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.     

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.     

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...     

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.     

In vitro detection of functional humoral immunocompetence in juvenile chinook salmon (Oncorhynchus tshawytscha) using flow cytometry

A flow cytometric (FCM) assay for detection of immunomodulatory effects of environmental factors on the humoral response of chinook salmon (Oncorhynchus tshawytscha) is described and validated. This technique combines exposure of whole animals or leucocyte cultures to immunomodulatory agents/conditions with in vitro mitogenic activation of B-lymphocytes. The proportion of leucocytes undergoing blastogenesis following in vitro stimulation with lipopolysaccharide (LPS) is quantified by FCM analysis of forward and side scatter properties. In addition, binding of a fluorescein isothiocyanate labelled anti-rainbow trout immunoglobulin M monoclonal antibody (anti-RBT SIgM-FITC), quantified by FCM analysis, is used to determine the ability of the lymphoblasts to express surface immunoglobulin M (SIgM).Through a series of calibration steps, it was confirmed that anti-RBT IgM-FITC was specific for B-lymphocyte SIgM in chinook salmon. Binding of anti-RBT IgM-FITC to chinook salmon SIgM positive leucocytes was effectively blocked with salmon serum and an isotype control was established. B-lymphocytes were partially removed from a population of leucocytes through adherence to a nylon wool column, which then demonstrated a consequent reduction in anti-RBT IgM-FITC binding. Using anti-RBT IgM-FITC as a marker, the distribution of resting lymphocytes expressing SIgM in lymphoid tissues of juvenile chinook salmon was described. The mean percentage of SIgM positive cells in spleen, pronephros and blood were found to be 62.1 (+/-2.82), 34.8 (+/-1.86) and 56.7% (+/-4.7) of all viable leucocytes, respectively. In a time-course experiment for optimal in vitro activation of leucocytes for this assay, blastogenesis and up-regulation of SIgM expression of splenic leucocytes were observed through FCM by 4 days post in vitro stimulation with LPS, continued through 7 days, but was no longer visible by 10 days post stimulation. Using this assay, reduced expression of SIgM in splenic and pronephric B-lymphocytes was detected following in vitro exposure to physiologically relevant stress concentrations of cortisol in conjunction with mitogenic stimulation. This technique will be a useful addition to the assays already available in the rapidly growing field of fish immunology.     

Growth estimates from otolith increment widths of juvenile chinook salmon (Oncorhynchus tshawytscha) reared in changing environments

For otolith increments to provide useful estimates of fish growth, otolith growth must covary closely with somatic growth. We reared groups of juvenile chinook salmon ( Oncorhynchus tshawytscha Walbaum) for 70 days, changing ration or temperature during a 20-day treatment period. Restricted rations halted somatic growth, however increment widths decreased gradually; somatic growth was overestimated from increment width. Otolith growth followed changes in water temperature more closely than changes in ration, supporting a hypothesized effect of metabolic rate on otolith growth. Increment growth was only loosely coupled to fish growth rate, and may also be affected by past growth histories. For juvenile fish, increment widths may not be sensitive indicators of short-term changes in growing conditions.     

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.     

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.     

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.     

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.     

Vulnerability to predation and physiological stress responses of experimentally descaled juvenile chinook salmon,Oncorhynchus tshawytscha

Synopsis Juvenile salmonids,Oncorhynchus spp., commonly encounter conditions (e.g., during hatchery release and dam passage) that result in damage to the skin, scale, and slime complex. We conducted laboratory experiments to determine if descaling of juvenile chinook salmon,O. tshawytscha, increased their vulnerability to predation, and to assess the physiological stress responses elicited by descaling. Salmon were experimentally descaled on either 10% or 20% of their total body area. When offered equal numbers of control and descaled juvenile chinook salmon, northern squawfish,Ptychocheilus oregonensis, did not consume significantly more of either prey type (48–60% of consumed prey were descaled). Juvenile chinook salmon descaled on 10% of their body area did show significant physiological stress responses, however. Mean concentrations of plasma cortisol peaked 1 h after descaling, and returned to control levels by 12 h. Plasma glucose peaked 3 h post-treatment and remained elevated for 24 h. Plasma lactate increased immediately following treatment and returned to undisturbed control levels by 3 h. The osmoregulatory response of plasma potassium was highly variable, but plasma sodium decreased immediately and remained low for 24 h. The observed physiological responses suggest that descaling of juvenile chinook salmon could result in decreased resistance to disease and other stressors encountered in the field, possibly leading to reduced performance capacity and lowered survival.     

