Effects of photoperiod manipulation on the appearance of precociously maturing males and smolts in freshwater and maturing fish in seawater in masu salmon Oncorhynchus masou from Hokkaido

Concentrations of several trace elements (Cu, Pb, Cd, Ni, and Zn) were measured in masu salmon (Oncorhynchus masou), the smallest in size and warmest-water species of Pacific salmon. The analyzed masu specimens came to spawn in the Bakhura River, the southeastern coast of Sakhalin Island, and were collected at the local fish hatchery. It has been found that the level of the trace elements, of which two (Pb and Cd) are toxic, in masu salmon meets the sanitary standards of the Russian Federation for seafood. A comparison of the microelement compositions of masu and pink salmon, which also came to spawn in the southeast of Sakhalin Island (Firsovka River), has shown that masu accumulates mainly Zn, Cu, and Ni during growth and feeding, while pink accumulates Pb and Cd.KeywordsPacific salmonMasu salmon (Oncorhynchus masou)Heavy metalsSea of OkhotskSoutheastern coast of Sakhalin Island

In Atlantic salmon, as in most salmonids, males can mature early in the life cycle, as small freshwater fish, termed parr, and/or undergo a sea migration before maturing as full-size adults. The alternative life histories are contingent on environmental and social circumstances, such as growth rate, territory quality or any other factor that affects the individual's state. In order to model the choice of life history in this group of commercially valuable species, it is necessary to understand not only the relative contribution of the different male types to subsequent generations, but also to know the factors that affect reproductive success in each type. In this paper we present the results of a study designed to investigate the factors that affect the reproductive success of mature parr. We used highly polymorphic minisatellite DNA markers to analyse paternity in a series of mating experiments where the number and body size of parr were manipulated. The fraction of eggs fertilized by mature parr ranged from 26 to 40 per cent, with individual parr fertilizing up to 26 per cent of the eggs. A strong positive correlation was found between parr size and reproductive success. The relative success of parr decreased with increasing parr number. Data from this and other studies on variation in the timing and degree of parr reproductive success are discussed in relation to the evolution of male mating strategies and life history in salmonids.

A total of 92 masu salmon (Oncorhynchus masou) which had developed silvering were divided into 4 groups. Three of these groups were transplanted from fresh water. The feeding conditions varied with the group. Viable counts were determined in the intestinal contents or slime of these salmon, in their ambient waters and in their diets. Over 1500 strains were isolated from the above samples. The composition of the microflora was determined according to the scheme of Shewan et al. (1960). The results are summarized as follows: 1) Microbial viable counts in the intestinal contents or slime of the fish which were transplanted under normal feeding conditions were nearly constant while those which were transplanted without being fed decreased rapidly. This decreasing tendency was found in both fresh and sea water reared non-feeding fish. 2) The predominant genus in the intestinal microflora of the fresh water fish was Aeromonas, while in sea water fish it was Vibrio. Upon transplanting the fish from fresh water into sea water fish it was Vibrio. Upon transplanting the fish from fresh water into sea water, Aeromonas of the terrestrial type was gradually replaced by Pseudomonas as the proportion of sea water in the rearing water increased. This was followed by further replacement by Vibrio of the halophilic type which became predominant in the intestinal microflora.

Effects of environmental salinity, biopsy, and GH and IGF-I administration on the expression of immune and osmoregulatory genes in the gills of Atlantic salmon (Salmo salar)

Captive animals are exposed to various stressors originating from their artificial environment. Although stress can initially be an adaptive physiological response, persistent stress generally decreases immune function and makes animals more susceptible to pathogens. Environmental enrichment is a strategy that has been widely used to mitigate excessive stress and enhance animal welfare, but the effectiveness of individual enrichment components remains controversial. Hence, this study aimed to reveal the effect of shelter enrichment on stress levels and parasite infections in Masu Salmon Oncorhynchus masou. By using a single component, the study will help to provide a better understanding of the underlying mechanism of the enrichment effect. We explored whether shelter-enriched conditions relieved stress in Masu Salmon by evaluating plasma cortisol levels. Fish provided with shelters were expected to experience less aggression from conspecifics and less stress from human disturbance. Moreover, we conducted a parasite challenge test using Ichthyobodo sp. to determine whether the presence of shelters inhibited infection by mitigating stress levels. Fish in enriched tanks had significantly lower basal cortisol levels than those in control tanks, suggesting that they were less affected by stressors. Although the difference was not significant, fish in enriched tanks also tended to experience less infection by the parasites than fish in control tanks. Given these results, simple shelter enrichment appears to be an effective method for mitigating stress among captive fish and reducing stress intensity or frequency. In addition, shelter enrichment could be used to inhibit infectious diseases, although further work is needed to prove the effect. These findings confirm that shelter enrichment has measurable effects on the factors related to fish welfare, which should encourage the adoption of enrichment in fish farms.

