Correlation among dominance status, metabolic rate and otolith size in masu salmon

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.     

Differences of tributyltin accumulation in the masu salmon Oncorhynchus masou between sea‐run and freshwater‐resident types

In order to examine the accumulation pattern of tributyltin (TBT) in relation to the migratory pattern, TBT concentrations in muscles and strontium (Sr) concentrations in otoliths were determined in both sea‐run and freshwater‐resident masu salmon Oncorhynchus masou . Both TBT and Sr concentrations in sea‐run masu salmon were significantly higher than those of freshwater‐residents.     

Differential dispersal patterns of male and female masu salmon fry

Behavioural experiments using artificial and natural stream channels were undertaken to determine whether there were differences in dispersal between newly emerged male and female masu salmon. Eyed eggs from a cross with wild spawners were planted in the middle pool of an artificial channel. After emergence, more males than females moved into an upstream trap, while fewer males moved downstream. In a natural stream, eyed eggs were marked with alizarin complexone to distinguish them from naturally spawned eggs and these were planted into artificial redds. More newly emerged male fry remained at the planted site than female fry. In contrast, more females moved downstream than males. These results imply that differences in dispersal patterns between male and female masu salmon fry are genetically controlled.     

Status-dependent female mimicry in landlocked red-spotted masu salmon

We observed transient status-dependent female mimicry related to reproductive behaviour in landlocked red-spotted masu salmon (Oncorhynchus masou ishikawae). In females, the lower half of the lateral flank underwent solid blackening as an ovipositional character, and 70% of subordinate males showed a similar lateral pattern (M-type). In contrast, most of the paired males and 30% of subordinate males showed a different lateral pattern in which a melanic stripe ran along the lateral-line organ (N-type). It was possible to bring an M-type pseudo-fish model into closer proximity to a pair than an N-type model. In addition, the rate of successful sperm release in subordinate M-type males was higher than that in subordinate N-type males. Males could transform between N- and M-types according to fluctuations in competitive status among the males. A digital video image related to the article is available at .     

Age composition of masu salmon smolts in northern Japan

Smolts of anadromous masu salmon Oncorhynchus masou aged 3+ years were found in a northern Japanese river. This is the first recording of 3+ year smolts in Japan. These fish appeared to originate in the cold upper river where 2+ year parr were found during autumn.     

Electrophoretic changes in olfactory system proteins in masu salmon during parr-smolt transformation

Changes in cytosolic proteins of the olfactory system (olfactory epithelium, olfactory nerve and olfactory bulb) and the telencephalon of masu salmon Oncorhynchus masou were analysed by two-dimensional polyacrylamide gel electrophoresis during parr-smolt transformation; parr, pre-smolt and full-smolt stages. In the olfactory system, several protein spots appeared and disappeared in the course of smolting. One protein spot in particular with an estimated molecular weight of 27 kDa and isoelectric point of 5.6 (M27) disappeared in common with the olfactory system during smolting. The disappearance of M27 was also observed in the telencephalon. These proteins, which appeared and disappeared, may reflect the changes in olfactory function during smolting. Simultaneously, the present study confirmed a salmonid olfactory specific protein of 24 kDa (N24), which existed in the olfactory system but not in the telencephalon, as a single neutral protein spot in masu salmon.     

Latitudinal variation in egg size and number in anadromous masu salmon Oncorhynchus masou

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

Preferable summering habitat of returning adult masu salmon in the natal stream

The preferable summering habitat of returning adult masu salmon Oncorhynchus masou Brevoort was examined in 31 study pools of a small stream in northern Hokkaido, Japan. A total of 23 salmon were captured and were present in only 10 of the study pools. Female salmon were found to be significantly larger than male salmon. Habitat characteristics were expressed by six environmental variables: mean water depth, maximal cover size, mean current velocity, pool volume, substratum coarseness and cover area. There were no significant differences between the pools with salmon and those without salmon in terms of substratum coarseness and cover area. However, significant differences were found in mean water depth, maximal cover size, mean current velocity and pool volume. The analysis found that the pools with salmon were discriminated from those without salmon at a level of 93.5% which was determined by the six environmental variables. Our results revealed that the returning adult salmon selectively used the pools with greater depth, large cover and slow current velocity as their preferable summering habitat. The results also found that the summering habitat of salmon can be estimated by measuring only six environmental variables. Therefore, these variables can be considered useful parameters in determining the summering habitat components of returning adult masu salmon and can contribute to the conservation and restoration strategy of this species by providing a better understanding of the relationship between fish and their habitat.     

Relationship between eyed-egg percentage and levels of cortisol and thyroid hormone in masu salmon Oncorhynchus masou

A possible cause of the low eyed-egg percentage that afflicts pond-reared masu salmon Oncorhynchus masou at a fish hatchery (Kumaishi Fish Hatchery, Hokkaido, Japan) was investigated. Serum cortisol levels of masu salmon during the spawning period were compared between individuals from Kumaishi and those from another station (Mori). Cortisol, thyroxine (T₄) and triiodothyronine (T₃) hormones were also measured in eyed eggs and their levels were compared to the eyed-egg percentage. Serum cortisol levels were significantly higher at Kumaishi than at Mori in May and July of the breeding season. At the Kumaishi station, there was a linear positive relationship between serum cortisol and fertilized-egg cortisol levels of female masu salmon. As cortisol levels in both ovulating females and eyed eggs increased, the eyed-egg percentage decreased, indicating a highly significant negative relationship between cortisol levels and eyed-egg percentage. In contrast, as T₃ and T₄ levels in eyed eggs increased, the eyed-egg percentage increased. The strong positive correlation between high cortisol levels in serum and in eyed eggs at Kumaishi indicates that the frequently higher cortisol levels may have caused the lowering of the eyed-egg percentage.     