Multigenerational outbreeding effects in Chinook salmon (Oncorhynchus tshawytscha)

Outbreeding, mating between genetically divergent individuals, may result in negative fitness consequences for offspring via outbreeding depression. Outbreeding effects are of notable concern in salmonid research as outbreeding can have major implications for salmon aquaculture and conservation management. We therefore quantified outbreeding effects in two generations (F₁ hybrids and F₂ backcrossed hybrids) of Chinook salmon (Oncorhynchus tshawytscha) derived from captively-reared purebred lines that had been selectively bred for differential performance based on disease resistance and growth rate. Parental lines were crossed in 2009 to create purebred and reciprocal hybrid crosses (n = 53 families), and in 2010 parental and hybrid crosses were crossed to create purebred and backcrossed hybrid crosses (n = 66 families). Although we found significant genetic divergence between the parental lines (F_ST = 0.130), reciprocal F₁ hybrids showed no evidence of outbreeding depression (hybrid breakdown) or favorable heterosis for weight, length, condition or survival. The F₂ backcrossed hybrids showed no outbreeding depression for a suite of fitness related traits measured from egg to sexually mature adult life stages. Our study contributes to the current knowledge of outbreeding effects in salmonids and supports the need for more research to better comprehend the mechanisms driving outbreeding depression.     

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.     

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.     

Antibody-producing cells correlated to body weight in juvenile chinook salmon (Oncorhynchus tshawytscha) acclimated to optimal and elevated temperatures.

The immune response of juvenile chinook salmon (Oncorhynchus tshawytscha) ranging in weight from approximately 10 to 55 g was compared when the fish were acclimated to either 13 or 21 degrees C. A haemolytic plaque assay was conducted to determine differences in the number of antibody-producing cells (APC) among fish of a similar age but different body weights. Regression analyses revealed significant increases in the number of APC with increasing body weight when fish were acclimated to either water temperature. These results emphasise the importance of standardising fish weight in immunological studies of salmonids before exploring the possible effects of acclimation temperatures.     

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 deterministic model for the dynamics of furunculosis in chinook salmon Oncorhynchus tshawytscha

Studies were undertaken to determine the parameters of transmission of Aeromonas salmonicida in chinook salmon Oncorhynchus tshawytscha, and to develop a deterministic model of the dynamics of experimental furunculosis. For determination of disease transmission coefficient (beta), disease-related mortality rate (alpha) and natural mortality rate (gamma), fish in 70 tanks (approximately 42 fish tank(-1)) were each exposed to a single infectious donor fish, 7 tanks were randomly selected daily and all individuals were examined for the presence of A. salmonicida in the kidney. The proportion of susceptible (S), infected (I) and removed (R, dead) individuals were determined daily. The parameters beta, alpha, gamma, reproductive ratio (R0) and threshold density were estimated to be 0.0214 infected ind. d(-1), 0.29 infected ind. d(-1), 0.00015 ind. d(-1), 3.23 and 13.56 ind., respectively. Using these parameters, a deterministic disease model of A. salmonicida infection as a cause of furunculosis was constructed. The net rate at which new individuals became infected (the incidence rate) per unit time was proportional to S x I x beta. The model-produced data for S were significantly associated with experimental data (r2 = 0.92). In brief, a simple SIR (susceptible-infected-removed) model was successfully utilized to simulate observed data     

Plasma profiles of the sex steroids and gonadotropins in maturing female spring chinook salmon (Oncorhynchus tshawytscha)

Plasma testosterone concentrations were low through the spring and early summer, concentrations began rising in late July and reached maximum levels by ovulation in September. Plasma concentrations of 11-ketotestosterone were low throughout sexual maturation until ovulation when a significant increase occurred. Plasma androstendione and 17β-estradiol concentrations were high throughout sexual maturation, and decreased significantly at ovulation. Plasma 17α,20β-dihydroxy-4-pregnen-3-one concentrations were low throughout maturation, and increased significantly at ovulation. Plasma gonadotropin I concentrations paralleled those of estradiol and exceeded gonadotropin II levels prior to ovulation. Plasma concentrations of gonadotropin II were low throughout the spring and summer, increasing dramatically at ovulation.     

Isolation and characterization of a trypsin fraction from the pyloric ceca of chinook salmon (Oncorhynchus tshawytscha)

A trypsin fraction was isolated from the pyloric ceca of New Zealand farmed chinook salmon (Oncorhynchus tshawytscha) by ammonium sulfate fractionation, acetone precipitation and affinity chromatography. The chinook salmon enzyme hydrolyzed the trypsin-specific synthetic substrate benzoyl-dl-arginine-p-nitroanilide (dl-BAPNA), and was inhibited by the general serine protease inhibitor phenyl methyl sulfonyl fluoride (PMSF), and also by the specific trypsin inhibitors — soybean trypsin inhibitor (SBTI) and benzamidine. The enzyme was active over a broad pH range (from 7.5 to at least pH 10.0) at 25 °C and was stable from pH 4.0 to pH 10.0 when incubated at 20 °C, with a maximum at pH 8.0. The optimum temperature for the hydrolysis of dl-BAPNA by the chinook salmon enzyme was 60 °C, however, the enzyme was unstable at temperatures above 40 °C. The molecular mass of the chinook salmon trypsin was estimated as 28 kDa by SDS–PAGE.