Control of maturation in masu salmon by manipulation of photoperiod

The effect of photoperiod manipulation on growth, smoltification and maturation was assessed in Atlantic salmon through an observational study performed in a commercial RAS facility from June (25.0 ± 11.0 g) to October 2018 (151.1 ± 25.5 g). Half of the commercial cohort 721 was raised in continuous light LD24:0 (LL) while the other received a 6-week LD12:12 winter signal (WS) for smoltification from 12 August to 21 September. Parameters related to growth (body weight, gene expression of pituitary gh1 and gh2, and liver ghr1, igf1 and igfbp1a), smoltification (condition factor, plasma sodium and cortisol, gill NKA activity and nka1a, nka1b and nkcc1a expression) and maturation (GSI and pituitary fshb and lhb expression) were analysed. Afterwards, a multivariate analysis was performed on production data from five commercial cohorts raised in the facility (including 721) to identify variables potentially linked to early maturation in RAS. Results from the observational study indicated weak compensatory growth and slightly better smoltification in WS, although signs of size-induced smoltification were present in LL. Smoltification indicators were poor in both treatments, suggesting that smolts may not be yet ready for seawater. No maturation was observed in any photoperiod treatment; however, the multivariate assessment suggested that such lack of maturation might be rather linked to the low mean temperature and SGR experienced by our cohort.

Effects of photoperiod manipulation on development of seawater tolerance in Arctic charr

Masu salmon (Oncorhynchus masou) are the only Pacific salmon endemic to Asia. Some researchers prefer to categorize these salmon into 4 subspecies (masu-Oncorhynchus masou masou, amago-Oncorhynchus masou ishikawae, Biwa-Oncorhynchus masou subsp., and Formosan-Oncorhynchus masou formosanus), while others prefer individual species designations. Even though the masu salmon fishery is thousands of years old, classification of the diversity within the masu salmon species complex remains elusive. In this study, a genetic map and reference genome assembly were generated for 1 species/subspecies (masu) to provide resources for understanding the species complex. In O. m. masou, the sex chromosome was determined to be chromosome 7. Resequenced genomes from 2 other putative subspecies (amago and Biwa) provided evidence that they do not share the same sex chromosome. Principal component and admixture analyses clustered the amago and Biwa salmon close together. This supported previous findings of a close relationship between amago and Biwa salmon and a more distant relationship to masu salmon for both. Additional analyses of the masu salmon species complex will benefit from using the new reference genome assembly.

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.

This study analysed the relationship between body size and the commencement of smoltification in masu salmon Oncorhynchus masou. Juveniles of a strain of O. masou were reared under a simulated natural photoperiod (SNP) and an advanced photoperiod (AP), in which LD15:9 was interrupted by LD9:15 from June to October to induce spring smoltification. Regardless of the photoperiod, the underyearlings were just shorter than 12 cm in fork length (LF) in October. When the photoperiod in the AP group was switched from short to long days in late October, juvenile O. masou exceeded 12 cm and exhibited increased gill Na+/K+-ATPase (NKA) activity in December. These changes were accompanied by an increase in circulating levels of insulin-like growth factor (IGF)-1 and IGF-binding protein-2b. In contrast, fish under SNP remained below 12 cm and had low gill NKA activity and hormone/protein levels from October to January. Underyearlings of the same strain reared in a larger tank under a natural photoperiod grew well and exhibited increased gill NKA activity in July when they exceeded 12 cm. These results suggest that an LF of 12 cm is the threshold size above which juvenile O. masou increase gill NKA activity throughout the year. This implies that they may cease their growth during autumn and winter to avoid exceeding the threshold size required to commence smoltification during spring. Although the threshold size may be related to rearing density or tank size, the findings of the present study suggest that increases in gill NKA activity during summer and the following spring are tightly linked to body size in O. masou.

