A single-locus minisatellite discriminates chinook salmon (Oncorhynchus tshawytscha) populations

A knowledge of genetic structure in natural populations is often necessary for conservation and management purposes, especially in declining Pacific salmon populations. To test for genetic differentiation between nine populations of chinook salmon, Oncorhynchus tshawytscha, from south-western British Columbia, Canada, DNA was extracted from 603 fish and hybridized with a single-locus minisatellite probe. Multivariate statistical analyses of the resulting allele size data permitted successful overall population identification of 52% (individual population range: 24–78%; P < 0.005), indicating a high level of genetic differentiation among the nine populations. Two of the nine populations were further analysed using data from a second minisatellite locus. The discrimination success rate improved from 81.1% (one-locus analyses) to 90.0% (two-locus analyses), indicating the potential for greatly increased resolution gained by the addition of more loci. These results indicate that variation at minisatellite loci can be used for assessing population-level genetic structure, even with artificial gene flow.     

Chains of single-domain magnetite particles in chinook salmon,Oncorhynchus tshawytscha

Summary Although the presence of magnetite in their tissues is correlated with the ability of different species to detect magnetic fields, proof that the magnetite is involved in magnetoreception has not yet been provided. Using the approach employed to localize and isolate magnetic particles in the yellowfin tuna, we found that single-domain magnetite occurs in chains of particles in tissue contained within the dermethmoid cartilage of adult chinook salmon,Oncorhynchus tshawytscha. The particles are present in sufficient numbers to provide the adult fish with a very sensitive magnetoreceptor system. Magnetite in the chinook can be correlated with responses to magnetic fields in a congeneric species, the sockeye salmon. Based on the presence of the chains of particles, we propose behavioral experiments that exploit the responses of sockeye salmon fry to magnetic fields to test explicit predictions of the ferromagnetic magnetoreception hypothesis.     

Identification of the Y chromosome in chinook salmon (Oncorhynchus tshawytscha)

The Y chromosome in chinook salmon, Oncorhynchus tshawytscha, was identified using fluorescence in situ hybridization (FISH) with a probe to a male-specific repetitive sequence isolated from this species. The probe highlights the distal end of the short arm of an acrocentric chromosome with a DAPI-bright interstitial band of variable size. The proximal portion of the short arm of the Y chromosome contains 5S rDNA sequences, which are also found on the short arms of six other acrocentric chromosomes in this species.     

Molecular evolution at Mhc genes in two populations of chinook salmon Oncorhynchus tshawytscha

The DNA sequences of four exons of the MHC (major histocompatibilty complex) were examined in chinook salmon ( Oncorhynchus tshawytscha ) from an interior (Nechako River) and a coastal (Harrison River) population in the Fraser River drainage of British Columbia. Mhc class I A1, A2 and A3 sequences and a class II B1 sequence were obtained by PCR from each of 16–20 salmon from each population. The class I A1 and a pair of linked A2–A3 exons were derived from two different classical salmonid class I genes, Sasa-A and Onmy-UA , respectively. Allelic variation for B1, A1 and A2 was characterized by the high levels of nonsynonymous substitution indicative of the effects of natural selection on Mhc domains that contain peptide binding regions. The number of alleles detected at each of the four exons ranged from three ( B1 ) to 22 ( A1 ), but levels of nucleotide sequence divergence at all four exons were low relative to classical mammalian Mhc genes. The nucleotide similarity among alleles ranged between 89 and 99% over all exons, and all four domains possessed only two major sequence motifs. Allelic distributions at B1, A1 and A3 confirmed the genetic distinctiveness of the Harrison and Nechako chinook salmon populations revealed in previous studies. The two major allelic motifs of B1 and A1 segregated strongly between the populations. In spite of evidence that allelic diversity at these chinook salmon Mhc exons has been generated by selection, the level and distribution of diversity in the two salmon populations strongly reflected the demographic history of the species, which has been characterized by repeated bottlenecks and isolation-by-distance in glacial refugia.     

An intranuclear microsporidium associated with acute anemia in the chinook salmon, Oncorhynchus tshawytscha

An intranuclear microsporidium is described from hemoblastic cells of the chinook salmon, Oncorhynchus tshawytscha. The infection is associated with an acute anemia in the fish. Up to 47% of the hemoblast nuclei were infected in anemic fish. The organisms, found only in spleen and kidney tissues, were 1-2 microns in diameter and consisted of vegetative and early sporulation forms. This microsporidium differs from known species which parasitize fish in its tissue location; however, the absence of mature spores and other life cycle stages precludes determination of its precise taxonomic identity.     

Experimental horizontal transmission of Enterocytozoon salmonis to chinook salmon, Oncorhynchus tshawytscha.

