Comparative anatomy of the dorsal hump in mature Pacific salmon

Mature male Pacific salmon (Genus Oncorhynchus ) demonstrate prominent morphological changes, such as the development of a dorsal hump. The degree of dorsal hump formation depends on the species in Pacific salmon. It is generally accepted that mature males of sockeye (O. nerka ) and pink (O. gorbuscha ) salmon develop most pronounced dorsal humps. The internal structure of the dorsal hump in pink salmon has been confirmed in detail. In this study, the dorsal hump morphologies were analyzed in four Pacific salmon species inhabiting Japan, masu (O. masou ), sockeye, chum (O. keta ), and pink salmon. The internal structure of the dorsal humps also depended on the species; sockeye and pink salmon showed conspicuous development of connective tissue and growth of bone tissues in the dorsal tissues. Masu and chum salmon exhibited less‐pronounced increases in connective tissues and bone growth. Hyaluronic acid was clearly detected in dorsal hump connective tissue by histochemistry, except for in masu salmon. The lipid content in dorsal hump connective tissue was richer in masu and chum salmon than in sockeye and pink salmon. These results revealed that the patterns of dorsal hump formation differed among species, and especially sockeye and pink salmon develop pronounced dorsal humps through both increases in the amount of connective tissue and the growth of bone tissues. In contrast, masu and chum salmon develop their dorsal humps by the growth of bone tissues, rather than the development of connective tissue.     

Functional response of juvenile pink and chum salmon: effects of consumer size and two types of zooplankton prey

Feeding rate experiments were conducted for pink salmon Oncorhynchus gorbuscha fry [mean fork length ( L _F) 39 mm], juveniles (103–104 mm L _F) and juvenile chum salmon Oncorhynchus keta (106–107 mm L _F). Fishes were presented with small copepod ( Tisbi sp.) or larger mysid shrimp ( Mysidopsis bahia ) prey at varying densities ranging from 1 to 235 prey l⁻¹ in feeding rate experiments conducted at water temperatures ranging from 10·5 to 12·0° C under high light levels and low turbidity conditions. Juvenile pink and chum salmon demonstrated a type II functional response to mysid and copepod prey. Mysid prey was readily selected by both species whereas the smaller bodied copepod prey was not. When offered copepods, pink salmon fry fed at a higher maximum consumption rate (2·5 copepods min⁻¹) than larger juvenile pink salmon (0·4 copepods min⁻¹), whereas larger juvenile chum salmon exhibited the highest feeding rate (3·8 copepods min⁻¹). When feeding on mysids, the maximum feeding rate for larger juvenile pink (12·3 mysids min⁻¹) and chum (11·5 mysids min⁻¹) salmon were similar in magnitude, and higher than feeding rates on copepods. Functional response models parameterized for specific sizes of juvenile salmon and zooplankton prey provide an important tool for linking feeding rates to ambient foraging conditions in marine environments, and can enable mechanistic predictions for how feeding and growth should respond to spatial-temporal variability in biological and physical conditions during early marine life stages.     

Differences in host selection of actinospores of two myxosporeans, Myxobolus arcticus and Thelohanellus hovorkai

We investigated the host selection mechanism of actinospore stages of 2 myxosporeans, Myxobolus arcticus and Thelohanellus hovorkai, infecting masu salmon (Oncorhynchus masou) and common carp (Cyprinus carpio), respectively. Discharge of the polar filaments and sporoplasm release by M. arcticus actinospores occurred within the first 5 min of exposure to skin mucus of masu salmon. The actinospores also reacted to the mucus of nonsusceptible fish, i.e., sockeye salmon (Oncorhynchus nerka) and goldfish (Carassius auratus), although the reactivity was comparatively lower. After exposure of masu, and sockeye and chum salmon (Oncorhynchus keta) to M. arcticus actinospores, the penetration of sporoplasms was observed in the fins and gills of masu and sockeye salmon to a similar extent and to a lesser extent in chum salmon. Thelohanellus hovorkai actinospores exhibited a slow response of sporoplasm release to common carp mucus as well as penetration into the gills of common carp. Neither chemoresponse to mucus of nonsusceptible fish (goldfish and sockeye salmon) nor sporoplasm invasion in goldfish was observed for T. hovorkai actinospores. These results indicate notable differences in the host selection at the time of entry between M. arcticus and T. hovorkai; the former responds quickly to fish mucus with low host specificity, whereas the latter was highly host specific in a dilatory reaction.     