We studied biochemically and cytologically the contribution of lysosomal hyrdolytic pathway to glycogen degradation in the striated muscle of diploid (control) masu salmon Oncorhynchus masou. The data were compared with those of triploid masu salmon or mouse skeletal muscle. Acid α-glucosidase activity, when measured using maltose or glycogen as substrate, was about 10 times higher in diploid masu salmon than in mice, but it was 50% lower inversely when 4-methy-lumbelliferyl α-D-glucoside (4MUαG) as substrate. Total phosphorylase activity in masu salmon muscle was about 7-fold lower than that in mouse muscle. The result suggests that lysosomal hydrolytic pathway of glycogenolysis is relatively active in the fish muscle. We observed glycogeno-somes (glycogen-containing autophagic vacuoles) in the masu salmon muscle but not in mouse muscle, mimicking cytology of human glycogen storage disease type II (GSD II) lacking lysosomal acid α-glucosidase. However, the mechanism of the glycogenosome formation in masu salmon muscle cell can not be explained analogically as in GSD II, since acid α-glucosidase activity was not deficient. There was no clear differences in ultrastructure of the muscle cell or biochemical measurements between diploid and triploid masu salmon.

We investigated the seasonal distribution and migration patterns of hatchery-reared masu salmon Oncorhynchus masou in the area surrounding Hokkaido, the northern island of Japan. From 1993 to 1999, we tagged and released 313,000 yearling masu salmon smolts into the coastal waters of the Sea of Japan adjacent to southwestern Hokkaido. From May of the release year through July of the following year, a total of 1,693 fish were recaptured in the coastal waters of northern Japan. Just after their release, masu salmon juveniles had two migration routes; one group moved northward along the western side of Hokkaido in the Sea of Japan, while the second group passed through the Tsugaru Strait and then moved eastward into the Pacific Ocean. It was estimated that both groups reached the Sea of Okhotsk at 30–40 d postrelease. The summer distribution of tagged fish was unknown because none were recaptured. Young fish appeared in the Sea of Okhotsk again in autumn and then migrated southward, retracing the above two routes. They reached the Tsugaru Strait and Pacific coastal waters ranging from the northeastern Shimokita Peninsula in Honshu to the Iburi district of Hokkaido. Tagged masu salmon juveniles were recaptured in these areas until the following March, which indicates that these sites were the most important overwintering areas. Adult fish rapidly returned to the area of release from April to June. Our results suggest that hatchery masu salmon migrated in the coastal waters of northern Japan within a relatively short period.

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.

Reproductive activities in vertebrates are regulated by an endocrine system, consisting of the brain-pituitary-gonad axis. In teleosts, gonadotropin-releasing hormone (GnRH) in the brain stimulates gonadotropin (GTH) release in the pituitary gland, but because of lack of the portal vessel, it is not known when and how much GnRH is released for the regulation of GTH release. There are multiple molecular types of GnRH in teleosts and several distinct populations of GnRH neurons in the brain. However, we do not know which types and populations of GnRH neurons regulate reproductive activities. Here we summarize our recent studies on GnRH and GTH in masu salmon Oncorhynchus masou and goldfish Carassius auratus. Immunocytochemistry showed the location and molecular types of GnRH neurons. Salmon (sGnRH) and chicken-II GnRH (cGnRH-II) neuronal fibers were widely distributed in the brain of both masu salmon and goldfish. Only sGnRH fibers were observed in the pituitary of masu salmon, whereas both sGnRH and cGnRH-II fibers were observed in the goldfish pituitary, indicating that species specific GnRH profiles are involved in the regulation of pituitary function in teleosts. A series of experiments in masu salmon and goldfish suggest that among GnRH neuron populations GnRH neurons in the ventral telencephalon and the hypothalamus regulate GTH release, and that GnRH of the terminal nerve origin is not essential to gonadal maturation and ovulation. The biological function of other GnRH neurons remains unkown. Two GTHs appear to be characteristic of teleost; however, regulation of reproduction by these GTHs is a question that remains to be elucidated. In salmonid species, it is proposed that GTH I stimulates early gonadal development, whereas GTH II acts in later stages. When GTH expression was examined in goldfish, both GTH I β and II β mRNA levels in the pituitary gland showed increases in accordance with gonadal development, unlike the sequential expression of GTH subunits in salmonids. The expression of these GTH subunit mRNAs were affected by water temperature, starvation, and steroid hormones in goldfish, but in what manner these two GTHs regulate gonadal development remains to be clarified.