Identification and characterization of pathogenic bacteria isolated from dead masu salmon Oncorhynchus masou masou and antibacterial activity of probiotic Lactococcus lactis subsp. lactis strain K-C2 against isolated pathogens

We focused on developing an epidemic prevention method for circulating masu salmon aquaculture using the probiotic Lactococcus lactis strain K-C2. This study aimed to evaluate the antibacterial activities of strain K-C2 against pathogens isolated from dead Yamame and masu salmon. First, we identified pathogenic bacteria that were isolated from dead masu salmon based on phylogenetic analysis of the 16S rRNA gene sequence and biological and biochemical characterization. The antibacterial activity of strain K-C2 against seven pathogenic strains isolated from dead masu salmon in this study and two strains of Aeromonas salmonicida previously isolated from dead Yamame was tested using a double agar plate method. The results of a BLAST search using 16S rRNA partial sequence data (1200–1300 bp) revealed that six of the former strains and one of the latter strains showed high similarity to Vibrio anguillarum and Tenacibaculum maritimum, respectively. Strain K-C2 showed antibacterial activity against all pathogenic bacteria. In this study, pathogenic bacteria were newly isolated from dead seawater-acclimated masu salmon, and strain K-C2 was found to have antibacterial effects against these pathogens.

     

Importance of instream cover for young masu salmon, Oncorhynchus masou, in autumn and winter

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

Recovery of white-spotted charr Salvelinus leucomaenis following the removal of stocked red-spotted masu salmon Oncorhynchus masou ishikawae in a small headwater tributary of Lake Biwa,central Japan

The possible recovery of a white-spotted charr population in a small tributary of a river to Lake Biwa, following removal of previously stocked red-spotted masu salmon, was investigated by electrofishing. We captured 30 red-spotted masu salmon and two hybrids of these two species in 2014. Prior to the stocking of red-spotted masu salmon, the estimated number of white-spotted charr?≥?100 mm standard length (SL) in the tributary was more than 200 individuals in 2005, which had reduced to fewer than 30 individuals by 2014 when 30 red-spotted masu salmon plus two charr/masu salmon hybrids were captured. However, no red-spotted masu salmon were captured from 2015 to 2017, indicating the success of a red-spotted masu salmon removal program. The estimated number of white-spotted charr?≥?100 mm SL ranged from 25 to 91 individuals between 2015 and 2017, implying a great reduction of red-spotted masu. The estimated population size of the former increased further to 171–221 individuals in 2021, comparable to 2005. These results suggest that the white-spotted charr population declined due to stocking of red-spotted masu salmon, and recovered following removal of the latter.

     

Commercial harvest of hatchery-reared masu salmon Oncorhynchus masou estimated by a coast-wide sampling program in Hokkaido, northern Japan, and the two-stage sampling schemes of landings

SUMMARY: This paper evaluates the stocking effectiveness of masu salmon Oncorhynchus masou in Hokkaido, northern Japan, through a coast-wide two-stage sampling survey of commercial landings. From January to June 1994–1996, commercial landings of masu salmon at 33–36 fish markets were sampled at 7–10 days intervals, and 60 866–72 124 fish were examined for marks indicated by fin clips. Based on the survey data, numbers of total and hatchery-reared masu salmon landed were estimated. To examine the structure of the errors, stratification of fish markets was implemented on the basis of geography and magnitudes of landings, and the stratification improved accuracy and precision of the estimates. Accuracy of the estimated numbers of total fish was evaluated by being compared to the true numbers of masu salmon landings reported by fishermen's cooperative associations. Estimates of total masu salmon landings were within ± 10% of the true numbers. The estimated recovery rates (± SE) for hatchery-reared masu salmon smolts were variable ranging from 0.18 (± 0.06) to 3.50 (± 0.41)% among the stocked groups. An optimal sampling strategy was examined to obtain precise estimates for future studies.     

