observed between samples collected in 2008 and those in 2009. In 2007, decomposed pairwise regression analysis identified one population as an outlier population and, following exclusion of the outlier sample, IBD was observed in 2007. This population is small and physically isolated by waterfall, in which the effective population size or effective number of breeders would be expected to be small. 5. We found that fine-scale genetic structure in masu salmon populations was temporally unstable, due perhaps to genetic drift in small isolated populations, temporal changes in environmental conditions (e.g. drought), and a combination of these factors. This study also highlights that the genetic structure among populations could easily be affected by temporal and/or sampling issues. Studies focused on temporal genetic structuring are likely to be important for conservation of species inhabiting unstable or isolated habitats such as freshwater fish.

In Salmonidae, subordinate males are exposed to higher risks of sperm competition than dominant males and thus are expected to improve the sperm characteristics (sperm concentrations, sperm velocity and sperm longevity). In this study, we investigated the relationships between body size and secondary sexual characters (breeding colour, hump height and snout length), and sperm characteristics of one-year-old (newly matured) Dolly Varden char. Small males displayed higher sperm concentrations than large males. Moreover, males with dull breeding colours, but not with lesser snout length and hump height, displayed an increased sperm velocity compared to males with bright colours, suggesting a trade-off between sperm quantity and the investment in breeding colour. In addition, sperm longevity decreased as sperm swimming velocity increased. These findings indicate that small males with dull breeding colours improve the quantity and quality of their sperm to a great extent to enhance their chances of reproductive success.

To evaluate the influences of spatial scale on dispersal, the dispersal patterns of masu salmon Oncorhynchus masou masou were investigated at among-river (ca. < 43 km) and within-river levels in mid-western Hokkaido, Japan. A genetic differentiation (F (ST)) and assignment test showed that among-river dispersal was much less common (2.9 % of 339 individuals) than within-river dispersal (7.4 % of 190 individuals). We also found that there was no bias in dispersal at the among-river level, while anadromous males were more likely to disperse at the within-river level, suggesting that the dispersal patterns may be scale dependent.

To estimate the migration timing, length, and location of females with the growth hormone pseudogene (GHp), trace element analyses were performed on the otoliths of 40 wild adult masu salmon, Oncorhynchus masou (12 males, 19 normal females, and nine females with GHp [all fish were 3- or 4-years-old]) caught during the reproductive season (September) from 2004 to 2010 in the Atsuta River in Japan. Sr concentrations at the end of the otolith translucent area formed during the winter suddenly increased in all individuals, suggesting that they had migrated in the spring. In addition, the estimated migration length and location of females with GHp indicated that they had spent a fixed period of 1year in the ocean, suggesting that females with GHp have a similar migratory history as that of normal females.

For bears, numerous associations between biotic and abiotic factors have been reported to correlate with the timing of den entry and emergence; however, an analysis showing which factors influence the timing of den entry and emergence has not been performed enough. In this study, a generalized linear mixed model was generated using 66 entry dates for 26 females and 40 entry dates for 26 males, and 56 emergence dates for 26 females and 25 emergence dates for 18 males between 1999 and 2012. Regarding factors for den entry, the average temperature in November and mast production of Mongolian oaks were significant for both males and females. For the date of den emergence, the average temperature in March affected strongly. For males, good mast production of Mongolian oaks in the previous year was found to be associated with early den emergence. For females, the presence of newborns had a significant influence on their den entry and emergence. This study indicated that denning behavior appears to be regulated by several abiotic and biotic factors and regulation factors are sex specific.

The proportions of individual masu salmon Oncorhynchus masou whose genotypic and phenotypic sex differed were compared among juvenile and adult fish in three rivers, and genotypically male but phenotypically female individuals were observed 6-16% more often among adults than among juveniles. This suggests that during the transition from juvenile to adult, survival rates of genotypically male but phenotypically female individuals are higher than those of normal females. In contrast, genotypically female but phenotypically male individuals were only found in the juvenile period, which suggests that they exhibit a decreased survival rate in comparison with normal males.