Enterocytozoon salmonis was transmitted to chinook salmon Oncorhynchus tshawytscha by feeding tissues infected with the parasite and by cohabitation of noninfected fish with experimentally infected fish. Affected fish (dead and survivors) in both transmission trials had gross and microscopic signs of the disease and merogonic and sporogonic stages of the parasite. There were no morbidities or mortalities, or evidence of the parasite among control fish in either study. Results suggest that the parasite may be contracted by indirect contact among healthy and infected fish held in crowded ponds or net pens or by direct ingestion of spores found in the water.     

Prey preferences of a riverine population of juvenile chinook salmon, Oncorhynchus tshawytscha

The diet of juvenile chinook salmon and the foods available to them were studied during spring and summer in a large, braided, New Zealand river. During both sampling periods fish and potential prey were collected at dawn and dusk. Analysis showed that in spring the feeding rate increased at dawn, when aquatic taxa comprised the majority of their prey. Prey of terrestrial origin dominated the diet at dusk in summer but formed only about 1% of the diet during spring, when few such prey were available.
During spring the fish selectively preyed upon larger nymphs of the mayflies Deleatidium spp. However, in summer chironomids, other dipterans, and trichopterans were consumed to the exclusion of Deleatidium . Most of the chironomids and trichopterans taken were pupae or emerging adults and it is suggested that this may reflect differences in vulnerability during the diurnal emergence period.     

Outbreaks of phaeohyphomycosis in the chinook salmon (Oncorhynchus tshawytscha) caused by Phoma herbarum

Phoma herbarum has been associated with two outbreaks of systemic mycosis in hatchery-reared chinook salmon (Oncorhynchus tshawytscha) fingerlings. Affected fish exhibited abnormal swimming behavior, exophthalmia, multiple rounded areas of muscle softening, protruded hemorrhagic vents, and abdominal swelling. In all affected fish, swimbladders were filled with whitish creamy viscous fungal mass, surrounded by dark red areas in swimbladder walls, kidneys, and musculature. Clinical and histopathological examinations suggest that the infection may have started primarily in the swimbladder and then spread to the kidneys, gastrointestinal tract, and surrounding musculature. Consistent microscopical findings included broad septate branched fungal hyaline hyphae, 5–12 μm in diameter within the swimbladder, stomach, and often within and adjacent to blood vessels. Profuse growths of woolly brown fungal colonies were obtained from swimbladders and kidneys on Sabouraud medium. On corn meal agar the formation of pycnidia, characteristic of Phoma spp., was detected within 10 days of incubation. Morphological and molecular analyses identified this fungus as Phoma herbarum. This report underscores systemic fungal infections as a threat to raceway-raised salmon.     

Evidence for a carrier state of infectious hematopoietic necrosis virus in chinook salmon Oncorhynchus tshawytscha.

In British Columbia, Canada, infectious hematopoietic necrosis virus (IHNV) is prevalent in wild sockeye salmon Oncorhynchus nerka and has caused disease in seawater net-pen reared Atlantic salmon Salmo salar. In this study, chinook salmon Oncorhynchus tshawytscha experimentally exposed to an isolate of IHNV found in British Columbia became carriers of the virus. When Atlantic salmon were cohabited with these virus-exposed chinook salmon, IHNV was isolated from the Atlantic salmon. Identification of chinook salmon populations that have been exposed to IHNV may be difficult, as virus isolation was successful only in fish that were concurrently infected with either Renibacterium salmoninarum or Piscirickettisia salmonis. Also, IHNV-specific antibodies were detected in only 2 of the 70 fish experimentally exposed to the virus. Two samples collected from chinook salmon exposed to IHNV while at a salt water net-pen site had a seroprevalence of 19 and 22%; however, the inconsistencies between our laboratory and field data suggest that further research is required before we can rely on serological analysis for identifying potential carrier populations. Because of the difficulty in determining the exposure status of populations of chinook salmon, especially if there is no concurrent disease, it may be prudent not to cohabit Atlantic salmon with chinook salmon on a farm if there is any possibility that the latter have been exposed to the virus.     

Innate susceptibility differences in chinook salmon Oncorhynchus tshawytscha to Loma salmonae (Microsporidia)

Loma salmonae (Putz, Hoffman and Dunbar, 1965) Morrison & Sprague, 1981 (Microsporidia) is an important gill pathogen of Pacific salmon Oncorhynchus spp. in the Pacific Northwest. Three strains of chinook salmon O. tshawytscha were infected in 2 trials with L. salmonae by feeding of macerated infected gill tissue or per os as a gill tissue slurry. Intensity of infection was significantly higher in the Northern stream (NS) strain as compared to the Southern coastal (SC) and a hybrid (H) strain derived from these 2 strains. Both wet mount and histological enumeration of intensity of infection demonstrated strain differences. Survival in the NS strain was significantly lower than the other strains. The NS strain may represent a naive strain and be less able to mount an effective immune response against the parasite.     