Differentiated predation risk on hatchery-reared juvenile masu salmon by white-spotted charr with different body sizes

Predation after release is one of the major concerns of hatchery fish propagation. However, size-specific interaction between predator and prey on the survival of hatchery-released salmonid fish is largely unknown. To understand the size-selective predation risk, 24-h predation experiments were conducted on masu salmon Oncorhynchus masou in tanks. Four ranges of fork length (FL) were examined for masu salmon as a prey, in combination with three ranges of FL for white-spotted charr Salvelinus leucomaenis as a predator. The results show that not only predator and prey sizes, but also interaction between prey size and predator size, strongly affected the survival rate of masu salmon. Predation on masu salmon with the FL exceeding 40% of the FL of white-spotted charr was rare in the experiment. A logistic regression suggests that 37% relative FL of masu salmon to white-spotted charr results in the 50% survival of masu salmon. Our results suggest that adjusting relative size of hatchery fish to the size of local fish predators at the time of hatchery release will have a significant impact on the survival of hatchery fish in the wild. From this perspective, site-specific, adaptive management might be important to improve the effectiveness of hatchery fish propagation.     

Serum thyroxine,estradiol-17β, and testosterone profiles during the parr-smolt transformation of masu salmon,Oncorhynchus masou

Changes in serum thyroxine (T₄), estradiol-17β (E₂) and testosterone (T) levels during the parr-smolt transformation (smoltification) were investigated in the masu salmon (Oncorhynchus masou) in 1985 and 1987. T₄ showed a peak in levels at the early stage of smoltification and E₂ and T levels peaked almost at the same time. There were no significant differences between the concentrations of serum hormones in female and males. During smoltification, germ cells in the peri-nucleolus and spermatogonia stage were present in the ovary and testis, respectively. These results suggest that E₂ and T may be involved in smoltification in the masu salmon.     

Effects of smolt size and timing of migration on recovery rate of wild masu salmon <Emphasis Type="Italic">Oncorhynchus masou</Emphasis>

Little is known about the survival rate of wild masu salmon Oncorhynchus masou. To examine the effects of smolt length and migration timing on the recovery rate of wild masu salmon, we reanalyzed past tagging and recovery data (1993–1994). The tagging study was conducted in the Shokanbetsu River, northern Japan; 863 wild masu salmon smolts were captured, tagged, and released in a downstream site, and a total of 19 fish were recovered in coastal fisheries and in the natal river the following year. The data were analyzed by a logistic regression analysis with recapture as a response variable and tagging date and smolt length as explanatory variables; the tagging date had a significant effect on the recapture rate, whereas the effect of smolt size was not significant. Despite the small number of recaptures, this study indicates that migration timing is a factor affecting the marine survival of wild masu salmon smolts, although this conclusion has been repeatedly documented for other species of Pacific salmon Oncorhynchus spp.     

Detection of hybrids between masu salmon <Emphasis Type="Italic">Oncorhynchus masou masou</Emphasis> and amago salmon <Emphasis Type="Italic">O. m. ishikawae</Emphasis> occurred in the Jinzu River using a random amplified polymorphic DNA technique

Identification of masu salmon Oncorhynchus masou masou and amago salmon O. m. ishikawae was accomplished using a random amplified polymorphic DNA (RAPD) technique. Of 80 primers screened, three primers amplified subspecies-specific fragments (OPA11-1095 ^*, OPB5-1618^* and OPD5-2038 ^*). Based on fragment patterns, 33 hybrids were detected among 150 upstream-migrating individuals in the Jinzu River, central Japan, a long-time habitat of masu salmon, to which amago salmon were recently introduced. All of the individuals examined in the 2000–2002 cohorts were identified as F ₁ and F _x as well as pure masu and amago forms. These results indicated continuing hybridization, the genetic pollution of the native masu salmon population by amago salmon possibly being a serious problem in the Jinzu River. Both standard length and body weight in F ₁ hybrids tended to be less than in pure masu salmon. However, F _x hybrids showed similar body sizes to masu salmon.     

An additional life-history tactic of masu salmon: Migration of parr to coastal habitats

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.     