Debris flow has potentially pronounced effects on streams and fish populations. In August 2010, a severe flood, which was coupled with wide-ranging landslides and massive debris flows, occurred in the Atsuta River. In this study, we evaluated the effects of floods on salmonid populations by examining the abundance and density of masu salmon Oncorhynchus masou and rainbow trout O. mykiss in the Atsuta River and comparing these values in the post-flood period to those in the pre-flood period. The estimated abundance and density of masu salmon did not change between the pre- and post-flood periods. In contrast, the density of adult (age-1and older) rainbow trout decreased dramatically. Although the density of age-0 rainbow trout decreased after the floods, there was no significant difference between the pre- and post-flood values. These results suggest that the effects of severe floods differ among species and these differences may be attributed to differences between masu salmon and rainbow trout in the timing of life-history events, such as fry emergence and spawning.

Most comparative studies on the sperm traits (sperm concentration, swimming speed, and longevity) of salmonid fish involve males with alternative life histories (anadromous versus resident forms) that exhibit different reproductive strategies. However, males exhibit a dominance hierarchy within the same life history, in which smaller males have a tendency to be lower rank. This phenomenon prompted us to compare the sperm traits of large anadromous and small resident males in masu salmon Oncorhynchus masou, and to identify the size-dependent sperm traits within these two life history groups. We detected a significant difference in sperm concentration between the two groups, with resident males having higher sperm concentrations than anadromous males. For resident males, we detected a significantly negative correlation between body size and sperm swimming speed, i.e., faster sperm speed was associated with smaller body size. These findings support earlier data and suggest that smaller or younger males utilize more energy to produce high concentration and high speed sperms.

Delineating population genetic structure and gene flow is a critical for identifying evolutionary processes and effective conservation. In this study, we examined the genetic structure of masu salmon (Oncorhynchus masou) populations within the Atsuta River, Hokkaido, Japan. In addition, to infer whether dispersers contribute to next generation's gene pool, a comparison between the genetic structure of juvenile and adult masu salmon was conducted. Significant genetic differentiation and isolation by distance were observed among populations of both juveniles and adults. The degree of genetic differentiation was larger among the juvenile samples, especially among geographically distant samples, than among the adults. These results suggest that a number of relatively stable discrete populations are maintained in the river and that the reproductive success of dispersers to distant tributaries was relatively low among distant tributaries.

This study compared the growth rates of female masu salmon Oncorhynchus masou, who possessed a male-specific gene marker, the growth hormone pseudogene (GHp), and normal females, as estimated from their scale growth. There was a difference between the growth rates of GHp-positive females and those of normal females of the same age during the ocean period, although their growth rates during the river period were similar. These results suggest that GHp-positive salmonid females exhibit male-like characteristics such as reduced feeding activity during the ocean period, which depresses their growth.

We performed microsatellite paternity analyses of two Asian black bear Ursus thibetanus littermates with a known mother. Using Cervus 3.0, we identified the probable father among 17 candidate fathers for Cub 1, but failed to identify a probable father for Cub 2. Cub 1 and her probable father shared at least one allele at all 13 loci. In contrast, Cub 2 and the probable father of Cub 1 had mismatching alleles at eight loci, indicating that the two cubs are offspring of different fathers. This is probably the first evidence for multiple paternity in Asian black bears.

To clarify the spatial distribution of masu salmon Oncorhynchus masou with incongruent genotypic and phenotypic sex, we compared the phenotypic sex determined by gonad analysis with the genotypic sex identified by a male-specific genetic marker, Growth hormone pseudogene (GHp), in 584 individuals inhabiting 23 rivers in Hokkaido, Japan. We found that the genotypic sex is different from the phenotypic sex for 39 individuals in 10 rivers. Among them, 35 individuals (89.7 %) were GHp + females phenotypically. The opposite feature was noted in the remaining four individuals (10.3 %). This study also revealed that GHp + phenotypic females are distributed mainly in the Sea of Japan, whereas GHp- phenotypic male tend to appear in the Sea of Okhotsk. In addition, there was a significant negative correlation between the distance from the Bishabetsu River, where frequency of GHp + phenotypic females was the highest, and the frequency of those individuals in each river. These results suggest that the distribution of masu salmon with incongruent genotypic and phenotypic sex would be related to the movement of individuals among the populations and regions.

