Smolt Stage Research Articles

Developing molecular classifiers to detect environmental stressors, smolt stages and morbidity in coho salmon, Oncorhynchus kisutch

Aquatic species are increasingly confronted with environmental stressors because of climate change. Although molecular technologies have advanced our understanding of how organisms respond to stressors in laboratory settings, the ability to detect physiological responses to specific stressors under complex field conditions remains underdeveloped. This research applied multi-stressor challenge trials on coho salmon, employing the “Salmon Fit-Chips” genomic tool and a random forest-based classification model to develop classifiers predictive for chronic thermal and hypoxic stress, as well as salinity acclimation, smolt stage and morbidity status. The study also examined how smolts and de-smolts (smolts not having entered SW during the smolt window) responded transcriptionally to exposure to saltwater. Using RF classifiers optimized with 4 to 12 biomarkers, we identified transcriptional signatures that accurately predicted the presence of each stressor and physiological state, achieving prediction accuracy rates between 86.8 % and 100 %, regardless of other background stressors present. Stressor recovery time was established by placing fish back into non-stressor conditions after stress exposure, providing important context to stressor detections in field applications. Recovery from thermal and hypoxic stress requires about 3 and 2 days, respectively, with >3 days needed for re-acclimation to freshwater for seawater acclimated fish. The study also found non-additive (synergistic) effects of multiple stressors on mortality risk. Importantly, osmotic stress associated with de-smolts was the most important predictor of mortality. In saltwater, de-smolts exposed to salinity, high temperature, and hypoxia experienced a 9-fold increase in mortality compared to those only exposed to saltwater, suggesting a synergistic response to multiple stressors. These findings suggest that delays in hatchery releases to support release of larger fish need to be carefully scrutinized to ensure fish are not being released as de-smolts, which are highly susceptible to additional climate-induced stressors like rising temperatures and reduced dissolved oxygen levels in the marine environment.

Metabolomic responses of Atlantic salmon (Salmo salar) cultured during the pre-smolt, smolt and post-smolt stages

Atlantic salmon (Salmo salar) is an anadromous salmonid that commonly undergoes a transfer from land-based freshwater hatcheries to ocean-based sea pens for cultivation. For investigating the metabolomic response properties in the cultured Atlantic salmon (∼ 400 g, n = 8 in each group) during the pre-smolt, smolt and post-smolt stages, we performed LC-MS/MS metabolomics to assess the metabolomic responses on the brain and skin of Atlantic salmon. A total of 153 and 115 differentially expressed metabolites (DEMs) exhibited significant variations in the brain and skin, respectively. These metabolites were mainly involved in three metabolic categories of amino acids, organic acids and fatty acids. In the brain, metabolites associated with cortisol synthesis and secretion and biosynthesis of cofactors were highly induced in the smolt stage compared to the pre-smolt stage, while in the post-smolt stage, levels of lipid and amino acid notably reduced. Moreover, 3-(3-hydroxyphenyl) propanoic acid, mannose 6-phosphate, cortisol, thymine, and guanine, as candidate smolting metabolites in the brain, displayed life stage-dependent alterations. In the skin, 3-methoxytyramine, L-sepiapterin, and nucleotide metabolism differed significantly during smoltification. Overall, these results reveal diverse life-stage-associated metabolomic responses in Atlantic salmon and provide important information for the aquaculture of Atlantic salmon.

Precocious Maturation of Hatchery‐Raised Spring Chinook Salmon as Age‐2 Minijacks Is Not Detectably Affected by Sire Age