Resistance to reinfection in chinook salmon Oncorhynchus tshawytscha to Loma salmonae (Microsporidia).

Chinook salmon Oncorhynchus tshawytscha were experimentally infected per os with Loma salmonae and held in flow-through seawater tanks at 12 to 14 degrees C. The fish exhibited 100% infection when first examined at 7 wk post initial exposure (p.e.), and by 20 wk p.e. they had completely recovered from gill infections. The recovered fish were then re-exposed the following week. All of these fish showed strong protection to new L. salmonae infections, while na?ve fish exposed to the same inoculum developed the infection. Most of the re-exposed fish exhibited a few free spores or spores within phagocytes in the kidney interstitium at 20 to 29 wk p.e., but xenomas were not detected in either the gills or visceral organs. The kidney is the primary site of reticulo-endothelial activity, and thus these spores were likely deposited in the kidney by entrapment by fixed macrophages. It is possible that these spores provide immunologic stimuli to reinforce the resistance to new L. salmonae infections.     

p,p'-DDE depresses the immune competence of chinook salmon (Oncorhynchus tshawytscha) leukocytes

p,p'-DDE, the main metabolite of DDT, is still detected in aquatic environments throughout the world. Here, the effects and mechanisms by which p,p'-DDE exposure might affect the immune system of chinook salmon (Oncorhynchus tshawytscha) was studied. Isolated salmon splenic and pronephric leukocytes were incubated with different concentrations of p,p'-DDE, and cell viability, induction of apoptosis, and mitogenic responses were measured by flow cytometry and Alamar Blue assay. p,p'-DDE significantly reduced cell viability and proliferation and increased apoptosis. The effect of p,p'-DDE on pronephric leukocytes was more severe than on splenic leukocytes, likely because pronephric leukocytes had a higher proportion of granulocytes, cells that appear more sensitive to p,p'-DDE. The effect of p,p'-DDE on leukocytes appeared to vary between developmental stages or seasonal differences. The mitogenic response of leukocytes of chinook salmon exposed to p,p'-DDE in vivo exhibited a biphasic dose-response relationship. Only leukocytes isolated from salmon treated with 59 ppm p,p'-DDE had a significantly lower percentage of Ig+ blasting cells than controls, although the response was biphasic. These results support the theory that exposure to chemical contaminants could lead to an increase in disease susceptibility and mortality of fish due to immune suppression.     

Isolation and characterization of Edwardsiella tarda from fall chinook salmon (Oncorhynchus tshawytscha).

A new bacterial pathogen of chinook salmon (oncorhynchus tshawytscha) was isolated from fish in Oregon's Rogue River. The bacteria are biochemically and serologically related to strains of Edwardsiella tarda. Initially isolated from chinook salmon, the bacteria were also pathogenic for steelhead and rainbow trout (Salmo gairdneri), and channel catfish (Ictalurus punctatus). The 50% lethal doses for chinook salmon, steelhead trout, and channel catfish injected intraperitoneally and maintained in 18 degrees C water were 4.1 x 10(6), 5.6 x 10(6), and 4.0 x 10(5) respectively. When chinook salmon and rainbow trout were injected intraperitoneally and held in 12 degrees C water, the mean lethal doses were 6.4 x 10(7) and 1.7 x 10(6), respectively. The invasiveness of the organism was low in steelhead trout exposed to the bacteria by the waterborne route. The optimum growth temperature of the bacteria in brain heart infusion broth was approximately 35 degrees C. The guanine plus cytosine content of DNA obtained from E. tarda isolated from salmon was 59 mol%.     

Environmental correlates of life-history variation in juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Throughout its native North Pacific, the chinook salmon, Oncorhynchus tshawytscha (Walbaum), exists as twolife-history types that aredistinguished by the age at which juvenile salmon migrate to sea as smolts. 'Stream-type' chinook migrate seaward after I or more years of feeding in fresh water, whereas 'ocean-type' fish migrate to sea as newly emerged fry or after 2–3 months in fresh water. Stream-type chinook predominate in populations distant from the sea south of 56° N, and in both inland and coastal populations north of this point. By contrast, ocean-type chinook predominate in coastal populations south of 56° N, but are rare in populations in more northerly latitudes. Stream-type populations are associated with areas of low 'growth opportunity' (as measured by temperature and photoperiod regimes) and/or areas distant from the sea compared to ocean-type. Geographic variability in juvenile life history is suggested to result, in part, from environmental modulation of smolting timing via differences in growth opportunity among geo-graphic areas. In addition, differences in migration distance and temperature regime may result in selection for different sizes at migration among populations which, through differences in growth opportunity, might promote geographic variability in age at seaward migration.     