Urine of ovulated female masu salmon attracts immature male parr treated with methyltestosterone

Methyltestosterone-treated immature male masu salmon Oncorhynchus masou parr were attracted to both the urine of ovulated females and the ether soluble basic substances extracted from the urine, but not to immature female urine. It is suggested that the male response to the sex attractant (releaser pheromone) in the urine is under the control of androgens.     

A releaser pheromone that attracts males in the urine of mature female masu salmon

A releaser pheromone of masu salmon Oncorhynchus masou was investigated using Y-maze behavioural experiments. During the reproductive season, urine of mature females contains a releaser pheromone which acts as a sex attractant for spermiating mature male parr. The releaser pheromone in mature female urine is one or more low molecular weight substances (less than 10 000) soluble in ether under basic conditions. The attractant was not present in either the coelomic fluid of ovulated females nor in neutral or acidic extracts of female urine which contain free steroids and F-type prostaglandins, respectively.     

The influence of metabolic rate on otolith increment width in Atlantic salmon parr, Salmo salar L.

The influence of metabolic rate on otolith accretion in Atlantic salmon parr was investigated by comparing oxygen consumption rate and increment width in fast and slow growing individuals. Increment width was found to be positively correlated to mean daily oxygen consumption in both fast growing (S1) and slow growing (S2) parr. The results support previous suggestions that a process related to metabolic rate, rather than somatic growth, governs the rate of otolith accretion.     

Dispersal of wild and domestic masu salmon fry (Oncorhynchus masou) in an artificial channel

The day and night pattern of upstream and downstream dispersal of masu salmon fry of wild and domestic origin was compared in artificial channels (45 m long), for two ages of planting: unfed alevins and eyed eggs. Early dispersal was important for the wild stock (48–50%) compared with the domestic one (16–36%). More wild fry moved downstream than upstream, and more domestic fry dispersed upstream. Upstream movement in wild and domestic fry was more active by day than by night, except for wild fry planted as eyed eggs, where upstream migration was higher at night. In contrast, downstream movement in wild and domestic fry was more common by night than by day, but daylight catches were not negligible for the wild stock.     

Ascending speed and nocturnal activity of adult masu salmon (Oncorhynchus masou Brevoort) during upstream migration

We released five adult masu salmon (Oncorhynchus masou) tagged with external transmitters to track their ascending behaviour. The signals of all specimens were recorded in the upper area of the river system. Two patterns of ascending behaviour were recognized: ascending upward immediately after release and ascending during increased river discharge. The fastest ascending speed was about 1000 m h^–1. Active movements were detected at night. The signal recording duration at each receiver for each fish was generally brief. Most fish did not stay at the pools where the receivers were installed.     

Effects of body size and predators on intracohort competition in wild and domesticated juvenile salmon in a stream

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, but 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.     

Genetic structure of masu salmon (Oncorhynchus masou) populations in Hokkaido, northernmost Japan, inferred from mitochondrial DNA variation

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.     

Reproduction ecology of masu salmon <Emphasis Type="Italic">Oncorhynchus masou</Emphasis> in the Kol basin (Western Kamchatka)

In the Kol basin, one of the typical salmon rivers of Western Kamchatka, special traits of reproduction ecology of masu salmon Oncorhynchus masou are studied (characteristics and behavior of spawners during spawning, localization of spawning grounds, topography and hydrology of redds, and interaction with other species of salmonids). The masu salmon spawns in the Kol basin in tributaries of the upper reaches of the river and only in downwelling. Spawning grounds of masu samlmon are confined to stretches with overhanging banks, log jams creating shelters for spawners and favorable hydrological conditions for spawning and egg development. All over its range, masu salmon requires similar conditions for reproduction. In the north of its range, in Kamchatka, masu salmon retains the properties of the most warm-water species in the genus Oncorhynchus and selects for spawning the grounds characterized by the highest temperature in the period of spawning and development.     

Latitudinal variation in sexual size dimorphism of sea-run masu salmon, Oncorhynchus masou

Sexual size dimorphism (SSD), a difference in body size between the sexes, occurs in many animal species. Although the larger sex is often considered invariable within species, patterns of selection may result in interpopulation variation or even reversal of SSD. We evaluated correlations between latitude and female body size, male body size, and relative body size (male body size/female body size) in 22 populations (ranging from 37 degrees N to 49 degrees N) of sea-run masu salmon (Oncorhynchus masou) that spawn in rivers along the Sea of Japan coast. Male size and the relative body size increased with latitude, but female size did not correlate with latitude. In addition, increase in male size with latitude was sufficient to result in a reversal of SSD, the switch-point being around 45 degrees N. We suggest that the positive correlation between latitude and male size is due to increasing operational sex ratios or sexual selection on sea-run male body size that result from sex-biased patterns of anadromy. In conclusion, our study provides the first example of predictable geographic variation in SSD shaped by apparent patterns of sexual selection.