Physiological mechanisms of imprinting and homing migration in Pacific salmon Oncorhynchus spp

After several years of feeding at sea, salmonids have an amazing ability to migrate long distances from the open ocean to their natal stream to spawn. Three different research approaches from behavioural to molecular biological studies have been used to elucidate the physiological mechanisms underpinning salmonid imprinting and homing migration. The study was based on four anadromous Pacific salmon Oncorhynchus spp., pink salmon Oncorhynchus gorbuscha, chum salmon Oncorhynchus keta, sockeye salmon Oncorhynchus nerka and masu salmon Oncorhynchus masou, migrating from the North Pacific Ocean to the coast of Hokkaido, Japan, as well as lacustrine O. nerka and O. masou in Lake Toya, Hokkaido, where the lake serves as the model oceanic system. Behavioural studies using biotelemetry techniques showed swimming profiles from the Bering Sea to the coast of Hokkaido in O. keta as well as homing behaviours of lacustrine O. nerka and O. masou in Lake Toya. Endocrinological studies on hormone profiles in the brain-pituitary-gonad axis of O. keta, and lacustrine O. nerka identified the hormonal changes during homing migration. Neurophysiological studies revealed crucial roles of olfactory functions on imprinting and homing during downstream and upstream migration, respectively. These findings are discussed in relation to the physiological mechanisms of imprinting and homing migration in anadromous and lacustrine salmonids.     

Microsporidiosis of Sakhalin Salmonidae: the incidence and dynamics of the infection

Four-year summer observations have shown that G. takedai distribution in Salmonidae of Sakhalin is limited by the rivers entering the Aniva Bay. Maximum infection of pink salmon (Oncorhynchus gorbuscha) amounts to 70-100%, that of masu salmon (Oncorhynchus masou) to 9.0-45.6% and was recorded in September. Dynamics and possible ways of infection are discussed.     

Paternal effect on otolith size at emergence in masu salmon

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.     

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.     

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.     

Cloning and sequence analyses of vasotocin and isotocin precursor cDNAs in the masu salmon, Oncorhynchus masou: evolution of neurohypophysial hormone precursors.

We have cloned and determined the nucleotide sequences of cDNAs encoding precursors of neurohypophysial hormones, vasotocin (VT) and isotocin (IT), from the hypothalamus of masu salmon, Oncorhynchus masou. The deduced amino acid sequences of masu salmon VT and IT precursors (proVT-I and proIT-I) are highly homologous to those of chum salmon proVT-I and proIT-I, respectively. The VT and IT precursors are composed of a signal peptide, hormone and neurophysin (NP), the middle portion of which is highly conserved among vertebrates. Both the NPs extend about 30 amino acids at the C-terminal. The extended C-terminals have a leucin-rich segment in the carboxyl-terminal, as copeptin of vasopressin precursor. Southern bot analysis showed the presence of two types of proVT genes (proVT-I and proVT-II) and proIT genes (proIT-I and proIT-II) in an individual masu salmon, as in a chum salmon. Southern blot analysis with proVT probes further suggested that at least two different types of proVT-I genes exist in a single masu salmon. Northern blot analysis indicated that proVT-I and proIT-I genes are expressed in the hypothalamus, whereas proVT-II and proIT-II genes are not expressed. Evolutionary distance between proVT-I and proIT-I genes was statistically estimated based on synonymous nucleotide substitution in the coding region of the cDNAs. The magnitude of distance between masu salmon proVT-I and proIT-I genes suggested that the highly conserved central portion of NPs resulted from a gene conversion event. Between masu salmon and chum salmon, evolutionary distance for proVT-I genes is about 6-fold larger than that for proIT-I genes.     

Cloning and characterization of an odorant receptor in five Pacific salmon

The olfactory system of fish is extremely important as it is able to recognize and distinguish a vast of odorous molecules involved in wide ranges of behaviors including reproduction, homing, kin recognition, feeding and predator avoidance; all of which are paramount for their survival. We cloned and characterized one type olfactory receptors (ORs) from five congeneric salmonids: lacustrine sockeye salmon (Oncorhynchus nerka), pink salmon (O. gorbuscha), chum salmon (O. keta), masu salmon (O. masou) and rainbow trout (O. mykiss). Lacustrine sockeye salmon olfactory receptor 1 (LSSOR1) showed high sequence homology to the OR subfamily, and was expressed only in the olfactory epithelium (as indicated by PCR amplified genomic DNA and cDNA). OR genes from the five salmonids examined all showed strong homology (96-99%) to each other. Hypervariable regions, believed to be ligand-binding pockets, showed homologous completely matched amino acid sequences except for one amino acid in pink salmon olfactory receptor 1 (PSOR1), revealing that these ORs may be well conserved among salmon species. These results suggest that the isolated 5 salmonid ORs might play an important role in salmon life cycles.     