Purification and characteristics of trypsin from masu salmon (<Emphasis Type="Italic">Oncorhynchus masou</Emphasis>) cultured in fresh-water

Trypsin from the pyloric ceca of masu salmon (Oncorhynchus masou) cultured in fresh water was purified by a series of chromatographies including Sephacryl S-200, Sephadex G-50 and diethylaminoethyl cellulose to obtain a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS–PAGE) and native PAGE. The molecular mass of the purified trypsin was estimated to be approximately 24,000 Da by SDS–PAGE. The enzyme activity was strongly inhibited by phenylmethylsulfonyl fluoride, soybean trypsin inhibitor, and N ^α -p-tosyl-l-lysine chloromethyl ketone. Masu salmon trypsin was stabilized by calcium ion. The optimum pH of the masu salmon trypsin was around pH 8.5, and the trypsin was unstable below pH 5.0. The optimum temperature of the masu salmon trypsin was around 60°C, and the trypsin was stable below 50°C, like temperate-zone and tropical-zone fish trypsins. The N-terminal 20 amino acid sequence of the masu salmon trypsin was IVGGYECKAYSQPHQVSLNS, and its charged amino acid content was lower than those of trypsins from frigid-zone fish and similar to those of trypsins from temperate-zone and tropical-zone fish. In the phylogenetic tree, the masu salmon trypsin was classified into the group of the temperate-zone fish trypsin.     

Politics and pride: Maintaining genetic novelty may be detrimental for the conservation of Formosa landlocked salmon Oncorhynchus formosanus

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Habitat niche separation of the nonnative rainbow trout and native masu salmon in the Atsuta River,Hokkaido, Japan

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.     

Hatch timing of two subarctic salmonids in a stream network estimated by otolith increments

Hatch timing in autumn-spawning stream salmonids is poorly understood in the subarctic region because snow cover prevents direct sampling of cryptic early life stages. Otolith micro-increment analysis was used to infer hatch dates of white-spotted charr Salvelinus leucomaenis (Pallas) and masu salmon Oncorhynchus masou (Brevoort) in a mainstem-tributary network in northern Japan. Accuracy and precision were validated by ageing hatchery individuals with known hatch date ranges. In July 2018, 93 wild young-of-the-year white-spotted charr and 81 masu salmon were collected and aged. Masu salmon hatched, on average, 24 days earlier (mean = February 8) than white-spotted charr (March 4), and hatch dates spanned a minimum of 2 months for each species. In masu salmon, hatch dates of individuals collected in the mainstem were nearly 3 weeks earlier than those in a tributary. This study provided knowledge on intra- and inter-specific variation in hatch timing of native salmonids in a subarctic stream network.     

Color vision, spectral sensitivity, accommodation, and visual acuity in juvenile masu salmon Oncorhynchus masou masou

ABSTRACT:   Color vision, spectral sensitivity, accommodation, and visual acuity were examined in juvenile masu salmon Oncorhynchus masou masou to obtain fundamental information about the visual system. Two types of S-potentials were recorded from 415 horizontal cells in isolated retinas from 34 cultured freshwater masu salmon (114–219 mm standard length, SL). Although horizontal cells recording S-potentials were not identified, the horizontal cells were confirmed because their responses were maintained while the stimulus remained. The variety of chromaticity (C)-type S-potentials indicated well-developed color vision. The analysis of the luminosity (L)-type S-potentials indicated that the peak spectral sensitivity was at 522 nm. S-potentials were also recorded in response to ultraviolet light. The direction and extent of lens movement induced by electrical stimulation was measured in 12 cultured masu salmon (99.0–142.5 mm SL). The results indicated that the visual axis was upward and forward, and that the range of accommodation was from 0.79 × SL in front of the eye to infinity. In histological analysis of the retinas of five wild smolts (100–118 mm SL), the maximum cone densities (276–345 cones/0.01 mm²) were detected in the ventral to temporal regions. The visual acuities assessed by histological methods were 0.069–0.075.     