AbstractJuvenile males produced in spring Chinook Salmon Oncorhynchus tshawytscha hatchery programs can exhibit high rates of maturation in freshwater as 2‐year‐old “minijacks.” This phenomenon is associated with high feeding rates and increased size and/or growth that juveniles experience in the hatchery environment, though studies also support a genetic component affecting age of maturation among salmonids, including precocious maturation in freshwater. This prompted a study to test whether the age of natural‐origin spring Chinook Salmon broodstock affects the rate at which their hatchery‐raised male progeny mature as age‐2 minijacks. In three consecutive brood years, we factorially mated age‐4 adult females with age‐3, age‐4, and age‐5 adult (jacks) male broodstock. In the latter two brood years, we also incorporated age‐1 precocious parr (microjacks) as sires. After communal rearing to the smolt stage (age 1), male juveniles were characterized as immature or as maturing minijacks based on plasma 11‐ketotestosterone concentration, and each was identified to its respective full‐sib progeny group via genetic parentage analysis. A generalized linear mixed model, performed for each brood year separately, was used to characterize expected precocious maturation rates by sire age, while controlling for potential effects of smolt body weight and individual parent identities. Multiple comparisons across sire ages within brood years were used to evaluate relative rates of precocious maturation. Estimates of the probability of minijack maturation among families within sire ages and brood years varied from as much as 0% to 100%, and no consistent effect of sire age on precocious maturation rate was observed. Exploratory analyses investigating additional effects of egg size, dam length, and spawn date also failed to identify consistent predictors of precocious maturation. Instead, variability was largely attributed to both dam‐ and sire‐specific effects, indicating a heritable component to precocious maturation, though not detectably associated with other measured attributes.

The influences of pre- and post-smolt captive rearing environments on growth, maturation, body size, and reproductive success of steelhead (Oncorhynchus mykiss) released as adults

Conservation hatcheries designed to aid in recovery of imperiled fish population often implement atypical rearing and release strategies. We evaluated a conservation hatchery program for steelhead (Oncorhynchus mykiss) that hydraulically removed naturally spawned eggs and captively reared them in different freshwater hatcheries to the smolt stage and reared smolts in both fresh water and seawater to sexual maturity, before releasing the sexually maturing adults onto the spawning grounds. The adult steelhead added to the spawning population, accounting for most of the adults observed during snorkel observations. They produced 32% of the juvenile offspring sampled, and females were 2.9 times more successful than males. Reproductive success was positively correlated with female body size, which was influenced by pre-smolt and post-smolt rearing conditions and their effects on growth rate and age-at-maturity. Juvenile offspring of the released adults showed size and age differences from offspring of naturally returning steelhead, but exhibited very similar early marine survival rates.

Delayed maturity does not offset negative impact afflicted by ectoparasitism in salmon

The negative effects of parasitism on host population dynamics may be mediated by plastic compensatory life-history changes in hosts. Theory predicts that hosts should shift their life-history towards early reproduction in response to virulent pathogens to maximize reproduction before death. However, for sublethal infections that affect growth, hosts whose fecundity is correlated with body size are predicted to shift towards delayed reproduction associated with larger body size and higher fecundity. This has been observed in Atlantic salmon and parasitic sea lice, via mark-recapture studies that recover mature fish from paired groups of control and parasiticide-treated smolts. We investigated whether such louse-induced changes to age at maturity can offset some of the negative effect of mortality on population growth rate in salmon using a structured population matrix model. Model results show that delayed maturity can partially compensate for reduced survival. However, this only occurs when marine survival is moderate to poor and growth conditions at sea are good. Also, the impact of delayed maturity on population growth when parameterizing the model with empirical data is negligible compared with effects of direct mortality. Our model thus suggests that management that works on minimizing the effect of sea lice from fish farms on wild salmon should focus mainly on correctly quantifying the effect of parasite-induced mortality during the smolt stage if the goal is to maximize population growth rate.

Dynamics of BK channel expression in gills during smoltification of Atlantic Salmon under farm conditions

Smoltification is a crucial step in the aquaculture of Atlantic salmon. Currently, the Na+/K+ ATPase (NKA) test is used to determine whether fish have adapted to seawater when moved from freshwater. The mortality rate of unadapted fish in Chile is ~10% after smoltification. During this period, BK channel expression decreases in gills after two weeks in saline water under laboratory conditions. However, these findings have not yet been tested on an industrial scale. We characterized the changes in mRNA levels of BK channels in gills from Salmo salar during smoltification using salmon farming protocols and compare these results against gill NKA activity and found a significant decrease in expression of BK channel transcripts during the smolt stage. We also characterized the dynamic of the BK channel expression (mRNA) versus NKA activity by using transcriptomic and RT-PCR analyses under an industrial setting. BK channels are involved in a variety of physiological processes, and our results indicate that BK potassium channel expression in gills could form the basis of a new complementary test to determine the level of smoltification more precisely.