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.     

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.     

The effects of introduced salmonids on two native stream‐dwelling salmonids through interspecific competition

Using an artificial stream, habitat use by two sympatric native salmonids in the presence and absence of introduced salmonid species was investigated experimentally. When only native white‐spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou were sympatric, they occupied different microhabitats. In the presence of introduced brown trout Salmo trutta or rainbow trout Oncorhynchus mykiss , however, white‐spotted charr and masu salmon were observed to use a similar habitat and interspecific competition between white‐spotted charr and masu salmon was initiated. The study suggested that the coexistence of native salmonids was negatively affected through interspecific competition between native and introduced salmonids.     

Identification of the sex chromosome pair in chum salmon (Oncorhynchus keta) and pink salmon (Oncorhynchus gorbuscha)

Fluorescence in situ hybridization (FISH) using a probe to the male-specific GH-Y (growth hormone pseudogene) was used to identify the Y chromosome in the karyotypes of chum salmon (Oncorhynchus keta) and pink salmon (Oncorhynchus gorbuscha). The sex chromosome pair is a small acrocentric chromosome pair in chum salmon and the smallest metacentric chromosome pair in pink salmon. Both of these chromosome pairs are morphologically different from the sex chromosome pairs in chinook salmon (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch). The 5S rRNA genes are on multiple chromosome pairs including the sex chromosome pair in chum salmon, but at the centromeres of two autosomal metacentric pairs in pink salmon. The sex chromosome pairs and the chromosomal locations of the 5S rDNA appear to be different in all five of the North American Pacific salmon species and rainbow trout. The implications of these results for evolution of sex chromosomes in salmonids are discussed.     

The lateral line system and its innervation in the masu salmon Oncorhynchus masou masou (Salmonidae)

Ichthyological Research - The lateral line system and its innervation were examined in the masu salmon Oncorhynchus masou masou. The species has 8 cephalic canals (supraorbital, infraorbital, otic,...     

Composition changes in retinal pigments according to habitat of chum and pink salmon

Juvenile chum Oncorhynchus keta and pink Oncorhynchus gorbuscha salmon change the composition of their visual pigments when they move from coastal waters to offshore in the Okhotsk Sea. Levels of the visual pigment rhodopsin were found to be high on the coast, while porphyropsin levels were high in offshore waters in both species. These facts suggest visual function may be similar, allowing them to coexist in the same sea area.     

Isolation, primary structure, and biological and immunological properties of pink and chum salmon insulins

1. Insulins have been isolated from islet tissue of pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon. The primary structure of chum and pink salmon insulins was found to be identical. Compared to the amino acid sequence of human insulin, the salmon insulins under study differed at 14 positions. 2. Biological activity of pink salmon insulin was 83% of that of standard porcine insulin. 3. The immunological properties of fish insulins were investigated in specific radioimmunoassay (RIA) systems, based on porcine and pink salmon insulins. 4. A significant difference in the antigenic determinants of these fish and mammalian hormones was revealed.     

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.     

Chemiluminescent immunoassay (CLIA) for salmon growth hormone (GH)

A highly sensitive and specific chemiluminescent immunoassay (CLIA) was developed for quantification of growth hormone (GH) in salmonid species. The CLIA for salmon GH was performed using the sandwich method with anti-GH IgG as the first antibody and chemiluminescent acridinium ester-labelled specific anti-GH F(ab′)₂ as the second antibody. The measurable range of salmon GH in the CLIA was 39–1250 pg/mL using a short assay (1 day) protocol and 3.9–125 pg/mL in a longer (2-day) assay. The dilution curve in the CLIA of serum from masu salmon (Oncorhynchus masou) was parallel to the standard curve of recombinant chum salmon (Oncorhynchus keta) GH. Seasonal changes of serum GH levels were measured in 1 year-old masu salmon cultivated in a pond from March to November. Their serum GH levels increased during smoltification from March to April, achieved a maximum level of 21 ng/mL in August, and then declined gradually to 11 ng/mL in October. © 1997 John Wiley & Sons, Ltd.