Dam construction dramatically influences riverine ecosystems, with habitat fragmentation being one of the most serious impacts. This habitat fragmentation is particularly relevant for anadromous species such as salmonids. We examined the effects of habitat fragmentation on masu salmon (Oncorhynchus masou) populations in Hokkaido, Japan. Specifically, we sampled from 15 locations located above and below a dam region in the Uryu River system, and analyzed the genetic structure of the populations using 10 microsatellite loci. No indication of a significant reduction in genetic diversity, estimated by allelic richness and heterozygosity, was observed within the above-dam region compared to the below-dam region. However, we also found that reducing the number of alleles had occurred within the above-dam region. The analysis of molecular variance and multidimensional scaling analysis indicated significant genetic differentiation between regions and within each region. A significant relationship between genetic and geographic distance was observed in the below-dam region, while no signal of isolation by distance was detected in the above-dam region. This study suggests a possibility of ongoing loss of alleles coinciding with habitat fragmentation caused by anthropogenic environmental changes such as water-level regulation, which negatively impacts genetic structure.

The Asian black bear inhabiting the Shimokita Peninsula, the northernmost part of Honshu, Japan, is categorized as one of the endangered local populations (LPs) in the list of threatened species under the Endangered Species Act. The Shimokita population has become geographically isolated and fragmented due to recent human activity. In this study, we conducted DNA sampling between 2005-2007 of two neighboring bear populations across two regions, Shimokita and Tsugaru, and analyzed the genetic characteristics from the mitochondria] control region (approximately 700 bp) and seven microsatellite loci. Based on the analysis of both microsatellite markers and the mitochondria] control region, we found that the genetic diversity of the Shimokita population was lower than that of the Tsugaru population. An assignment test showed that three of 112 individual bears migrated between subpopulations, suggesting the low level of gene flow between subpopulations. The extent of genetic differentiation between populations (Fst) was 0.163 in microsatellites and 0.443 in mtDNA. These findings indicate that the Shimokita population is genetically isolated from other populations by habitat fragmentation due to human activity and historical events.

Identifying the patterns and processes driving dispersal is critical for understanding population structure and dynamics. In many organisms, sex-biased dispersal is related to the type of mating system. Considerably, less is known about the influence of life-history variability on dispersal. Here we investigated patterns of dispersal in masu salmon (Oncorhynchus masou) to evaluate influences of sex and life history on dispersal. As expected, assignment tests and isolation by distance analysis revealed that dispersal of marine-migratory masu salmon was male-biased. However, dispersal of resident and migratory males did not follow our expectation and marine-migratory individuals dispersed more than residents. This may be because direct competition between marine-migratory and resident males is weak or that the cost of dispersal is smaller for marine-migratory individuals. This study revealed that both sex and migratory life-history influence patterns of dispersal at a local scale in masu salmon.

The genetic sex of mature masu salmon Oncorhynchus masou (212 males and 243 females) collected from three rivers located in the mid-western part of Hokkaido was determined using the male-specific genetic marker growth hormone pseudogene (GHp). A total of 72 phenotypic females were found to have GHp, whereas none of the phenotypic males lacked GHp. The occurrence of females with incongruence between genotypic and phenotypic sex varied with the river and reached the highest rate of 67%. In contrast, none of the phenotypic males were found to have a female genotype. Comparison of adult body size showed that females with GHp were significantly smaller than those without GHp. Moreover, comparison of secondary characteristics revealed that both upper jaw length and head length were greater in females with GHp than in those without GHp. These results suggest that genetic changes (e.g. transposition and mutation) may also affect morphological characteristics.