Identification of Hypoxia-Specific Biomarkers in Salmonids Using RNA-Sequencing and Validation Using High-Throughput qPCR.

Identifying early gene expression responses to hypoxia (i.e., low dissolved oxygen) as a tool to assess the degree of exposure to this stressor is crucial for salmonids, because they are increasingly exposed to hypoxic stress due to anthropogenic habitat change, e.g., global warming, excessive nutrient loading, and persistent algal blooms. Our goal was to discover and validate gill gene expression biomarkers specific to the hypoxia response in salmonids across multi-stressor conditions. Gill tissue was collected from 24 freshwater juvenile Chinook salmon (Oncorhynchus tshawytscha), held in normoxia [dissolved oxygen (DO) > 8 mg L-1] and hypoxia (DO = 4‒5 mg L-1) in 10 and 18° temperatures for up to six days. RNA-sequencing (RNA-seq) was then used to discover 240 differentially expressed genes between hypoxic and normoxic conditions, but not affected by temperature. The most significantly differentially expressed genes had functional roles in the cell cycle and suppression of cell proliferation associated with hypoxic conditions. The most significant genes (n = 30) were selected for real-time qPCR assay development. These assays demonstrated a strong correlation (r = 0.88; P < 0.001) between the expression values from RNA-seq and the fold changes from qPCR. Further, qPCR of the 30 candidate hypoxia biomarkers was applied to an additional 322 Chinook salmon exposed to hypoxic and normoxic conditions to reveal the top biomarkers to define hypoxic stress. Multivariate analyses revealed that smolt stage, water salinity, and morbidity status were relevant factors to consider with the expression of these genes in relation to hypoxic stress. These hypoxia candidate genes will be put into application screening Chinook salmon to determine the identity of stressors impacting the fish.

Genetics of growth and survival under chronic heat stress and trade-offs with growth- and robustness-related traits in rainbow trout

Artificial selection can be advantageous for breeding fish that are better adapted to temperature changes induced by climate change. Selecting fish with better growth performance and survival under long term thermal stress would require an understanding of the level of additive genetic variation for these traits. Moreover, disentangling the genetic co-variation between growth- and robustness-related traits is key to understand potential antagonisms between selection for an increased performance and survivability. Thus, this study aimed at: i) assessing levels of genetic variation for growth and survival under chronic heat stress, and ii) evaluating the genetic relationship between these two traits and a broad range of growth- and robustness-related traits to identify potential trade-offs in rainbow trout. Phenotypic and pedigree records (n = 65,298 animals) from a rainbow trout breeding population were used. Growth and survival under chronic thermal stress were defined as the final body weight (FWHT) and days to death (TSR), after the experimental chronic exposure to high temperature (between 18 and 22 °C). Other growth-related traits analyzed here were: body weight at smolt stage (SW), harvest weight (HW), weight at processing plant (WPP), fillet weight (FW), weight of the eviscerated fish (GUW) and body weight at low temperature (FWLT). Robustness-related traits evaluated here were: resistance against Piscirickettsia salmonis (PSR) and Flavobacterium psychrophilum (FPR), and susceptibility to Caligus rogercresseyi (CS). The heritabilites estimated were: FWHT = 0.41 ± 0.14; TSR = 0.13 ± 0.05; SW = 0.54 ± 0.01; HW = 0.63 ± 0.01; WPP = 0.19 ± 0.02; FW = 0.03 ± 0.03; GUW = 0.18 ± 0.02; FWLT = 0.59 ± 0.17; FPR = 0.35 ± 0.05; PSR = 0.30 ± 0.05; and CS 0.08 ± 0.02. In general, strong compensations were identified between the robustness- and growth- related traits with genetic correlations that ranged from −0.28 to −0.98, except for resistance to F. psychrophilum and growth-related traits, which showed correlation values from 0.28 to 0.46. Genetic improvement for growth and survival under chronic upper thermal exposure is feasible. Compensations between growth- and robustness- related traits have to be accounted for when simultaneously including performance and disease resistance traits in the breeding objective.