Preliminary evaluation of praziquantel against metacercariae of Nanophyetus salmincola in chinook salmon (Oncorhynchus tshawytscha)

Praziquantel at dosages of 10, 20 or 100 mg/kg of body weight was evaluated against metacercariae of Nanophyetus salmincola in chinook salmon (Oncorhynchus tshawytscha). Ten salmon were used in each of four treated groups and 10 salmon were nontreated controls. Three wk after treatment, viability of metacercariae was determined by histologic evaluation, and by feeding the salmon to coyotes and subsequently determining the numbers of trematode eggs/g of feces and numbers of N. salmincola recovered at necropsy. Results of the experiment indicated that praziquantel at the dosages and routes of administration used was not effective against metacercariae in chinook salmon.     

Ecomorphological plasticity of juvenile fall-run chinook salmon (Oncorhynchus tshawytscha) in perennial and ephemeral streams

Environmental Biology of Fishes - In the Central Valley of California, environmental characteristics differ between perennial and ephemeral stream types and therefore present different challenges...     

Heritability and Y-chromosome influence in the jack male life history of chinook salmon (Oncorhynchus tshawytscha)

Jacking in chinook salmon (Oncorhynchus tshawytscha) is an alternative reproductive strategy in which males sexually mature at least 1 year before other members of their year class. We characterize the genetic component of this reproductive strategy using two approaches; hormonal phenotypic sex manipulation, and a half-sib breeding experiment. We 'masculinized' chinook salmon larvae with testosterone, reared them to first maturation, identified jacks and immature males based on phenotype, and genotyped all fish as male ('XY') or female ('XX') using PCR-based Y-chromosome markers. The XY males had a much higher incidence of jacking than the XX males (30.8% vs 9.9%). There was no difference in body weight, gonad weight, and plasma concentrations of testosterone and 17beta-estradiol between the two jack genotypes, although XY jacks did have a higher gonadosomatic index (GSI) than XX jacks. In the second experiment, we bred chinook salmon in two modified half-sib mating designs, and scored the number of jacks and immature fish at first maturation. Heritability of jacking was estimated using two ANOVA models: dams nested within sires, and sires nested within dams with one-half of the half-sib families common to the two models. The sire component of the additive genetic variance yielded a high heritability estimate and was significantly higher than the dam component (h(2)(sire) = 0.62 +/- 0.21; h(2)(dam) = -0.14 +/- 0.12). Our experiments both indicated a strong sex-linked component (Y-chromosome) to jacking in chinook salmon, although evidence for at least some autosomal contribution was also observed.     

A cohabitation challenge to compare the efficacies of vaccines for bacterial kidney disease (BKD) in chinook salmon Oncorhynchus tshawytscha

The relative efficacies of 1 commercial and 5 experimental vaccines for bacterial kidney disease (BKD) were compared through a cohabitation waterborne challenge. Groups of juvenile chinook salmon Oncorhynchus tshawytscha were vaccinated with one of the following: (1) killed Renibacterium salmoninarum ATCC 33209 (Rs 33209) cells; (2) killed Rs 33209 cells which had been heated to 37 degrees C for 48 h, a process that destroys the p57 protein; (3) killed R. salmoninarum MT239 (Rs MT239) cells; (4) heated Rs MT239 cells; (5) a recombinant version of the p57 protein (r-p57) emulsified in Freund's incomplete adjuvant (FIA); (6) the commercial BKD vaccine Renogen; (7) phosphate-buffered saline (PBS) emulsified with an equal volume of FIA; or (8) PBS alone. Following injection, each fish was marked with a subcutaneous fluorescent latex tag denoting its treatment group and the vaccinated fish were combined into sham and disease challenge tanks. Two weeks after these fish were vaccinated, separate groups of fish were injected with either PBS or live R. salmoninarum GL64 and were placed inside coated-wire mesh cylinders (liveboxes) in the sham and disease challenge tanks, respectively. Mortalities in both tanks were recorded for 285 d. Any mortalities among the livebox fish were replaced with an appropriate cohort (infected with R. salmoninarum or healthy) fish. None of the bacterins evaluated in this study induced protective immunity against the R. salmoninarum shed from the infected livebox fish. The percentage survival within the test groups in the R. salmoninarum challenge tank ranged from 59% (heated Rs MT239 bacterin) to 81% (PBS emulsified with FIA). There were no differences in the percentage survival among the PBS-, PBS/FIA-, r-p57- and Renogen-injected groups. There also were no differences in survival among the bacterin groups, regardless of whether the bacterial cells had been heated or left untreated prior to injection.