We characterized annual and seasonal home ranges (HR) of 27 female Asiatic black bears (Ursus thibetanus) during 2003-11 in Karuizawa, Nagano Prefecture, Japan. Mean annual HR size for bears calculated by the 95% fixed kernel method was 10.0 km(2) (SD = 8.5) and differed depending on the year. However, annual HR sizes were not affected by reproductive status (with or without cubs) or age (subadult or adult). Home ranges tended to be larger in summer than in autumn. We speculate that a contributing factor to differences in HR size was annual or seasonal fluctuation of food resources. Annual shifts in HR centers and HR overlaps were estimated to be 1.2 km (SD = 0.9) and 0.55 km (SD = 0.13), respectively. Seasonal shifts in HR centers and seasonal overlaps were 1.1 km (SD = 1.1) and 0.54 km (SD = 0.12), respectively. Together, these results suggest that female Asiatic black bears in our study area do not migrate long distances seasonally or annually.

Salmonid species have the potential to make a population genetic structure due to their homing behavior. However, the extent of genetic structuring would vary considerably because precision of homing varies within and among species. In addition to homing, several biotic and/or abiotic factors would also influence genetic structuring. A description of the genetic structure at various spatial scales and an understanding of the extent of genetic structuring could facilitate the identification of factors that affect genetic structuring. The knowledge of such factors is a fundamental requirement for the accurate inference of population dynamics, evolutionary processes, and conservation decisions. We focus on the factors that influence population genetic structuring and briefly describe the genetic structure of masu salmon (Oncorhynchus masou) populations at both regional and microgeographic scales. By analyzing mitochondrial DNA and microsatellite DNA variations, we found that masu salmon exhibit hierarchical genetic structuring and genetic differentiations not only at the regional scale but also at the microgeographic scale. These observations indicate that masu salmon would have the potential to make a population genetic structure at the microgeographic scale due to precise homing. Furthermore, it is also suggested that genetic structuring would be affected by several factors, such as refugia during glacial periods, ocean current, and dispersal patterns of each individual, and the effects of such factors may vary depending on the intended geographic scale. However, such intrinsic genetic structuring faces the danger of being eroded by anthropogenic effects, including habitat degradation, habitat fragmentation, and artificial release of hatchery-reared fish. In fact, the negative impact of damming would hold true for masu salmon. Our results indicate the possibility that the indigenous genetic structure that has been created over many years would be lost by human activities during short periods. © 2012 Nova Science Publishers, Inc. All rights reserved.

ニホンツキノワグマUrsus thibetanus japonicus(以下,クマ)の人の生活圏への出没には、いくつかの要因が考えられているが、その一つに里地里山(以下,里地)環境の荒廃が挙げられている。本研究は、クマの出没と里地環境との関係を解析し、クマの出没要因の解明を目的とした。本研究は、群馬県沼田市にて、2008年10月から12月の期間に行った。クマの出没に関連すると考えられる里地環境を畑地、水田、果樹園、不作付け地、荒地、林縁と定義し、これらの位置、作付け状況、管理状況を調査した。解析には、地理情報システムによって定量化した里地環境のデータと調査地におけるクマの出没地点データを用いた。解析は、巨視的解析と微視的解析に分け、それぞれ、作成した区画、クマの出没地点を基にして行った。巨視的な解析では、クマの出没と地形の複雑性(P<0.01)および土地利用における果樹園の割合(P<0.01)との間に正の相関が確認された。微視的な解析では、クマの出没が、果樹園(P<0.01)および藪化レベルが高い林縁周辺(P<0.05)に偏っていることが確認された。解析の結果、クマの出没には、地形の複雑な場所であることと、果樹園と藪の存在が影響していると考えられた。

To infer the distribution pattern of introduced rainbow trout, Oncorhynchus mykiss, we compared abiotic factors among tributaries with and without rainbow trout in the Atsuta River, Hokkaido, Japan. Rainbow trout were present in 10 of the 24 tributaries. Stepwise logistic regression analysis indicated that the occurrence of rainbow trout was more likely in low-gradient tributaries and was negatively correlated with elevation. Our results indicate that the successful establishment of rainbow trout can be predicted using abiotic factors, including elevation and gradient.