Passage through a hydropower plant affects the physiological and health status of Atlantic salmon smolts

Atlantic salmon is an anadromous species migrating from upper-reach nursery areas in rivers to the oceanic feeding areas at smolt stage and inversely at adult stage requiring unimpeded migration routes. However, dams associated with hydroelectric power plants (HPP) disrupt river connectivity and affect fish movement and survival. The objective of the current study was to evaluate the short and mid-term physiological and immune response of Atlantic salmon smolts after passing through Andenne HPP (Meuse River, Belgium). Several parameters were studied after an in situ deliberate passage including direct mortality and external damages, stress and immune biomarkers as plasma cortisol and glucose levels, complement and peroxidase activities, and immune and oxidative stress related gene expression 24 h, 72 h and 120 h after passage. Survival rate was lower and external damages were more important in fish that confronted the HPP compared to the control ones. Moreover, the passage through the turbine affected plasma glucose levels, complement and peroxidase activities and the expression of some immune genes such as lysg, igm and mpo in a timely manner suggesting that this passage can lead to a great energy expenditure and a disruption of innate immunity. Our observations can partially explain the delayed mortality observed in many studies leading to a poor success of restocking programs. HPPs not only have a direct impact in terms of mortalities and injuries but also an indirect one in terms of physiological and immune changes that can compromise Atlantic salmon smolts ability to escape successfully to the ocean.

Comparative transcriptome profiling of selected osmotic regulatory proteins in the gill during seawater acclimation of chum salmon (Oncorhynchus keta) fry

Salmonid fishes, chum salmon (Oncorhynchus keta) have the developed adaptive strategy to withstand wide salinity changes from the early life stage. This study investigated gene expression patterns of cell membrane proteins in the gill of chum salmon fry on the transcriptome level by tracking the salinity acclimation of the fish in changing environments ranging from freshwater (0 ppt) to brackish water (17.5 ppt) to seawater (35 ppt). Using GO analysis of DEGs, the known osmoregulatory genes and their functional groups such as ion transport, transmembrane transporter activity and metal ion binding were identified. The expression patterns of membrane protein genes, including pump-mediated protein (NKA, CFTR), carrier-mediated protein (NKCC, NHE3) and channel-mediated protein (AQP) were similar to those of other salmonid fishes in the smolt or adult stages. Based on the protein-protein interaction analysis between transmembrane proteins and other related genes, we identified osmotic-related genes expressed with salinity changes and analyzed their expression patterns. The findings of this study may facilitate the disentangling of the genetic basis of chum salmon and better able an understanding of the osmophysiology of the species.

Plasma proteome profiling of freshwater and seawater life stages of rainbow trout (Oncorhynchus mykiss).

The sea-run phenotype of rainbow trout (Oncorhynchus mykiss), like other anadromous salmonids, present a juvenile stage fully adapted to life in freshwater known as parr. Development in freshwater is followed by the smolt stage, where preadaptations needed for seawater life are developed making fish ready to migrate to the ocean, after which event they become post-smolts. While these three life stages have been studied using a variety of approaches, proteomics has never been used for such purpose. The present study characterised the blood plasma proteome of parr, smolt and post-smolt rainbow trout using a gel electrophoresis liquid chromatography tandem mass spectrometry approach alone or in combination with low-abundant protein enrichment technology (combinatorial peptide ligand library). In total, 1,822 proteins were quantified, 17.95% of them being detected only in plasma post enrichment. Across all life stages, the most abundant proteins were ankyrin-2, DNA primase large subunit, actin, serum albumin, apolipoproteins, hemoglobin subunits, hemopexin-like proteins and complement C3. When comparing the different life stages, 17 proteins involved in mechanisms to cope with hyperosmotic stress and retinal changes, as well as the downregulation of nonessential processes in smolts, were significantly different between parr and smolt samples. On the other hand, 11 proteins related to increased growth in post-smolts, and also related to coping with hyperosmotic stress and to retinal changes, were significantly different between smolt and post-smolt samples. Overall, this study presents a series of proteins with the potential to complement current seawater-readiness assessment tests in rainbow trout, which can be measured non-lethally in an easily accessible biofluid. Furthermore, this study represents a first in-depth characterisation of the rainbow trout blood plasma proteome, having considered three life stages of the fish and used both fractionation alone or in combination with enrichment methods to increase protein detection.