Terrestrial prey inputs to streams bordered by deciduous broadleaved forests,conifer plantations and clear‐cut sites in southwestern Japan: effects on the abundance of red‐spotted masu salmon

Terrestrial invertebrates falling from the riparian canopy are a major energy source for fishes in headwater streams. Because quantity and quality of such allochthonous resources can vary depending on riparian conditions, conversion of riparian forests to conifer plantations may affect stream productivity. We compared falling and drifting invertebrate abundances and the diet of masu salmon (Oncorhynchus masou) among stream reaches bordered by deciduous broadleaved forests, conifer plantations (Cryptomeria japonica), and clear‐cut sites in southwestern Japan. We also examined whether among‐reach variation in salmon abundance was related to the riparian vegetation types. The results indicated that, on an annual basis, falling inputs of terrestrial invertebrates at the broadleaved reaches were 2–4 times higher than those at the plantation and clear‐cut reaches. In nonwinter seasons, terrestrial invertebrates made up 40–60% and 30–90% of drift and masu salmon diets, respectively, and drifting invertebrate abundance was higher in the broadleaved reaches than in the plantation reaches. Furthermore, a multivariate analysis of salmon abundance showed that variation in salmon biomass was explained primarily by riparian vegetation type, with broadleaved and clear‐cut reaches having higher biomass than the plantation reaches. These results indicate that terrestrial invertebrates are an important resource for masu salmon, and suggest that streams bordered by conifer plantations receive lower terrestrial prey inputs, which results in lower salmon abundance. In regions where natural forests have been extensively converted to conifer plantations, forest management that allows and facilitates recovery of natural riparian stands is important.     

Movement of the fluvial form of masu salmon, <Emphasis Type="Italic">Oncorhynchus masou masou</Emphasis>, in a mountain stream in Kyushu,Japan

Using mark-recapture methods, the movements of the fluvial form of masu salmon (Oncorhynchus masou masou) in a mountain stream on the island of Kyushu, Japan, were studied. Most (78%) of the masu salmon were recaptured in the pool in which they had been originally caught and tagged. Of those that moved between pools, the proportion of individuals that moved during the breeding period was not significantly higher than the proportion that moved during the non-breeding period. However, during the breeding period, a higher proportion of larger salmon moved than did smaller fish. The proportion of mobile large males during breeding period was higher than that for small males. Also, it was found that a few individuals showed long-range movement in the autumn. As a long-term movement, 78 individual fish (65%) that were recaptured more than three times showed high sedentary tendencies. Sixteen individual mobile fish (13%) moved and returned to the original pool. Fluvial form of masu salmon in Kyushu show a high sedentary nature; however, large mature males seem to actively move in search of female during breeding period.     

Microsatellite variation reveals fine-scale genetic structure of masu salmon, Oncorhynchus masou, within the Atsuta River

Abstract –  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 ( F _ST) 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.     

Opsonization by fish antibody and complement in the immune phagocytosis by peritoneal exudate cells isolated from salmonid fishes

Abstract. Opsonic activities of fish antibody and complement in the phagocytic responses of fish leucocytes have been investigated using salmonid peritoneal exudate cells (PEC) and viable cells of Aeromonas salmonicida as antigenic particles. PEC were isolated from liquid paraffin-injected coho salmon, masu salmon and rainbow trout. After incubation at 20°C for 2 h, phagocytosis by PEC was evaluated. Phagocytic activity was attributed to neutrophil-like and macrophage-like cells, though in the absence of immune serum activity was low. The addition of fish antibody (heal-inactivated antiserum) slightly increased the activity. On the addition of normal serum as a complement source phagocytic activity was dramatically accelerated. This acceleration was interpreted as opsonization as a result of the classical pathway activation of fish complement by an antigen-antibody reaction. The complement of mammals (goat, guinea pig and rabbit) did not possess opsonic activity in the presence of fish antibody.     

Dominance and predator avoidance in domesticated and wild masu salmon Oncorhynchus masou

ABSTRACT:   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.     

Sex-specific growth pattern during early life history in masu salmon, Oncorhynchus masou

Abstract –  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.