The mechanisms by which nonnative species establish populations can be classified into two broad categories: they usurp the niches of native species through interspecific competition, or they avoid this intense interspecific competition by making use of minimal niche overlap with the native species. In this study, we considered how a nonnative salmonid species, the rainbow trout Oncorhynchus mykiss, established a population in the presence of the native salmonid species, the masu salmon O. masou, in Hokkaido, Japan. Circumstantial field evidence shows that the masu salmon exceeds the rainbow trout in abundance and suggests that these species use different types of cover habitat ( rainbow trout abundance increases with increasing abundance of large woody debris aggregates, whereas masu salmon abundance increases with increasing abundance of undercut banks). These results imply that the rainbow trout established a population due to minimal niche overlap with the masu salmon, and not by competitive exclusion of the native species.

To describe in detail the population genetic structure of masu salmon Oncorhynchus masou, seven anadromous populations in the Atsuta River were investigated by analysis of eight polymorphic microsatellite loci. Exact test and genetic differentiation (FST) revealed that significant differentiation was observed at the within-river scale. Principal components analysis revealed that the spatial pattern of genetic composition was related to geographical location of each population. Furthermore, a significant correlation was observed between genetic differentiation and geographical distance, indicating that dispersal is less likely to occur between distant populations. The results of this study imply that masu salmon have potential to show clear genetic structure at the microgeographical scale (21 km) due to precise homing behaviour. © 2008 Blackwell Munksgaard.

Genetic structure of masu salmon Oncorhynchus masou populations in Hokkaido was examined by analysing mtDNA NADH dehydrogenase Subunit 5 gene (561 bp) of 382 individuals collected from 12 rivers, in which there were no records of artificial release. Analysis of molecular variance showed that between groups level and between populations within-group level explained each c. 10% of genetic variance. In neighbour-joining tree, four populations located in southern Hokkaido were clustered into a single group, however, other populations did not form any clear clusters. Fu's F-s Tajima's D and a mismatch distribution test indicated a sudden expansion of population in the entire population of Hokkaido and the northernmost Population of Chiraibetsu, which was genetically close to the Southern Hokkaido group. The Sea of Japan and southern rivers, including those of southern Hokkaido, seem to have served as refugia for masu salmon during glacial periods, and their dispersal and straying in interglacial periods affected the genetic structure of masu salmon Populations in Hokkaido. (c) 2007 The Authors Journal compilation (c) 2007 The Fisheries Society of the British Isles.

Male body size has been emphasized as an important factor contributing to the breeding success of individuals. However, the operational sex ratio (OSR: ratio of mature females to males) during the breeding season significantly change due to differences in the breeding timing and period within and between males and females and may influence the selective advantage of the male large body size for mates. We examined the reproductive ecology of masu salmon Oncorhynchus masou, inhabiting Lake Toya, Hokkaido, northern Japan. Precipitation triggered the upstream migration, although males migrated into the river earlier than females. As a result, the OSR in the river changed markedly during the breeding season, as did the size structure of males in the river. Large males migrated into the river earlier than the smaller males. Differential male survival resulted in the different population size structure between the early and late periods. Under these conditions, we analyzed which males were more successful in releasing the more sperm throughout the breeding season by estimating the decrease in the sperm content in male carcasses. The body size and sperm-releasing success of the males were not related. With a weak male-biased OSR and synchronous timing of reproduction in females, males that had entered the river succeeded in releasing the sperm regardless of body size. Such a fluctuating advantage for different body sizes likely contributes to the maintenance of the size variation in the male salmon.

To examine whether or not sex-specific growth occurs in fry of masu salmon Oncorhynchus masou, 90 half-sibling fry chosen from three different classes of egg size (small, medium and large) and emergence time (period from fertilisation to first feeding; early, middle and late) were raised in a tank for 60 days. In all four replications, male fry grew faster than females. The difference in growth between sexes was greater among siblings hatched from larger eggs. Fast-growing, young male masu salmon show a precocious form that tends to mature without migrating to sea. Slower growing males tend to show the anadromous form, which is only shown in females. Sex-specific life histories may influence the sex-specific growth patterns during early life history.

Brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, have been introduced to freshwaters in Hokkaido, Japan. Today, it is recognized that these introduced salmonids have negative impacts on native salmonids such as white-spotted charr, Salvelinus leucomaenis, and masu salmon, O. masou. In particular, interspecific competition may be an important mechanism that could contribute to the exclusion for native salmonids. In this study, experimental pairwise contests were conducted to compare interference competitive ability between native and introduced salmonids. We demonstrated that brown trout were competitively superior to white-spotted charr and masu salmon whereas rainbow trout were superior to white-spotted charr. We suggest that introduced brown trout negatively impact both white-spotted charr and masu salmon, and introduced rainbow trout negatively impact white-spotted charr.

Otolith size of emerging masu salmon Oncorhynchus masou fry was measured as a surrogate measure for standard metabolic rate in a controlled breeding study. Otolith size differed significantly among half siblings from the same dam, Which suggested that standard metabolic rate of the offspring and hence their growth rate were probably paternally inherited. (C) 2003 The Fisheries Society of the British Isles.

Dominance, aggression and predator avoidance were compared among farmed, sea-ranched and wild juvenile masu salmon Oncorhynchus masou in laboratory experiments. Domesticated fish (farmed and sea-ranched), which had been exposed to artificial selection, were not dominant against wild fish in pairwise contests, nor did they show greater aggressiveness. Farmed fish did show greater feeding than wild fish. Under chemically simulated predation risk, farmed fish were more willing to leave cover and feed than wild fish, indicating reduced predator avoidance in the farmed fish. Our results indicate that selection for fast growth (domestication) in masu salmon favors fish that respond to food quickly and ignore predation risk.

We observed the spawning behaviors of male masu salmon (Oncorhynchus masou) to examine whether the life history forms of males were related to the reproductive tactics. Standard males (mean ± SD = 43.0 ± 7.1 cm), which migrate into the lake and return to the natal creek for spawning, fought with other males to become the dominant male in the reproductive group. The male that became dominant in the reproductive group intensively attacked males of a similar size and defended an area around a spawning female against other males. Puerile males (22.3 ± 6.3 cm), which grow in the lake and mature while retaining parr marks, attempted to gain a higher hierarchical position in the subordinate group, while being attacked frequently by the dominant male. Precocious males (mature male parr, 11.5 ± 3.3 cm), which mature without migrating to the lake, had a tendency to occupy positions near the females and attempted to use sneaking mating tactics. These males were not attacked by the dominant male. This indicates that the different life history forms of males are associated with different reproductive tactics.

Competition between masu salmon (Oncorhynchus masou) of wild and aquaculture origin was investigated. Fry were individually marked and released in stream enclosures with and without a piscivorous predator. The aim was to assess the effects of predators and salmon body size on survival and growth of the two types of fish under natural conditions. The presence of predaceous Japanese huchen (Hucho perryi) resulted in lower mean growth rates of surviving fry. Relatively large fish survived and grew better than relatively small fish in the absence of predators, bur not in their presence. This probably indicates an indirect effect of predation risk on within-cohort competitive behavior among salmon juveniles, with larger fish forced to give up their position as superior competitors. Domesticated fish survived in larger numbers and grew much faster than wild fish, irrespective of predator presence. Comparison with similar field studies revealed a pattern that the pre-experimental environment influenced the outcome of competition between wild and domesticated juvenile salmon. Domesticated fish were superior competitors even in the absence of an initial size advantage, which commonly gives a further advantage to hatchery-raised fish in natural streams. Therefore, caution dictates to avoid the release or escape of such fish into the wild.

In spatial competition between individuals, neither fish sex nor body mass affected dominance status in masu salmon Oncorhynchus masou. In contrast, resting metabolic rate (M-R) was significantly correlated with dominance status, indicating that a high metabolic rate can increase the dominance rank of juvenile salmon. Whole animal growth rate was significantly correlated with M-R, but not with initial body weight. This suggests that the body size of masu salmon is not a cause, but rather a consequence, of dominance status which is closely related to M-R. The increment width between otolith daily rings was also significantly correlated with M-R. Thus, the size of the otolith may indicate the degree of M-R. (C) 1998 The Fisheries Society of the British Isles.