Effects of dietary glutamate and succinate on growth performance and mitochondrial respiration in heart and liver of Atlantic salmon (Salmo salar) smolts

The smolt stage of salmon has challenges in reaching adequate growth rates due to the changing environmental conditions at sea. Therefore, it is necessary to provide adequate diets to achieve sufficient growth. This study determined the impacts of glutamate and succinate (1% each) supplemented diet on the growth of Atlantic salmon smolts along with characterization of mitochondrial respiration using high-resolution respirometry technique. Results indicated that there was no significant difference in growth response between the treatment and control groups. Maximum oxidative phosphorylation (OXPHOS) was reached after addition of succinate. Analysis of heart homogenates revealed a significant difference in LEAK respiration state (P = 0.005). No significant difference was recorded between the diet groups for liver homogenates. Differences between heart and liver respiration revealed that mitochondrial activity is organ dependent.

The expression pattern of calcium signaling-related genes during smoltification of Salmo salar in productive conditions.

Variations in the mRNA expression of hepatic and muscle genes that are related to calcium signaling were analyzed by real-time qPCR in farmed Atlantic salmon (Salmo salar L. 1758) to determine changes in expression between parr and smolt stages. These organs were selected due to their close relationship with calcium signaling and metabolism (e.g., glycolysis, oxidative phosphorylation, muscle contraction). Differential expression between smolt and parr specimens and between organs was observed. Compared to parr specimens, smolts exhibited upregulated expression of the calcitonin receptor precursor, calcitonin receptor, calcitonin isoform, parathyroid hormone, and calmodulin in the liver. This pattern was inverse in muscle, with the exception of calmodulin, which was significantly upregulated in smolts compared to parr. Additionally, plasma calcium was decreased in the smolt condition. This study is the first to characterize the expression pattern of calcium signaling-related genes in the liver and muscle of parr and smolt S. salar. However, further functional studies are required to obtain a wider understanding about the physiological changes that accompany the productive conditions during smoltification.

Impact of dietary phosphorous in diploid and triploid Atlantic salmon (Salmo salar L.) with reference to early skeletal development in freshwater

In order to assess the effect of dietary phosphorus (P) in reducing vertebral malformations and improving freshwater (FW) performance in triploid Atlantic salmon (Salmo salar), both triploid and diploid Atlantic salmon were fed three different dietary P inclusion levels (low: 4.9, medium: 7.7, and high: 9.7 g available P kg−1) from first feeding until smolt. Somatic and skeletal response was assessed at fry (~0.5 g), parr (~5 g) and smolt (~45 g) stages. Triploid parr initially grew faster on the high P diet, while groups fed low P resulted in a significantly higher weight at smolt. Image analysis of double stained Alcian blue and Alizarin red S fry revealed that low P fed triploid fish presented less well mineralised vertebrae, and significantly more malformed vertebrae in both parr and smolt stages following x-ray radiographic assessment. Triploid parr fed high and medium P had similar numbers of malformed vertebrae relative to their diploid counterparts but greater numbers than at smolt. Low P fed triploids had the highest prevalence of jaw and vertebral malformations as well as the highest number of deformed vertebrae in the central caudal vertebral region, which was more pronounced at parr than at smolt. Shorter vertebrae dorso-ventral lengths were observed throughout the spinal column (R1–R4) in parr fed low P and only in the caudal region (R3) at smolt. In parr, both ploidies showed reduced phosphate homeostasis protein fgf23 gene expression in vertebrae when fed low P diets, while triploids showed greater down-regulation of osteogenic factors (alp, opn and igf1r) between diets relative to diploids, suggesting possible greater active suppression of mineralisation and reduced osteogenic potential in triploids. No effects of diet or ploidy on gene expression were evident at smolt. Comparisons between development stages suggest early P supplementation in triploids is crucial for skeletal development. Ultimately, reducing vertebral deformities observed at smolt with higher P supplementation in triploids could contribute towards improving skeletal performance and welfare of the stocks in the marine phase.

