Experimental observation of the prior residence advantage in masu salmon in an experimental channel

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 individual9s 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.

We have revealed several unique characteristics of germ cell development using rainbow trout, including the fact that spermatogonia transplanted into the peritoneal cavity of newly hatched embryos migrate toward recipient gonads, that spermatogonia transplanted into female recipients start oogenesis and produce functional eggs and that diploid germ cells transplanted into triploid trout can complete gametogenesis. By combining these unique features of fish germ cells, we established allogeneic and xenogeneic transplantation systems for spermatogonia in several fish species. Spermatogonia isolated from the mature testes of vasa-green fluorescent protein (Gfp) transgenic rainbow trout were transplanted into the peritoneal cavity of triploid masu salmon newly hatched embryos. These spermatogonia migrated toward recipient salmon genital ridges with extending pseudopodia and were subsequently incorporated into them. We further confirmed that the donor-derived spermatogonia resumed gametogenesis and produced sperm and eggs in male and female salmon recipients, respectively. By inseminating the resulting eggs and sperm, we obtained only rainbow trout offspring in the F1 generation, suggesting that the triploid salmon recipients produced functional gametes derived only from donor trout. We further confirmed that this intra-peritoneal transplantation of germ cells is applicable to several marine fishes, which could be of benefit in the production of bluefin tuna that has a large broodstock (>100 kg) and is difficult to maintain in captivity. Gamete production of bluefin tuna could be more easily achieved by generating a surrogate species, such as mackerel, that can produce tuna gametes.

Summary1. 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 <5% if the carrying capacity increased from 141 to 303 for masu salmon and from 94 to 125 for charr, by allowing fish passage at the lower end of the habitat, and if annual adult survival rate increased by 0.04. Adult survival rate was the principal factor associated with population persistence.5. To conserve isolated populations of stream‐dwelling salmonids, we recommend (i) assessing the distribution of remnant native and non‐native fish populations, (ii) that fishing regulations are modified to improve adult survival and population persistence and (iii) that fragmented reaches be reconnected to adjacent habitat, for example by removing or modifying artificial barriers to increase the carrying capacity of the isolated populations. Reconnection of fragmented reaches should, however, be avoided if it results in non‐native fish invading isolated populations.

Diploid and triploid fish produced in crosses between female masu salmon (MS) Oncor-hynchus masou and males of six species, namely, MS, rainbow trout (RT) O. mykiss, brown trout (BN) Salmo trutra, Japanese common charr (JC) Salvelinus leucomaenis pluvius, white-spotted char (WS) Salvelinus leucomaenis leucomaenis, and brook trout (BK) Salvelinus fontinalis, were subjected to efficiency tests for aquaculture. Diploids and triploids of MS×MS, MS×JC, MS×WS and MS×BK, in addition to triploids of MS×RT and MS×BN, survived beyond the swimming-up stage, and thereafter the survival rate of every allotriploid group clearly exceeded that of their diploid hybrid counterparts. The daily growth rates of immature 1-year-old diploids and triploids of MS×JC and MS×WS and triploids of MS×BN were superior under controlled feeding, but inferior at satiation level to those of diploid and triploid MS×MS. Triploids appeared to perform better than diploids at satiation level in crosses of MS×JC and MS×WS. During the 6 months of accelerated gonadal development preceding the spawning season, the survival and growth rates of hybrids remarkably surpassed those of MS×MS for both ploidy levels.

SummaryInteractions between captive‐reared and wild salmonids are frequent because hatcheries annually rear millions of fish for release in conservation programmes while many thousands of domesticated fish escape from fish farms. However, the outcome of competition between captive‐reared and wild fish is not clear: wild fish may be smaller and less aggressive than hatchery fish, but they have more local experience and a prior residence advantage. Moreover, it is important to know whether any competitive differences are genetic (due to the process of domestication) or due to the rearing environment.We therefore examined the factors influencing competition for feeding territories in juvenile Atlantic salmon. We studied the effect of domestication by using three independent stocks of both domesticated and wild‐origin fish, all of which were reared in a common hatchery environment. We also used fish from the same wild stocks that had been living in the wild. Territorial contests were staged in stream tank compartments between pairs of fish differing in origin or rearing environment; the relative importance of body size and prior residence was also assessed.All three stocks of domesticated fish were generally dominant over wild‐origin fish when both had been raised in a common hatchery environment. If the wild‐origin fish were given a 2‐day period of prior residence on the territory this asymmetry in dominance was reversed. However, domesticated fish did not gain any additional advantage from being prior residents. The relative body size of the two contestants had a negligible effect on contest outcomes.Truly wild fish (i.e. those of wild origin that had also grown up in the wild) were generally dominant over domesticated or wild‐origin fish that had been hatchery‐reared. Differences in body size between contestants had no effect on the outcome.Synthesis and applications. These results show that, while juvenile farmed Atlantic salmon are inherently more aggressive than wild‐origin fish, the hatchery environment reduces their ability to compete for territories with wild resident fish. Rearing salmon in conventional hatcheries for later release into the wild where natural populations already exist may not be a prudent conservation measure; it is preferable to plant eggs or first‐feeding fry rather than attempt to ‘help’ the fish by rearing them through the early life stages.

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.

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

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.

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.

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.

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.

The behavior of carotenoids in the course of maturation of the masu salmon was investigated in comparison with that of the chum salmon.The behavior of carotenoids in the masu salmon during anadromous migration was similar to that of the chum salmon. With advancing maturation, the amount of carotenoids decreased in the muscle (0.76→0.06mg/100g) and increased in the skin (1.17→2.80mg/100g) and ovaries (1.24→1.72mg/100g)As the matured masu salmon, the absorption spectra of crude carotenoids extracted from the muscle and the skin changed. In the ovaries, there was no changed observed.With the advance of maturation, the astaxanthin rate in the muscle was largely reduced (80→55%). In the skin, the salmoxanthin rate decreased (39→22%) but the astaxanthin rate increased considerably (2→22%). In the ovaries, the composition of carotenoid did not change.In the masu salmon during the anadromous migration period, astaxanthin was presumed to be converted into zeaxanthin, as was presumed for the chum salmon during the spawning period.

An ectoparasitic ciliate, Chilodonella piscicola (=C. cyprini), became epizootic and caused chronic mortalities in juvenile masu (cherry) salmon Oncorhynchus masou and pink salmon O. gorbuscha being reared at the Nemuro Hatchery, eastern Hokkaido, Japan, in the spring of 1988. Cumulative mortalities amounted to 20% in a masu salmon population over 5 months and 10% in a pink salmon population over 2 months. Transmission experiments in masu salmon fry confirmed that C. piscicola can cause severe proliferation of the gill epithelium in the absence of other stressors. The infected fish exhibited reduced growth and chronic mortalities after the mean intensity had reached a peak (619 parasites per fish) at week 4 postinfection. The marked epithelial hyperplasia was followed by intense fusion of adjacent gill lamellae and filaments. Respiratory failure due to gill epithelial hyperplasia may have been a primary cause of the deaths.