Rearing strategies alter patterns of size-selective mortality and heritable size variation in steelhead trout (Oncorhynchus mykiss)

Steelhead trout (Oncorhynchus mykiss) reared under two different regimes: high food ration for 1 year (S1; typical strategy) or low ration for 2 years (S2) were subjected to a seawater challenge during the corresponding spring outmigration period. The S1smolts were smaller and suffered greater seawater challenge mortality (23.9% compared with 0.7% for the S2smolts) that was significantly and negatively related to body size. Heritability of body size was similar for the two treatments during the parr stage (fork length: S1= 0.181, S2= 0.245; mass: S1= 0.372; S2= 0.447), but higher for the S1treatment during the smolt stage for length (S1= 0.212, S2= 0.002) and body mass (S1= 0.145, S2= 0.015). Strong family effects for both traits and significant family by environment interactions for parr mass and smolt length indicated significant phenotypic plasticity. A genetic response to size-selective mortality caused by insufficient growth opportunity in the S1treatment is plausible and may affect fitness in the natural environment through effects on correlated traits.

Experimental Evidence for Olfactory Imprinting by Sockeye Salmon at Embryonic and Smolt Stages

AbstractAnadromous salmonids have an extraordinary ability to migrate back to their natal streams to spawn as adults, but the mechanisms underlying this ability are not completely known. Many experiments indicate that salmon imprint on natal odors at the smolt stage prior to seaward migration, but the life history and population genetics of some species, notably Sockeye Salmon Oncorhynchus nerka, suggest that imprinting also occurs during the period between hatching and emergence from the gravel as fry. To test the hypothesis that Sockeye Salmon imprint during this period, we exposed juveniles to a mixture of odorants during either the alevin or smolt stage. The smolt exposure group was further divided into different exposure durations (6 weeks, 1 week, and 1 d) to evaluate the duration of odor exposure needed for imprinting during that stage. Imprinting was assessed by testing fish as mature adults in two‐choice mazes containing unfamiliar water with and without the mixture of odorants. Fish exposed either as alevins or for 6 weeks as smolts both spent significantly more time in the odor‐scented arm than control fish (unexposed to the odors as juveniles). Fish exposed to odors for 1 week or 1 d as smolts showed similar but weaker responses. Concurrent measures of gill Na+/K+‐ATPase activity and plasma thyroxine confirmed that the fish exposed as smolts were undergoing parr–smolt transformation during exposure. We conclude that Sockeye Salmon imprinted as both alevins and smolts and that longer periods of odor exposure yielded greater behavioral responses to odors as adults, though specific times within the parr–smolt transformation period may be more sensitive to imprinting than others.Received June 21, 2016; accepted September 13, 2016 Published online December 2, 2016

Identification and expressional analysis of NLRC5 inflammasome gene in smolting Atlantic salmon (Salmo salar)

The NOD-like receptors (NLRs) were recently identified as an intracellular pathogen recognition receptor family in vertebrates. While the immune system participation of NLRs has been characterized and analyzed in various mammalian models, few studies have considered NLRs in teleost species. Therefore, this study analyzed the Atlantic salmon (Salmo salar) NLRC5. Structurally, Atlantic salmon NLRC5 presented leucine-rich repeat subfamily genes. Phylogenetically, NLRC5 was moderately conserved between S. salar and other species. Real-time quantitative PCR revealed NLRC5 expression in almost all analyzed organs, with greatest expressions in the head kidney, spleen, and hindgut. Furthermore, NLRC5 gene expression decreased during smolt stage. These data suggest that NLRC5 participates in the Atlantic salmon immune response and is regulated, at least partly, by the smoltification process, suggesting that there is a depression of immune system from parr at smolt stage. This is the first report on the NLRC5 gene in salmonid smolts.

Effects of Recombinant Aquaporin 3 and Seawater Acclimation on the Expression of Aquaporin 3 and 8 mRNAs in the Parr and Smolt Stages of Rainbow Trout, Oncorhynchus mykiss

This study aimed to examine the role of two aquaporin isoforms (AQP3 and AQP8) in response to the hyperosmotic challenge of transitioning from freshwater (FW) to seawater (SW) during parr and smoltification (smolt) using the rainbow trout, Oncorhynchus mykiss. We examined the changes in the expression of AQPs mRNAs in the gills and intestine of the parr and smolt stages of rainbow trout transferred from FW to SW using quantitative real-time PCR in an osmotically changing environment [FW, SW, and recombinant AQP3 (rAQP3) injection at two dosage rates]. Correspondingly, AQPs were greater during smoltification than during parr stages in the rainbow trout. Plasma osmolality and gill Na⁺/K⁺-ATPase activity increased when the fish were exposed to SW, but these parameters decreased when the fish were exposed to SW following treatment with rAQP3 during the transition to seawater. Our results suggest that AQPs play an important role in water absorbing mechanisms associated with multiple AQP isoforms in a hyperosmotic environment.

Changes of physiological rhythms of N-methyl-d-aspartate receptors in the chum salmon Oncorhynchus keta: effect of seawater acclimation during the parr-smolt transformation

We examined changes on N-methyl-d-aspartate receptors (NRs) in different growth stages (early parr, parr, and early smolt) of chum salmon, Oncorhynchus keta, during parr-smolt transformation from freshwater to seawater. Expression levels of NR genes mRNA and concentration of cortisol, T3, T4, dopamine and Na+/K+-ATPase activity significantly increased at salinity change condition. Moreover, in cultured brain cells, NRs were significantly lower in all groups treated with MK-801 (an antagonist of NRs) than in the early parr stage group in the FW treatment. We confirmed that the reduction in mRNA expression levels of NRs increased from the early parr to the early smolt stage. The information reported here should be taken into account in future studies on the relationship between memory factors of natal streams and homing mechanisms in Salmonidae.

Profiles of hypothalamus–pituitary–interrenal axis gene expression in the parr and smolt stages of rainbow trout, Oncorhynchus mykiss: Effects of recombinant aquaporin 3 and seawater acclimation

The objective of this investigation was to quantify how the hypothalamus–pituitary–interrenal (HPI) axis in the rainbow trout, Oncorhynchus mykiss (parr/smolt), responds to salinity changes during transfer from freshwater (FW) to seawater (SW) and recombinant aquaporin 3 (rAQP3) injection. mRNA expression levels of HPI axis genes [corticotropic-releasing hormone (CRH) and adrenocorticotropic hormone (ACTHα and ACTHβ)] significantly increased when the fish were transferred from FW to SW (parr: 16.4-, 13.2-, 21.4-, and 11.9-fold higher than FW; smolt: 2.3-, 2.7-, 13.6-, and 6.2-fold higher than FW, respectively). Furthermore, and the plasma ACTH, Na+, Cl−, and K+ levels were the highest at 50% SW. Moreover, these parameters were significantly lower in the rAQP3-treated group than those in the control (parr: 2.0-, 2.4-, 2.1-, and 2.0-fold lower than SW; smolt: 4.2-, 1.9-, 2.4-, and 2.3-fold lower than SW, respectively). Hence, HPI axis genes may play a role in SW adaptation during migration from FW to SW environments. We showed that there was a negative correlation between rAQP3, HPI axis genes, and ion levels when the fish were transferred to SW, with levels being significantly lower in the rAQP3-injected group. Hence, cortisol appears to be a stress hormone and plasma Na+ and Cl− levels significantly increased when the fish were transferred to SW, with levels being significantly lower in the rAQP3-treated group. These results indicate that rAQP3 modulates the HPI axis and ion transportation in rainbow trout.